在專案中做了一個關於viewpager滾動翻頁，因為做的是平板介面，所以左右滾動非常不靈敏，非的滾動很大距離才可以，為此我們需要重寫viewpager修改minOageOffset的值，預設是0.6f,這裡我們設定為0.3f就大大提高了靈敏度。此值不易修改需要重寫viewpager 這裡為MinOffsetViewPager，此外還要重寫他的adapter，這裡為MinOffsetPagerAdapter

專案目錄，如果裡面還巢狀一個Fragment就用第一個MFragmetPagerAdpter，沒有就直接用。畫不多說直接上程式碼，導包還是自己導！

MinOffsetFragmentPagerAdapter

：

MinOffsetViewPager：

 */
/*就是重寫了系統自帶的viewpager，還必須重寫他的adapter才能自定義改他的滑動距離
* */
public class MinOffsetViewPager extends ViewGroup {
    private static final String TAG = "ViewPager";
    private static final boolean DEBUG = false;

    private static final boolean USE_CACHE = false;

    private static final int DEFAULT_OFFSCREEN_PAGES = 1;
    private static final int MAX_SETTLE_DURATION = 600; // ms
    private static final int MIN_DISTANCE_FOR_FLING = 25; // dips

    private static final int DEFAULT_GUTTER_SIZE = 16; // dips

    private static final int MIN_FLING_VELOCITY = 400; // dips

    static final int[] LAYOUT_ATTRS = new int[]{
            android.R.attr.layout_gravity
    };

    /**
     * Used to track what the expected number of items in the adapter should be.
     * If the app changes this when we don't expect it, we'll throw a big obnoxious exception.
     */
    private int mExpectedAdapterCount;
    /** 最小可切換Page的偏移量,原來是的0.6f，因為我們是大螢幕，所以修改成0.3f，這樣左右滑動的靈敏度就出來了**/
    private float minPageOffset = 0.3f;

    public void setMinPageOffset(float pageOffset){
        minPageOffset = pageOffset;
    }

    static class ItemInfo {
        Object object;
        int position;
        boolean scrolling;
        float widthFactor;
        float offset;
    }

    private static final Comparator<ItemInfo> COMPARATOR = new Comparator<ItemInfo>() {
        @Override
        public int compare(ItemInfo lhs, ItemInfo rhs) {
            return lhs.position - rhs.position;
        }
    };

    private static final Interpolator sInterpolator = new Interpolator() {
        @Override
        public float getInterpolation(float t) {
            t -= 1.0f;
            return t * t * t * t * t + 1.0f;
        }
    };

    private final ArrayList<ItemInfo> mItems = new ArrayList<ItemInfo>();
    private final ItemInfo mTempItem = new ItemInfo();

    private final Rect mTempRect = new Rect();

    MinOffsetPagerAdapter mAdapter;
    int mCurItem;   // Index of currently displayed page.
    private int mRestoredCurItem = -1;
    private Parcelable mRestoredAdapterState = null;
    private ClassLoader mRestoredClassLoader = null;

    private Scroller mScroller;
    private boolean mIsScrollStarted;

    private PagerObserver mObserver;

    private int mPageMargin;
    private Drawable mMarginDrawable;
    private int mTopPageBounds;
    private int mBottomPageBounds;

    // Offsets of the first and last items, if known.
    // Set during population, used to determine if we are at the beginning
    // or end of the pager data set during touch scrolling.
    private float mFirstOffset = -Float.MAX_VALUE;
    private float mLastOffset = Float.MAX_VALUE;

    private int mChildWidthMeasureSpec;
    private int mChildHeightMeasureSpec;
    private boolean mInLayout;

    private boolean mScrollingCacheEnabled;

    private boolean mPopulatePending;
    private int mOffscreenPageLimit = DEFAULT_OFFSCREEN_PAGES;

    private boolean mIsBeingDragged;
    private boolean mIsUnableToDrag;
    private int mDefaultGutterSize;
    private int mGutterSize;
    private int mTouchSlop;
    /**
     * Position of the last motion event.
     */
    private float mLastMotionX;
    private float mLastMotionY;
    private float mInitialMotionX;
    private float mInitialMotionY;
    /**
     * ID of the active pointer. This is used to retain consistency during
     * drags/flings if multiple pointers are used.
     */
    private int mActivePointerId = INVALID_POINTER;
    /**
     * Sentinel value for no current active pointer.
     * Used by {@link #mActivePointerId}.
     */
    private static final int INVALID_POINTER = -1;

    /**
     * Determines speed during touch scrolling
     */
    private VelocityTracker mVelocityTracker;
    private int mMinimumVelocity;
    private int mMaximumVelocity;
    private int mFlingDistance;
    private int mCloseEnough;

    // If the pager is at least this close to its final position, complete the scroll
    // on touch down and let the user interact with the content inside instead of
    // "catching" the flinging pager.
    private static final int CLOSE_ENOUGH = 2; // dp

    private boolean mFakeDragging;
    private long mFakeDragBeginTime;

    private EdgeEffect mLeftEdge;
    private EdgeEffect mRightEdge;

    private boolean mFirstLayout = true;
    private boolean mNeedCalculatePageOffsets = false;
    private boolean mCalledSuper;
    private int mDecorChildCount;

    private List<OnPageChangeListener> mOnPageChangeListeners;
    private OnPageChangeListener mOnPageChangeListener;
    private OnPageChangeListener mInternalPageChangeListener;
    private List<OnAdapterChangeListener> mAdapterChangeListeners;
    private PageTransformer mPageTransformer;
    private int mPageTransformerLayerType;

    private static final int DRAW_ORDER_DEFAULT = 0;
    private static final int DRAW_ORDER_FORWARD = 1;
    private static final int DRAW_ORDER_REVERSE = 2;
    private int mDrawingOrder;
    private ArrayList<View> mDrawingOrderedChildren;
    private static final ViewPositionComparator sPositionComparator = new ViewPositionComparator();

    /**
     * Indicates that the pager is in an idle, settled state. The current page
     * is fully in view and no animation is in progress.
     */
    public static final int SCROLL_STATE_IDLE = 0;

    /**
     * Indicates that the pager is currently being dragged by the user.
     */
    public static final int SCROLL_STATE_DRAGGING = 1;

    /**
     * Indicates that the pager is in the process of settling to a final position.
     */
    public static final int SCROLL_STATE_SETTLING = 2;

    private final Runnable mEndScrollRunnable = new Runnable() {
        @Override
        public void run() {
            setScrollState(SCROLL_STATE_IDLE);
            populate();
        }
    };

    private int mScrollState = SCROLL_STATE_IDLE;

    /**
     * Callback interface for responding to changing state of the selected page.
     */
    public interface OnPageChangeListener {

        /**
         * This method will be invoked when the current page is scrolled, either as part
         * of a programmatically initiated smooth scroll or a user initiated touch scroll.
         *
         * @param position             Position index of the first page currently being displayed.
         *                             Page position+1 will be visible if positionOffset is nonzero.
         * @param positionOffset       Value from [0, 1) indicating the offset from the page at position.
         * @param positionOffsetPixels Value in pixels indicating the offset from position.
         */
        void onPageScrolled(int position, float positionOffset, int positionOffsetPixels);

        /**
         * This method will be invoked when a new page becomes selected. Animation is not
         * necessarily complete.
         *
         * @param position Position index of the new selected page.
         */
        void onPageSelected(int position);

        /**
         * Called when the scroll state changes. Useful for discovering when the user
         * begins dragging, when the pager is automatically settling to the current page,
         * or when it is fully stopped/idle.
         */
        void onPageScrollStateChanged(int state);
    }

    /**
     * Simple implementation of the {@link OnPageChangeListener} interface with stub
     * implementations of each method. Extend this if you do not intend to override
     * every method of {@link OnPageChangeListener}.
     */
    public static class SimpleOnPageChangeListener implements OnPageChangeListener {
        @Override
        public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
            // This space for rent
        }

        @Override
        public void onPageSelected(int position) {
            // This space for rent
        }

        @Override
        public void onPageScrollStateChanged(int state) {
            // This space for rent
        }
    }

    /**
     * A PageTransformer is invoked whenever a visible/attached page is scrolled.
     * This offers an opportunity for the application to apply a custom transformation
     * to the page views using animation properties.
     * <p>
     * <p>As property animation is only supported as of Android 3.0 and forward,
     * setting a PageTransformer on a ViewPager on earlier platform versions will
     * be ignored.</p>
     */
    public interface PageTransformer {
        /**
         * Apply a property transformation to the given page.
         *
         * @param page     Apply the transformation to this page
         * @param position Position of page relative to the current front-and-center
         *                 position of the pager. 0 is front and center. 1 is one full
         *                 page position to the right, and -1 is one page position to the left.
         */
        void transformPage(View page, float position);
    }

    /**
     * Callback interface for responding to adapter changes.
     */
    public interface OnAdapterChangeListener {
        /**
         * Called when the adapter for the given view pager has changed.
         *
         * @param viewPager  ViewPager where the adapter change has happened
         * @param oldAdapter the previously set adapter
         * @param newAdapter the newly set adapter
         */
        void onAdapterChanged(@NonNull MinOffsetViewPager viewPager,
                              @Nullable MinOffsetPagerAdapter oldAdapter, @Nullable MinOffsetPagerAdapter newAdapter);
    }

    /**
     * Annotation which allows marking of views to be decoration views when added to a view
     * pager.
     * <p>
     * <p>Views marked with this annotation can be added to the view pager with a layout resource.
     * An example being {@link }.</p>
     * <p>
     * <p>You can also control whether a view is a decor view but setting
     * {@link LayoutParams#isDecor} on the child's layout params.</p>
     */
    @Retention(RetentionPolicy.RUNTIME)
    @Target(ElementType.TYPE)
    @Inherited
    public @interface DecorView {
    }

    public MinOffsetViewPager(Context context) {
        super(context);
        initViewPager();
    }

    public MinOffsetViewPager(Context context, AttributeSet attrs) {
        super(context, attrs);
        initViewPager();
    }

    void initViewPager() {
        setWillNotDraw(false);
        setDescendantFocusability(FOCUS_AFTER_DESCENDANTS);
        setFocusable(true);
        final Context context = getContext();
        mScroller = new Scroller(context, sInterpolator);
        final ViewConfiguration configuration = ViewConfiguration.get(context);
        final float density = context.getResources().getDisplayMetrics().density;

        mTouchSlop = configuration.getScaledPagingTouchSlop();
        mMinimumVelocity = (int) (MIN_FLING_VELOCITY * density);
        mMaximumVelocity = configuration.getScaledMaximumFlingVelocity();
        mLeftEdge = new EdgeEffect(context);
        mRightEdge = new EdgeEffect(context);

        mFlingDistance = (int) (MIN_DISTANCE_FOR_FLING * density);
        mCloseEnough = (int) (CLOSE_ENOUGH * density);
        mDefaultGutterSize = (int) (DEFAULT_GUTTER_SIZE * density);

        ViewCompat.setAccessibilityDelegate(this, new MyAccessibilityDelegate());

        if (ViewCompat.getImportantForAccessibility(this)
                == ViewCompat.IMPORTANT_FOR_ACCESSIBILITY_AUTO) {
            ViewCompat.setImportantForAccessibility(this,
                    ViewCompat.IMPORTANT_FOR_ACCESSIBILITY_YES);
        }

        ViewCompat.setOnApplyWindowInsetsListener(this,
                new android.support.v4.view.OnApplyWindowInsetsListener() {
                    private final Rect mTempRect = new Rect();

                    @Override
                    public WindowInsetsCompat onApplyWindowInsets(final View v,
                                                                  final WindowInsetsCompat originalInsets) {
                        // First let the ViewPager itself try and consume them...
                        final WindowInsetsCompat applied =
                                ViewCompat.onApplyWindowInsets(v, originalInsets);
                        if (applied.isConsumed()) {
                            // If the ViewPager consumed all insets, return now
                            return applied;
                        }

                        // Now we'll manually dispatch the insets to our children. Since ViewPager
                        // children are always full-height, we do not want to use the standard
                        // ViewGroup dispatchApplyWindowInsets since if child 0 consumes them,
                        // the rest of the children will not receive any insets. To workaround this
                        // we manually dispatch the applied insets, not allowing children to
                        // consume them from each other. We do however keep track of any insets
                        // which are consumed, returning the union of our children's consumption
                        final Rect res = mTempRect;
                        res.left = applied.getSystemWindowInsetLeft();
                        res.top = applied.getSystemWindowInsetTop();
                        res.right = applied.getSystemWindowInsetRight();
                        res.bottom = applied.getSystemWindowInsetBottom();

                        for (int i = 0, count = getChildCount(); i < count; i++) {
                            final WindowInsetsCompat childInsets = ViewCompat
                                    .dispatchApplyWindowInsets(getChildAt(i), applied);
                            // Now keep track of any consumed by tracking each dimension's min
                            // value
                            res.left = Math.min(childInsets.getSystemWindowInsetLeft(),
                                    res.left);
                            res.top = Math.min(childInsets.getSystemWindowInsetTop(),
                                    res.top);
                            res.right = Math.min(childInsets.getSystemWindowInsetRight(),
                                    res.right);
                            res.bottom = Math.min(childInsets.getSystemWindowInsetBottom(),
                                    res.bottom);
                        }

                        // Now return a new WindowInsets, using the consumed window insets
                        return applied.replaceSystemWindowInsets(
                                res.left, res.top, res.right, res.bottom);
                    }
                });
    }

    @Override
    protected void onDetachedFromWindow() {
        removeCallbacks(mEndScrollRunnable);
        // To be on the safe side, abort the scroller
        if ((mScroller != null) && !mScroller.isFinished()) {
            mScroller.abortAnimation();
        }
        super.onDetachedFromWindow();
    }

    void setScrollState(int newState) {
        if (mScrollState == newState) {
            return;
        }

        mScrollState = newState;
        if (mPageTransformer != null) {
            // PageTransformers can do complex things that benefit from hardware layers.
            enableLayers(newState != SCROLL_STATE_IDLE);
        }
        dispatchOnScrollStateChanged(newState);
    }

    /**
     * Set a PagerAdapter that will supply views for this pager as needed.
     *
     * @param adapter Adapter to use
     */
    public void setAdapter(MinOffsetPagerAdapter adapter) {
        if (mAdapter != null) {
            mAdapter.setViewPagerObserver(null);
            mAdapter.startUpdate(this);
            for (int i = 0; i < mItems.size(); i++) {
                final ItemInfo ii = mItems.get(i);
                mAdapter.destroyItem(this, ii.position, ii.object);
            }
            mAdapter.finishUpdate(this);
            mItems.clear();
            removeNonDecorViews();
            mCurItem = 0;
            scrollTo(0, 0);
        }

        final MinOffsetPagerAdapter oldAdapter = mAdapter;
        mAdapter = adapter;
        mExpectedAdapterCount = 0;

        if (mAdapter != null) {
            if (mObserver == null) {
                mObserver = new PagerObserver();
            }
            mAdapter.setViewPagerObserver(mObserver);
            mPopulatePending = false;
            final boolean wasFirstLayout = mFirstLayout;
            mFirstLayout = true;
            mExpectedAdapterCount = mAdapter.getCount();
            if (mRestoredCurItem >= 0) {
                mAdapter.restoreState(mRestoredAdapterState, mRestoredClassLoader);
                setCurrentItemInternal(mRestoredCurItem, false, true);
                mRestoredCurItem = -1;
                mRestoredAdapterState = null;
                mRestoredClassLoader = null;
            } else if (!wasFirstLayout) {
                populate();
            } else {
                requestLayout();
            }
        }

        // Dispatch the change to any listeners
        if (mAdapterChangeListeners != null && !mAdapterChangeListeners.isEmpty()) {
            for (int i = 0, count = mAdapterChangeListeners.size(); i < count; i++) {
                mAdapterChangeListeners.get(i).onAdapterChanged(this, oldAdapter, adapter);
            }
        }
    }

    private void removeNonDecorViews() {
        for (int i = 0; i < getChildCount(); i++) {
            final View child = getChildAt(i);
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();
            if (!lp.isDecor) {
                removeViewAt(i);
                i--;
            }
        }
    }

    /**
     * Retrieve the current adapter supplying pages.
     *
     * @return The currently registered PagerAdapter
     */
    public MinOffsetPagerAdapter getAdapter() {
        return mAdapter;
    }

    /**
     * Add a listener that will be invoked whenever the adapter for this ViewPager changes.
     *
     * @param listener listener to add
     */
    public void addOnAdapterChangeListener(@NonNull OnAdapterChangeListener listener) {
        if (mAdapterChangeListeners == null) {
            mAdapterChangeListeners = new ArrayList<>();
        }
        mAdapterChangeListeners.add(listener);
    }

    /**
     * Remove a listener that was previously added via
     * {@link #addOnAdapterChangeListener(OnAdapterChangeListener)}.
     *
     * @param listener listener to remove
     */
    public void removeOnAdapterChangeListener(@NonNull OnAdapterChangeListener listener) {
        if (mAdapterChangeListeners != null) {
            mAdapterChangeListeners.remove(listener);
        }
    }

    private int getClientWidth() {
        return getMeasuredWidth() - getPaddingLeft() - getPaddingRight();
    }

    /**
     * Set the currently selected page. If the ViewPager has already been through its first
     * layout with its current adapter there will be a smooth animated transition between
     * the current item and the specified item.
     *
     * @param item Item index to select
     */
    public void setCurrentItem(int item) {
        mPopulatePending = false;
        setCurrentItemInternal(item, !mFirstLayout, false);
    }

    /**
     * Set the currently selected page.
     *
     * @param item         Item index to select
     * @param smoothScroll True to smoothly scroll to the new item, false to transition immediately
     */
    public void setCurrentItem(int item, boolean smoothScroll) {
        mPopulatePending = false;
        setCurrentItemInternal(item, smoothScroll, false);
    }

    public int getCurrentItem() {
        return mCurItem;
    }

    void setCurrentItemInternal(int item, boolean smoothScroll, boolean always) {
        setCurrentItemInternal(item, smoothScroll, always, 0);
    }

    void setCurrentItemInternal(int item, boolean smoothScroll, boolean always, int velocity) {
        if (mAdapter == null || mAdapter.getCount() <= 0) {
            setScrollingCacheEnabled(false);
            return;
        }
        if (!always && mCurItem == item && mItems.size() != 0) {
            setScrollingCacheEnabled(false);
            return;
        }

        if (item < 0) {
            item = 0;
        } else if (item >= mAdapter.getCount()) {
            item = mAdapter.getCount() - 1;
        }
        final int pageLimit = mOffscreenPageLimit;
        if (item > (mCurItem + pageLimit) || item < (mCurItem - pageLimit)) {
            // We are doing a jump by more than one page.  To avoid
            // glitches, we want to keep all current pages in the view
            // until the scroll ends.
            for (int i = 0; i < mItems.size(); i++) {
                mItems.get(i).scrolling = true;
            }
        }
        final boolean dispatchSelected = mCurItem != item;

        if (mFirstLayout) {
            // We don't have any idea how big we are yet and shouldn't have any pages either.
            // Just set things up and let the pending layout handle things.
            mCurItem = item;
            if (dispatchSelected) {
                dispatchOnPageSelected(item);
            }
            requestLayout();
        } else {
            populate(item);
            scrollToItem(item, smoothScroll, velocity, dispatchSelected);
        }
    }

    private void scrollToItem(int item, boolean smoothScroll, int velocity,
                              boolean dispatchSelected) {
        final ItemInfo curInfo = infoForPosition(item);
        int destX = 0;
        if (curInfo != null) {
            final int width = getClientWidth();
            destX = (int) (width * Math.max(mFirstOffset,
                    Math.min(curInfo.offset, mLastOffset)));
        }
        if (smoothScroll) {
            smoothScrollTo(destX, 0, velocity);
            if (dispatchSelected) {
                dispatchOnPageSelected(item);
            }
        } else {
            if (dispatchSelected) {
                dispatchOnPageSelected(item);
            }
            completeScroll(false);
            scrollTo(destX, 0);
            pageScrolled(destX);
        }
    }

    /**
     * Set a listener that will be invoked whenever the page changes or is incrementally
     * scrolled. See {@link OnPageChangeListener}.
     *
     * @param listener Listener to set
     * @deprecated Use {@link #addOnPageChangeListener(OnPageChangeListener)}
     * and {@link #removeOnPageChangeListener(OnPageChangeListener)} instead.
     */
    @Deprecated
    public void setOnPageChangeListener(OnPageChangeListener listener) {
        mOnPageChangeListener = listener;
    }

    /**
     * Add a listener that will be invoked whenever the page changes or is incrementally
     * scrolled. See {@link OnPageChangeListener}.
     * <p>
     * <p>Components that add a listener should take care to remove it when finished.
     * Other components that take ownership of a view may call {@link #clearOnPageChangeListeners()}
     * to remove all attached listeners.</p>
     *
     * @param listener listener to add
     */
    public void addOnPageChangeListener(OnPageChangeListener listener) {
        if (mOnPageChangeListeners == null) {
            mOnPageChangeListeners = new ArrayList<>();
        }
        mOnPageChangeListeners.add(listener);
    }

    /**
     * Remove a listener that was previously added via
     * {@link #addOnPageChangeListener(OnPageChangeListener)}.
     *
     * @param listener listener to remove
     */
    public void removeOnPageChangeListener(OnPageChangeListener listener) {
        if (mOnPageChangeListeners != null) {
            mOnPageChangeListeners.remove(listener);
        }
    }

    /**
     * Remove all listeners that are notified of any changes in scroll state or position.
     */
    public void clearOnPageChangeListeners() {
        if (mOnPageChangeListeners != null) {
            mOnPageChangeListeners.clear();
        }
    }

    /**
     * Sets a {@link PageTransformer} that will be called for each attached page whenever
     * the scroll position is changed. This allows the application to apply custom property
     * transformations to each page, overriding the default sliding behavior.
     * <p>
     * <p><em>Note:</em> By default, calling this method will cause contained pages to use
     * {@link View#LAYER_TYPE_HARDWARE}. This layer type allows custom alpha transformations,
     * but it will cause issues if any of your pages contain a {@link android.view.SurfaceView}
     * and you have not called {@link android.view.SurfaceView#setZOrderOnTop(boolean)} to put that
     * {@link android.view.SurfaceView} above your app content. To disable this behavior, call
     * {@link #setPageTransformer(boolean, PageTransformer, int)} and pass
     * {@link View#LAYER_TYPE_NONE} for {@code pageLayerType}.</p>
     *
     * @param reverseDrawingOrder true if the supplied PageTransformer requires page views
     *                            to be drawn from last to first instead of first to last.
     * @param transformer         PageTransformer that will modify each page's animation properties
     */
    public void setPageTransformer(boolean reverseDrawingOrder, PageTransformer transformer) {
        setPageTransformer(reverseDrawingOrder, transformer, View.LAYER_TYPE_HARDWARE);
    }

    /**
     * Sets a {@link PageTransformer} that will be called for each attached page whenever
     * the scroll position is changed. This allows the application to apply custom property
     * transformations to each page, overriding the default sliding behavior.
     *
     * @param reverseDrawingOrder true if the supplied PageTransformer requires page views
     *                            to be drawn from last to first instead of first to last.
     * @param transformer         PageTransformer that will modify each page's animation properties
     * @param pageLayerType       View layer type that should be used for ViewPager pages. It should be
     *                            either {@link View#LAYER_TYPE_HARDWARE},
     *                            {@link View#LAYER_TYPE_SOFTWARE}, or
     *                            {@link View#LAYER_TYPE_NONE}.
     */
    public void setPageTransformer(boolean reverseDrawingOrder, PageTransformer transformer,
                                   int pageLayerType) {
        final boolean hasTransformer = transformer != null;
        final boolean needsPopulate = hasTransformer != (mPageTransformer != null);
        mPageTransformer = transformer;
        setChildrenDrawingOrderEnabled(hasTransformer);
        if (hasTransformer) {
            mDrawingOrder = reverseDrawingOrder ? DRAW_ORDER_REVERSE : DRAW_ORDER_FORWARD;
            mPageTransformerLayerType = pageLayerType;
        } else {
            mDrawingOrder = DRAW_ORDER_DEFAULT;
        }
        if (needsPopulate) populate();
    }

    @Override
    protected int getChildDrawingOrder(int childCount, int i) {
        final int index = mDrawingOrder == DRAW_ORDER_REVERSE ? childCount - 1 - i : i;
        final int result =
                ((LayoutParams) mDrawingOrderedChildren.get(index).getLayoutParams()).childIndex;
        return result;
    }

    /**
     * Set a separate OnPageChangeListener for internal use by the support library.
     *
     * @param listener Listener to set
     * @return The old listener that was set, if any.
     */
    OnPageChangeListener setInternalPageChangeListener(OnPageChangeListener listener) {
        OnPageChangeListener oldListener = mInternalPageChangeListener;
        mInternalPageChangeListener = listener;
        return oldListener;
    }

    /**
     * Returns the number of pages that will be retained to either side of the
     * current page in the view hierarchy in an idle state. Defaults to 1.
     *
     * @return How many pages will be kept offscreen on either side
     * @see #setOffscreenPageLimit(int)
     */
    public int getOffscreenPageLimit() {
        return mOffscreenPageLimit;
    }

    /**
     * Set the number of pages that should be retained to either side of the
     * current page in the view hierarchy in an idle state. Pages beyond this
     * limit will be recreated from the adapter when needed.
     * <p>
     * <p>This is offered as an optimization. If you know in advance the number
     * of pages you will need to support or have lazy-loading mechanisms in place
     * on your pages, tweaking this setting can have benefits in perceived smoothness
     * of paging animations and interaction. If you have a small number of pages (3-4)
     * that you can keep active all at once, less time will be spent in layout for
     * newly created view subtrees as the user pages back and forth.</p>
     * <p>
     * <p>You should keep this limit low, especially if your pages have complex layouts.
     * This setting defaults to 1.</p>
     *
     * @param limit How many pages will be kept offscreen in an idle state.
     */
    public void setOffscreenPageLimit(int limit) {
        if (limit < DEFAULT_OFFSCREEN_PAGES) {
            Log.w(TAG, "Requested offscreen page limit " + limit + " too small; defaulting to "
                    + DEFAULT_OFFSCREEN_PAGES);
            limit = DEFAULT_OFFSCREEN_PAGES;
        }
        if (limit != mOffscreenPageLimit) {
            mOffscreenPageLimit = limit;
            populate();
        }
    }

    /**
     * Set the margin between pages.
     *
     * @param marginPixels Distance between adjacent pages in pixels
     * @see #getPageMargin()
     * @see #setPageMarginDrawable(Drawable)
     * @see #setPageMarginDrawable(int)
     */
    public void setPageMargin(int marginPixels) {
        final int oldMargin = mPageMargin;
        mPageMargin = marginPixels;

        final int width = getWidth();
        recomputeScrollPosition(width, width, marginPixels, oldMargin);

        requestLayout();
    }

    /**
     * Return the margin between pages.
     *
     * @return The size of the margin in pixels
     */
    public int getPageMargin() {
        return mPageMargin;
    }

    /**
     * Set a drawable that will be used to fill the margin between pages.
     *
     * @param d Drawable to display between pages
     */
    public void setPageMarginDrawable(Drawable d) {
        mMarginDrawable = d;
        if (d != null) refreshDrawableState();
        setWillNotDraw(d == null);
        invalidate();
    }

    /**
     * Set a drawable that will be used to fill the margin between pages.
     *
     * @param resId Resource ID of a drawable to display between pages
     */
    public void setPageMarginDrawable(@DrawableRes int resId) {
        setPageMarginDrawable(ContextCompat.getDrawable(getContext(), resId));
    }

    @Override
    protected boolean verifyDrawable(Drawable who) {
        return super.verifyDrawable(who) || who == mMarginDrawable;
    }

    @Override
    protected void drawableStateChanged() {
        super.drawableStateChanged();
        final Drawable d = mMarginDrawable;
        if (d != null && d.isStateful()) {
            d.setState(getDrawableState());
        }
    }

    // We want the duration of the page snap animation to be influenced by the distance that
    // the screen has to travel, however, we don't want this duration to be effected in a
    // purely linear fashion. Instead, we use this method to moderate the effect that the distance
    // of travel has on the overall snap duration.
    float distanceInfluenceForSnapDuration(float f) {
        f -= 0.5f; // center the values about 0.
        f *= 0.3f * (float) Math.PI / 2.0f;
        return (float) Math.sin(f);
    }

    /**
     * Like {@link View#scrollBy}, but scroll smoothly instead of immediately.
     *
     * @param x the number of pixels to scroll by on the X axis
     * @param y the number of pixels to scroll by on the Y axis
     */
    void smoothScrollTo(int x, int y) {
        smoothScrollTo(x, y, 0);
    }

    /**
     * Like {@link View#scrollBy}, but scroll smoothly instead of immediately.
     *
     * @param x        the number of pixels to scroll by on the X axis
     * @param y        the number of pixels to scroll by on the Y axis
     * @param velocity the velocity associated with a fling, if applicable. (0 otherwise)
     */
    void smoothScrollTo(int x, int y, int velocity) {
        if (getChildCount() == 0) {
            // Nothing to do.
            setScrollingCacheEnabled(false);
            return;
        }

        int sx;
        boolean wasScrolling = (mScroller != null) && !mScroller.isFinished();
        if (wasScrolling) {
            // We're in the middle of a previously initiated scrolling. Check to see
            // whether that scrolling has actually started (if we always call getStartX
            // we can get a stale value from the scroller if it hadn't yet had its first
            // computeScrollOffset call) to decide what is the current scrolling position.
            sx = mIsScrollStarted ? mScroller.getCurrX() : mScroller.getStartX();
            // And abort the current scrolling.
            mScroller.abortAnimation();
            setScrollingCacheEnabled(false);
        } else {
            sx = getScrollX();
        }
        int sy = getScrollY();
        int dx = x - sx;
        int dy = y - sy;
        if (dx == 0 && dy == 0) {
            completeScroll(false);
            populate();
            setScrollState(SCROLL_STATE_IDLE);
            return;
        }

        setScrollingCacheEnabled(true);
        setScrollState(SCROLL_STATE_SETTLING);

        final int width = getClientWidth();
        final int halfWidth = width / 2;
        final float distanceRatio = Math.min(1f, 1.0f * Math.abs(dx) / width);
        final float distance = halfWidth + halfWidth
                * distanceInfluenceForSnapDuration(distanceRatio);

        int duration;
        velocity = Math.abs(velocity);
        if (velocity > 0) {
            duration = 4 * Math.round(1000 * Math.abs(distance / velocity));
        } else {
            final float pageWidth = width * mAdapter.getPageWidth(mCurItem);
            final float pageDelta = (float) Math.abs(dx) / (pageWidth + mPageMargin);
            duration = (int) ((pageDelta + 1) * 100);
        }
        duration = Math.min(duration, MAX_SETTLE_DURATION);

        // Reset the "scroll started" flag. It will be flipped to true in all places
        // where we call computeScrollOffset().
        mIsScrollStarted = false;
        mScroller.startScroll(sx, sy, dx, dy, duration);
        ViewCompat.postInvalidateOnAnimation(this);
    }

    ItemInfo addNewItem(int position, int index) {
        ItemInfo ii = new ItemInfo();
        ii.position = position;
        ii.object = mAdapter.instantiateItem(this, position);
        ii.widthFactor = mAdapter.getPageWidth(position);
        if (index < 0 || index >= mItems.size()) {
            mItems.add(ii);
        } else {
            mItems.add(index, ii);
        }
        return ii;
    }

    void dataSetChanged() {
        // This method only gets called if our observer is attached, so mAdapter is non-null.

        final int adapterCount = mAdapter.getCount();
        mExpectedAdapterCount = adapterCount;
        boolean needPopulate = mItems.size() < mOffscreenPageLimit * 2 + 1
                && mItems.size() < adapterCount;
        int newCurrItem = mCurItem;

        boolean isUpdating = false;
        for (int i = 0; i < mItems.size(); i++) {
            final ItemInfo ii = mItems.get(i);
            final int newPos = mAdapter.getItemPosition(ii.object);

            if (newPos == MinOffsetPagerAdapter.POSITION_UNCHANGED) {
                continue;
            }

            if (newPos == MinOffsetPagerAdapter.POSITION_NONE) {
                mItems.remove(i);
                i--;

                if (!isUpdating) {
                    mAdapter.startUpdate(this);
                    isUpdating = true;
                }

                mAdapter.destroyItem(this, ii.position, ii.object);
                needPopulate = true;

                if (mCurItem == ii.position) {
                    // Keep the current item in the valid range
                    newCurrItem = Math.max(0, Math.min(mCurItem, adapterCount - 1));
                    needPopulate = true;
                }
                continue;
            }

            if (ii.position != newPos) {
                if (ii.position == mCurItem) {
                    // Our current item changed position. Follow it.
                    newCurrItem = newPos;
                }

                ii.position = newPos;
                needPopulate = true;
            }
        }

        if (isUpdating) {
            mAdapter.finishUpdate(this);
        }

        Collections.sort(mItems, COMPARATOR);

        if (needPopulate) {
            // Reset our known page widths; populate will recompute them.
            final int childCount = getChildCount();
            for (int i = 0; i < childCount; i++) {
                final View child = getChildAt(i);
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                if (!lp.isDecor) {
                    lp.widthFactor = 0.f;
                }
            }

            setCurrentItemInternal(newCurrItem, false, true);
            requestLayout();
        }
    }

    void populate() {
        populate(mCurItem);
    }

    void populate(int newCurrentItem) {
        ItemInfo oldCurInfo = null;
        if (mCurItem != newCurrentItem) {
            oldCurInfo = infoForPosition(mCurItem);
            mCurItem = newCurrentItem;
        }

        if (mAdapter == null) {
            sortChildDrawingOrder();
            return;
        }

        // Bail now if we are waiting to populate.  This is to hold off
        // on creating views from the time the user releases their finger to
        // fling to a new position until we have finished the scroll to
        // that position, avoiding glitches from happening at that point.
        if (mPopulatePending) {
            if (DEBUG) Log.i(TAG, "populate is pending, skipping for now...");
            sortChildDrawingOrder();
            return;
        }

        // Also, don't populate until we are attached to a window.  This is to
        // avoid trying to populate before we have restored our view hierarchy
        // state and conflicting with what is restored.
        if (getWindowToken() == null) {
            return;
        }

        mAdapter.startUpdate(this);

        final int pageLimit = mOffscreenPageLimit;
        final int startPos = Math.max(0, mCurItem - pageLimit);
        final int N = mAdapter.getCount();
        final int endPos = Math.min(N - 1, mCurItem + pageLimit);

        if (N != mExpectedAdapterCount) {
            String resName;
            try {
                resName = getResources().getResourceName(getId());
            } catch (Resources.NotFoundException e) {
                resName = Integer.toHexString(getId());
            }
            throw new IllegalStateException("The application's PagerAdapter changed the adapter's"
                    + " contents without calling PagerAdapter#notifyDataSetChanged!"
                    + " Expected adapter item count: " + mExpectedAdapterCount + ", found: " + N
                    + " Pager id: " + resName
                    + " Pager class: " + getClass()
                    + " Problematic adapter: " + mAdapter.getClass());
        }

        // Locate the currently focused item or add it if needed.
        int curIndex = -1;
        ItemInfo curItem = null;
        for (curIndex = 0; curIndex < mItems.size(); curIndex++) {
            final ItemInfo ii = mItems.get(curIndex);
            if (ii.position >= mCurItem) {
                if (ii.position == mCurItem) curItem = ii;
                break;
            }
        }

        if (curItem == null && N > 0) {
            curItem = addNewItem(mCurItem, curIndex);
        }

        // Fill 3x the available width or up to the number of offscreen
        // pages requested to either side, whichever is larger.
        // If we have no current item we have no work to do.
        if (curItem != null) {
            float extraWidthLeft = 0.f;
            int itemIndex = curIndex - 1;
            ItemInfo ii = itemIndex >= 0 ? mItems.get(itemIndex) : null;
            final int clientWidth = getClientWidth();
            final float leftWidthNeeded = clientWidth <= 0 ? 0 :
                    2.f - curItem.widthFactor + (float) getPaddingLeft() / (float) clientWidth;
            for (int pos = mCurItem - 1; pos >= 0; pos--) {
                if (extraWidthLeft >= leftWidthNeeded && pos < startPos) {
                    if (ii == null) {
                        break;
                    }
                    if (pos == ii.position && !ii.scrolling) {
                        mItems.remove(itemIndex);
                        mAdapter.destroyItem(this, pos, ii.object);
                        if (DEBUG) {
                            Log.i(TAG, "populate() - destroyItem() with pos: " + pos
                                    + " view: " + ((View) ii.object));
                        }
                        itemIndex--;
                        curIndex--;
                        ii = itemIndex >= 0 ? mItems.get(itemIndex) : null;
                    }
                } else if (ii != null && pos == ii.position) {
                    extraWidthLeft += ii.widthFactor;
                    itemIndex--;
                    ii = itemIndex >= 0 ? mItems.get(itemIndex) : null;
                } else {
                    ii = addNewItem(pos, itemIndex + 1);
                    extraWidthLeft += ii.widthFactor;
                    curIndex++;
                    ii = itemIndex >= 0 ? mItems.get(itemIndex) : null;
                }
            }

            float extraWidthRight = curItem.widthFactor;
            itemIndex = curIndex + 1;
            if (extraWidthRight < 2.f) {
                ii = itemIndex < mItems.size() ? mItems.get(itemIndex) : null;
                final float rightWidthNeeded = clientWidth <= 0 ? 0 :
                        (float) getPaddingRight() / (float) clientWidth + 2.f;
                for (int pos = mCurItem + 1; pos < N; pos++) {
                    if (extraWidthRight >= rightWidthNeeded && pos > endPos) {
                        if (ii == null) {
                            break;
                        }
                        if (pos == ii.position && !ii.scrolling) {
                            mItems.remove(itemIndex);
                            mAdapter.destroyItem(this, pos, ii.object);
                            if (DEBUG) {
                                Log.i(TAG, "populate() - destroyItem() with pos: " + pos
                                        + " view: " + ((View) ii.object));
                            }
                            ii = itemIndex < mItems.size() ? mItems.get(itemIndex) : null;
                        }
                    } else if (ii != null && pos == ii.position) {
                        extraWidthRight += ii.widthFactor;
                        itemIndex++;
                        ii = itemIndex < mItems.size() ? mItems.get(itemIndex) : null;
                    } else {
                        ii = addNewItem(pos, itemIndex);
                        itemIndex++;
                        extraWidthRight += ii.widthFactor;
                        ii = itemIndex < mItems.size() ? mItems.get(itemIndex) : null;
                    }
                }
            }

            calculatePageOffsets(curItem, curIndex, oldCurInfo);
        }

        if (DEBUG) {
            Log.i(TAG, "Current page list:");
            for (int i = 0; i < mItems.size(); i++) {
                Log.i(TAG, "#" + i + ": page " + mItems.get(i).position);
            }
        }

        mAdapter.setPrimaryItem(this, mCurItem, curItem != null ? curItem.object : null);

        mAdapter.finishUpdate(this);

        // Check width measurement of current pages and drawing sort order.
        // Update LayoutParams as needed.
        final int childCount = getChildCount();
        for (int i = 0; i < childCount; i++) {
            final View child = getChildAt(i);
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();
            lp.childIndex = i;
            if (!lp.isDecor && lp.widthFactor == 0.f) {
                // 0 means requery the adapter for this, it doesn't have a valid width.
                final ItemInfo ii = infoForChild(child);
                if (ii != null) {
                    lp.widthFactor = ii.widthFactor;
                    lp.position = ii.position;
                }
            }
        }
        sortChildDrawingOrder();

        if (hasFocus()) {
            View currentFocused = findFocus();
            ItemInfo ii = currentFocused != null ? infoForAnyChild(currentFocused) : null;
            if (ii == null || ii.position != mCurItem) {
                for (int i = 0; i < getChildCount(); i++) {
                    View child = getChildAt(i);
                    ii = infoForChild(child);
                    if (ii != null && ii.position == mCurItem) {
                        if (child.requestFocus(View.FOCUS_FORWARD)) {
                            break;
                        }
                    }
                }
            }
        }
    }

    private void sortChildDrawingOrder() {
        if (mDrawingOrder != DRAW_ORDER_DEFAULT) {
            if (mDrawingOrderedChildren == null) {
                mDrawingOrderedChildren = new ArrayList<View>();
            } else {
                mDrawingOrderedChildren.clear();
            }
            final int childCount = getChildCount();
            for (int i = 0; i < childCount; i++) {
                final View child = getChildAt(i);
                mDrawingOrderedChildren.add(child);
            }
            Collections.sort(mDrawingOrderedChildren, sPositionComparator);
        }
    }

    private void calculatePageOffsets(ItemInfo curItem, int curIndex, ItemInfo oldCurInfo) {
        final int N = mAdapter.getCount();
        final int width = getClientWidth();
        final float marginOffset = width > 0 ? (float) mPageMargin / width : 0;
        // Fix up offsets for later layout.
        if (oldCurInfo != null) {
            final int oldCurPosition = oldCurInfo.position;
            // Base offsets off of oldCurInfo.
            if (oldCurPosition < curItem.position) {
                int itemIndex = 0;
                ItemInfo ii = null;
                float offset = oldCurInfo.offset + oldCurInfo.widthFactor + marginOffset;
                for (int pos = oldCurPosition + 1;
                     pos <= curItem.position && itemIndex < mItems.size(); pos++) {
                    ii = mItems.get(itemIndex);
                    while (pos > ii.position && itemIndex < mItems.size() - 1) {
                        itemIndex++;
                        ii = mItems.get(itemIndex);
                    }
                    while (pos < ii.position) {
                        // We don't have an item populated for this,
                        // ask the adapter for an offset.
                        offset += mAdapter.getPageWidth(pos) + marginOffset;
                        pos++;
                    }
                    ii.offset = offset;
                    offset += ii.widthFactor + marginOffset;
                }
            } else if (oldCurPosition > curItem.position) {
                int itemIndex = mItems.size() - 1;
                ItemInfo ii = null;
                float offset = oldCurInfo.offset;
                for (int pos = oldCurPosition - 1;
                     pos >= curItem.position && itemIndex >= 0; pos--) {
                    ii = mItems.get(itemIndex);
                    while (pos < ii.position && itemIndex > 0) {
                        itemIndex--;
                        ii = mItems.get(itemIndex);
                    }
                    while (pos > ii.position) {
                        // We don't have an item populated for this,
                        // ask the adapter for an offset.
                        offset -= mAdapter.getPageWidth(pos) + marginOffset;
                        pos--;
                    }
                    offset -= ii.widthFactor + marginOffset;
                    ii.offset = offset;
                }
            }
        }

        // Base all offsets off of curItem.
        final int itemCount = mItems.size();
        float offset = curItem.offset;
        int pos = curItem.position - 1;
        mFirstOffset = curItem.position == 0 ? curItem.offset : -Float.MAX_VALUE;
        mLastOffset = curItem.position == N - 1
                ? curItem.offset + curItem.widthFactor - 1 : Float.MAX_VALUE;
        // Previous pages
        for (int i = curIndex - 1; i >= 0; i--, pos--) {
            final ItemInfo ii = mItems.get(i);
            while (pos > ii.position) {
                offset -= mAdapter.getPageWidth(pos--) + marginOffset;
            }
            offset -= ii.widthFactor + marginOffset;
            ii.offset = offset;
            if (ii.position == 0) mFirstOffset = offset;
        }
        offset = curItem.offset + curItem.widthFactor + marginOffset;
        pos = curItem.position + 1;
        // Next pages
        for (int i = curIndex + 1; i < itemCount; i++, pos++) {
            final ItemInfo ii = mItems.get(i);
            while (pos < ii.position) {
                offset += mAdapter.getPageWidth(pos++) + marginOffset;
            }
            if (ii.position == N - 1) {
                mLastOffset = offset + ii.widthFactor - 1;
            }
            ii.offset = offset;
            offset += ii.widthFactor + marginOffset;
        }

        mNeedCalculatePageOffsets = false;
    }

    /**
     * This is the persistent state that is saved by ViewPager.  Only needed
     * if you are creating a sublass of ViewPager that must save its own
     * state, in which case it should implement a subclass of this which
     * contains that state.
     */
    public static class SavedState extends AbsSavedState {
        int position;
        Parcelable adapterState;
        ClassLoader loader;

        public SavedState(Parcelable superState) {
            super(superState);
        }

        @Override
        public void writeToParcel(Parcel out, int flags) {
            super.writeToParcel(out, flags);
            out.writeInt(position);
            out.writeParcelable(adapterState, flags);
        }

        @Override
        public String toString() {
            return "FragmentPager.SavedState{"
                    + Integer.toHexString(System.identityHashCode(this))
                    + " position=" + position + "}";
        }

        public static final Creator<SavedState> CREATOR = new ClassLoaderCreator<SavedState>() {
            @Override
            public SavedState createFromParcel(Parcel in, ClassLoader loader) {
                return new SavedState(in, loader);
            }

            @Override
            public SavedState createFromParcel(Parcel in) {
                return new SavedState(in, null);
            }

            @Override
            public SavedState[] newArray(int size) {
                return new SavedState[size];
            }
        };

        SavedState(Parcel in, ClassLoader loader) {
            super(in, loader);
            if (loader == null) {
                loader = getClass().getClassLoader();
            }
            position = in.readInt();
            adapterState = in.readParcelable(loader);
            this.loader = loader;
        }
    }

    @Override
    public Parcelable onSaveInstanceState() {
        Parcelable superState = super.onSaveInstanceState();
        SavedState ss = new SavedState(superState);
        ss.position = mCurItem;
        if (mAdapter != null) {
            ss.adapterState = mAdapter.saveState();
        }
        return ss;
    }

    @Override
    public void onRestoreInstanceState(Parcelable state) {
        if (!(state instanceof SavedState)) {
            super.onRestoreInstanceState(state);
            return;
        }

        SavedState ss = (SavedState) state;
        super.onRestoreInstanceState(ss.getSuperState());

        if (mAdapter != null) {
            mAdapter.restoreState(ss.adapterState, ss.loader);
            setCurrentItemInternal(ss.position, false, true);
        } else {
            mRestoredCurItem = ss.position;
            mRestoredAdapterState = ss.adapterState;
            mRestoredClassLoader = ss.loader;
        }
    }

    @Override
    public void addView(View child, int index, ViewGroup.LayoutParams params) {
        if (!checkLayoutParams(params)) {
            params = generateLayoutParams(params);
        }
        final LayoutParams lp = (LayoutParams) params;
        // Any views added via inflation should be classed as part of the decor
        lp.isDecor |= isDecorView(child);
        if (mInLayout) {
            if (lp != null && lp.isDecor) {
                throw new IllegalStateException("Cannot add pager decor view during layout");
            }
            lp.needsMeasure = true;
            addViewInLayout(child, index, params);
        } else {
            super.addView(child, index, params);
        }

        if (USE_CACHE) {
            if (child.getVisibility() != GONE) {
                child.setDrawingCacheEnabled(mScrollingCacheEnabled);
            } else {
                child.setDrawingCacheEnabled(false);
            }
        }
    }

    private static boolean isDecorView(@NonNull View view) {
        Class<?> clazz = view.getClass();
        return clazz.getAnnotation(DecorView.class) != null;
    }

    @Override
    public void removeView(View view) {
        if (mInLayout) {
            removeViewInLayout(view);
        } else {
            super.removeView(view);
        }
    }

    ItemInfo infoForChild(View child) {
        for (int i = 0; i < mItems.size(); i++) {
            ItemInfo ii = mItems.get(i);
            if (mAdapter.isViewFromObject(child, ii.object)) {
                return ii;
            }
        }
        return null;
    }

    ItemInfo infoForAnyChild(View child) {
        ViewParent parent;
        while ((parent = child.getParent()) != this) {
            if (parent == null || !(parent instanceof View)) {
                return null;
            }
            child = (View) parent;
        }
        return infoForChild(child);
    }

    ItemInfo infoForPosition(int position) {
        for (int i = 0; i < mItems.size(); i++) {
            ItemInfo ii = mItems.get(i);
            if (ii.position == position) {
                return ii;
            }
        }
        return null;
    }

    @Override
    protected void onAttachedToWindow() {
        super.onAttachedToWindow();
        mFirstLayout = true;
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        // For simple implementation, our internal size is always 0.
        // We depend on the container to specify the layout size of
        // our view.  We can't really know what it is since we will be
        // adding and removing different arbitrary views and do not
        // want the layout to change as this happens.
        setMeasuredDimension(getDefaultSize(0, widthMeasureSpec),
                getDefaultSize(0, heightMeasureSpec));

        final int measuredWidth = getMeasuredWidth();
        final int maxGutterSize = measuredWidth / 10;
        mGutterSize = Math.min(maxGutterSize, mDefaultGutterSize);

        // Children are just made to fill our space.
        int childWidthSize = measuredWidth - getPaddingLeft() - getPaddingRight();
        int childHeightSize = getMeasuredHeight() - getPaddingTop() - getPaddingBottom();

        /*
         * Make sure all children have been properly measured. Decor views first.
         * Right now we cheat and make this less complicated by assuming decor
         * views won't intersect. We will pin to edges based on gravity.
         */
        int size = getChildCount();
        for (int i = 0; i < size; ++i) {
            final View child = getChildAt(i);
            if (child.getVisibility() != GONE) {
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                if (lp != null && lp.isDecor) {
                    final int hgrav = lp.gravity & Gravity.HORIZONTAL_GRAVITY_MASK;
                    final int vgrav = lp.gravity & Gravity.VERTICAL_GRAVITY_MASK;
                    int widthMode = MeasureSpec.AT_MOST;
                    int heightMode = MeasureSpec.AT_MOST;
                    boolean consumeVertical = vgrav == Gravity.TOP || vgrav == Gravity.BOTTOM;
                    boolean consumeHorizontal = hgrav == Gravity.LEFT || hgrav == Gravity.RIGHT;

                    if (consumeVertical) {
                        widthMode = MeasureSpec.EXACTLY;
                    } else if (consumeHorizontal) {
                        heightMode = MeasureSpec.EXACTLY;
                    }

                    int widthSize = childWidthSize;
                    int heightSize = childHeightSize;
                    if (lp.width != LayoutParams.WRAP_CONTENT) {
                        widthMode = MeasureSpec.EXACTLY;
                        if (lp.width != LayoutParams.MATCH_PARENT) {
                            widthSize = lp.width;
                        }
                    }
                    if (lp.height != LayoutParams.WRAP_CONTENT) {
                        heightMode = MeasureSpec.EXACTLY;
                        if (lp.height != LayoutParams.MATCH_PARENT) {
                            heightSize = lp.height;
                        }
                    }
                    final int widthSpec = MeasureSpec.makeMeasureSpec(widthSize, widthMode);
                    final int heightSpec = MeasureSpec.makeMeasureSpec(heightSize, heightMode);
                    child.measure(widthSpec, heightSpec);

                    if (consumeVertical) {
                        childHeightSize -= child.getMeasuredHeight();
                    } else if (consumeHorizontal) {
                        childWidthSize -= child.getMeasuredWidth();
                    }
                }
            }
        }

        mChildWidthMeasureSpec = MeasureSpec.makeMeasureSpec(childWidthSize, MeasureSpec.EXACTLY);
        mChildHeightMeasureSpec = MeasureSpec.makeMeasureSpec(childHeightSize, MeasureSpec.EXACTLY);

        // Make sure we have created all fragments that we need to have shown.
        mInLayout = true;
        populate();
        mInLayout = false;

        // Page views next.
        size = getChildCount();
        for (int i = 0; i < size; ++i) {
            final View child = getChildAt(i);
            if (child.getVisibility() != GONE) {
                if (DEBUG) {
                    Log.v(TAG, "Measuring #" + i + " " + child + ": " + mChildWidthMeasureSpec);
                }

                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                if (lp == null || !lp.isDecor) {
                    final int widthSpec = MeasureSpec.makeMeasureSpec(
                            (int) (childWidthSize * lp.widthFactor), MeasureSpec.EXACTLY);
                    child.measure(widthSpec, mChildHeightMeasureSpec);
                }
            }
        }
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);

        // Make sure scroll position is set correctly.
        if (w != oldw) {
            recomputeScrollPosition(w, oldw, mPageMargin, mPageMargin);
        }
    }

    private void recomputeScrollPosition(int width, int oldWidth, int margin, int oldMargin) {
        if (oldWidth > 0 && !mItems.isEmpty()) {
            if (!mScroller.isFinished()) {
                mScroller.setFinalX(getCurrentItem() * getClientWidth());
            } else {
                final int widthWithMargin = width - getPaddingLeft() - getPaddingRight() + margin;
                final int oldWidthWithMargin = oldWidth - getPaddingLeft() - getPaddingRight()
                        + oldMargin;
                final int xpos = getScrollX();
                final float pageOffset = (float) xpos / oldWidthWithMargin;
                final int newOffsetPixels = (int) (pageOffset * widthWithMargin);

                scrollTo(newOffsetPixels, getScrollY());
            }
        } else {
            final ItemInfo ii = infoForPosition(mCurItem);
            final float scrollOffset = ii != null ? Math.min(ii.offset, mLastOffset) : 0;
            final int scrollPos =
                    (int) (scrollOffset * (width - getPaddingLeft() - getPaddingRight()));
            if (scrollPos != getScrollX()) {
                completeScroll(false);
                scrollTo(scrollPos, getScrollY());
            }
        }
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        final int count = getChildCount();
        int width = r - l;
        int height = b - t;
        int paddingLeft = getPaddingLeft();
        int paddingTop = getPaddingTop();
        int paddingRight = getPaddingRight();
        int paddingBottom = getPaddingBottom();
        final int scrollX = getScrollX();

        int decorCount = 0;

        // First pass - decor views. We need to do this in two passes so that
        // we have the proper offsets for non-decor views later.
        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            if (child.getVisibility() != GONE) {
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                int childLeft = 0;
                int childTop = 0;
                if (lp.isDecor) {
                    final int hgrav = lp.gravity & Gravity.HORIZONTAL_GRAVITY_MASK;
                    final int vgrav = lp.gravity & Gravity.VERTICAL_GRAVITY_MASK;
                    switch (hgrav) {
                        default:
                            childLeft = paddingLeft;
                            break;
                        case Gravity.LEFT:
                            childLeft = paddingLeft;
                            paddingLeft += child.getMeasuredWidth();
                            break;
                        case Gravity.CENTER_HORIZONTAL:
                            childLeft = Math.max((width - child.getMeasuredWidth()) / 2,
                                    paddingLeft);
                            break;
                        case Gravity.RIGHT:
                            childLeft = width - paddingRight - child.getMeasuredWidth();
                            paddingRight += child.getMeasuredWidth();
                            break;
                    }
                    switch (vgrav) {
                        default:
                            childTop = paddingTop;
                            break;
                        case Gravity.TOP: