在方法的最後，看到了

也會明確當向右移動視圖時候，為什麽getScrollX()返回值會是負的了。以下做一個測試的demo，來練習一下這兩個方法的使用。

在MotionEvent.ACTION_UP事件中，也就是手指擡起時候，直接使用scrollTo()方法讓視圖回到初始位置。再強調一遍。註意無論是scrollBy()還是scrollTo()方法，都是對CusScrollView內容視圖進行移動。效果例如以下：

只是通過上面的樣例，發現一個問題就是滑動速度非常快，尤其是scrollTo()方法，差點兒是瞬間移動到指定位置。這樣倒不能說是缺點。只是在某些情況下，是希望能夠緩慢的移動或者有一個明顯的移動效果。就像側滑菜單那樣。仿佛有一個移動的動畫。

這時候Scroller閃亮登場了。

 /**
     * Called by a parent to request that a child update its values for mScrollX
     * and mScrollY if necessary. This will typically be done if the child is
     * animating a scroll using a [email protected] android.widget.Scroller Scroller}
     * object.
     */
    public void computeScroll() {
    }

 // Step 4, draw the children 
           dispatchDraw(canvas); 

/**
    * Called by draw to draw the child views. This may be overridden
    * by derived classes to gain control just before its children are drawn
    * (but after its own view has been drawn).
    * @param canvas the canvas on which to draw the view
    */ 
   protected void dispatchDraw(Canvas canvas) { 
 
   }

  @Override
    protected void dispatchDraw(Canvas canvas) {
        ...
        ...
        ...

        if ((flags & FLAG_USE_CHILD_DRAWING_ORDER) == 0) {
            for (int i = 0; i < count; i++) {
                final View child = children[i];
                if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
                    more |= drawChild(canvas, child, drawingTime);
                }
            }
        } else {
            for (int i = 0; i < count; i++) {
                final View child = children[getChildDrawingOrder(count, i)];
                if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
                    more |= drawChild(canvas, child, drawingTime);
                }
            }
        }

        // Draw any disappearing views that have animations
        if (mDisappearingChildren != null) {
            final ArrayList<View> disappearingChildren = mDisappearingChildren;
            final int disappearingCount = disappearingChildren.size() - 1;
            // Go backwards -- we may delete as animations finish
            for (int i = disappearingCount; i >= 0; i--) {
                final View child = disappearingChildren.get(i);
                more |= drawChild(canvas, child, drawingTime);
            }
        }
        ...
        ...
        ...

        }
    }

protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
        ...
        ...
        ...

    if (!concatMatrix && canvas.quickReject(cl, ct, cr, cb, Canvas.EdgeType.BW) &&
                (child.mPrivateFlags & DRAW_ANIMATION) == 0) {
            return more;
        }

        child.computeScroll();

        final int sx = child.mScrollX;
        final int sy = child.mScrollY;

        boolean scalingRequired = false;
        Bitmap cache = null;

        ...
        ...
        ...

}

也就是ViewGroup在分發繪制自己的孩子的時候，會對其子View調用computeScroll()方法。

public class Scroller  {
    private int mMode;

    private int mStartX;
    private int mStartY;
    private int mFinalX;
    private int mFinalY;

    private int mMinX;
    private int mMaxX;
    private int mMinY;
    private int mMaxY;

    private int mCurrX;
    private int mCurrY;
    private long mStartTime;
    private int mDuration;
    private float mDurationReciprocal;
    private float mDeltaX;
    private float mDeltaY;
    private boolean mFinished;
    private Interpolator mInterpolator;

    private float mVelocity;
    private float mCurrVelocity;
    private int mDistance;

    private float mFlingFriction = ViewConfiguration.getScrollFriction();

    private static final int DEFAULT_DURATION = 250;
    private static final int SCROLL_MODE = 0;
    private static final int FLING_MODE = 1;

    /**
     * Create a Scroller with the default duration and interpolator.
     */
    public Scroller(Context context) {
        this(context, null);
    }

    /**
     * Create a Scroller with the specified interpolator. If the interpolator is
     * null, the default (viscous) interpolator will be used. "Flywheel" behavior will
     * be in effect for apps targeting Honeycomb or newer.
     */
    public Scroller(Context context, Interpolator interpolator) {
        this(context, interpolator,
                context.getApplicationInfo().targetSdkVersion >= Build.VERSION_CODES.HONEYCOMB);
    }

    /**
     * Create a Scroller with the specified interpolator. If the interpolator is
     * null, the default (viscous) interpolator will be used. Specify whether or
     * not to support progressive "flywheel" behavior in flinging.
     */
    public Scroller(Context context, Interpolator interpolator, boolean flywheel) {
        mFinished = true;
        mInterpolator = interpolator;
        mPpi = context.getResources().getDisplayMetrics().density * 160.0f;
        mDeceleration = computeDeceleration(ViewConfiguration.getScrollFriction());
        mFlywheel = flywheel;

        mPhysicalCoeff = computeDeceleration(0.84f); // look and feel tuning
    }

    /**
     * Call this when you want to know the new location.  If it returns true,
     * the animation is not yet finished.
     */
    public boolean computeScrollOffset() {
        if (mFinished) {
            return false;
        }

        int timePassed = (int)(AnimationUtils.currentAnimationTimeMillis() - mStartTime);
   
        if (timePassed < mDuration) {
            switch (mMode) {
            case SCROLL_MODE:
                float x = timePassed * mDurationReciprocal;
   
                if (mInterpolator == null)
                    x = viscousFluid(x);
                else
                    x = mInterpolator.getInterpolation(x);
   
                mCurrX = mStartX + Math.round(x * mDeltaX);
                mCurrY = mStartY + Math.round(x * mDeltaY);
                break;
            case FLING_MODE:
                final float t = (float) timePassed / mDuration;
                final int index = (int) (NB_SAMPLES * t);
                float distanceCoef = 1.f;
                float velocityCoef = 0.f;
                if (index < NB_SAMPLES) {
                    final float t_inf = (float) index / NB_SAMPLES;
                    final float t_sup = (float) (index + 1) / NB_SAMPLES;
                    final float d_inf = SPLINE_POSITION[index];
                    final float d_sup = SPLINE_POSITION[index + 1];
                    velocityCoef = (d_sup - d_inf) / (t_sup - t_inf);
                    distanceCoef = d_inf + (t - t_inf) * velocityCoef;
                }

                mCurrVelocity = velocityCoef * mDistance / mDuration * 1000.0f;
               
                mCurrX = mStartX + Math.round(distanceCoef * (mFinalX - mStartX));
                // Pin to mMinX <= mCurrX <= mMaxX
                mCurrX = Math.min(mCurrX, mMaxX);
                mCurrX = Math.max(mCurrX, mMinX);
               
                mCurrY = mStartY + Math.round(distanceCoef * (mFinalY - mStartY));
                // Pin to mMinY <= mCurrY <= mMaxY
                mCurrY = Math.min(mCurrY, mMaxY);
                mCurrY = Math.max(mCurrY, mMinY);

                if (mCurrX == mFinalX && mCurrY == mFinalY) {
                    mFinished = true;
                }

                break;
            }
        }
        else {
            mCurrX = mFinalX;
            mCurrY = mFinalY;
            mFinished = true;
        }
        return true;
    }
   
    /**
     * Start scrolling by providing a starting point and the distance to travel.
     * The scroll will use the default value of 250 milliseconds for the
     * duration.
     *
     * @param startX Starting horizontal scroll offset in pixels. Positive
     *        numbers will scroll the content to the left.
     * @param startY Starting vertical scroll offset in pixels. Positive numbers
     *        will scroll the content up.
     * @param dx Horizontal distance to travel. Positive numbers will scroll the
     *        content to the left.
     * @param dy Vertical distance to travel. Positive numbers will scroll the
     *        content up.
     */
    public void startScroll(int startX, int startY, int dx, int dy) {
        startScroll(startX, startY, dx, dy, DEFAULT_DURATION);
    }

    /**
     * Start scrolling by providing a starting point, the distance to travel,
     * and the duration of the scroll.
     *
     * @param startX Starting horizontal scroll offset in pixels. Positive
     *        numbers will scroll the content to the left.
     * @param startY Starting vertical scroll offset in pixels. Positive numbers
     *        will scroll the content up.
     * @param dx Horizontal distance to travel. Positive numbers will scroll the
     *        content to the left.
     * @param dy Vertical distance to travel. Positive numbers will scroll the
     *        content up.
     * @param duration Duration of the scroll in milliseconds.
     */
    public void startScroll(int startX, int startY, int dx, int dy, int duration) {
        mMode = SCROLL_MODE;
        mFinished = false;
        mDuration = duration;
        mStartTime = AnimationUtils.currentAnimationTimeMillis();
        mStartX = startX;
        mStartY = startY;
        mFinalX = startX + dx;
        mFinalY = startY + dy;
        mDeltaX = dx;
        mDeltaY = dy;
        mDurationReciprocal = 1.0f / (float) mDuration;
    }

    /**
     * Start scrolling based on a fling gesture. The distance travelled will
     * depend on the initial velocity of the fling.
     *
     * @param startX Starting point of the scroll (X)
     * @param startY Starting point of the scroll (Y)
     * @param velocityX Initial velocity of the fling (X) measured in pixels per
     *        second.
     * @param velocityY Initial velocity of the fling (Y) measured in pixels per
     *        second
     * @param minX Minimum X value. The scroller will not scroll past this
     *        point.
     * @param maxX Maximum X value. The scroller will not scroll past this
     *        point.
     * @param minY Minimum Y value. The scroller will not scroll past this
     *        point.
     * @param maxY Maximum Y value. The scroller will not scroll past this
     *        point.
     */
    public void fling(int startX, int startY, int velocityX, int velocityY,
            int minX, int maxX, int minY, int maxY) {
        // Continue a scroll or fling in progress
        if (mFlywheel && !mFinished) {
            float oldVel = getCurrVelocity();

            float dx = (float) (mFinalX - mStartX);
            float dy = (float) (mFinalY - mStartY);
            float hyp = FloatMath.sqrt(dx * dx + dy * dy);

            float ndx = dx / hyp;
            float ndy = dy / hyp;

            float oldVelocityX = ndx * oldVel;
            float oldVelocityY = ndy * oldVel;
            if (Math.signum(velocityX) == Math.signum(oldVelocityX) &&
                    Math.signum(velocityY) == Math.signum(oldVelocityY)) {
                velocityX += oldVelocityX;
                velocityY += oldVelocityY;
            }
        }

        mMode = FLING_MODE;
        mFinished = false;

        float velocity = FloatMath.sqrt(velocityX * velocityX + velocityY * velocityY);
    
        mVelocity = velocity;
        mDuration = getSplineFlingDuration(velocity);
        mStartTime = AnimationUtils.currentAnimationTimeMillis();
        mStartX = startX;
        mStartY = startY;

        float coeffX = velocity == 0 ?
 1.0f : velocityX / velocity;
        float coeffY = velocity == 0 ? 1.0f : velocityY / velocity;

        double totalDistance = getSplineFlingDistance(velocity);
        mDistance = (int) (totalDistance * Math.signum(velocity));
       
        mMinX = minX;
        mMaxX = maxX;
        mMinY = minY;
        mMaxY = maxY;

        mFinalX = startX + (int) Math.round(totalDistance * coeffX);
        // Pin to mMinX <= mFinalX <= mMaxX
        mFinalX = Math.min(mFinalX, mMaxX);
        mFinalX = Math.max(mFinalX, mMinX);
       
        mFinalY = startY + (int) Math.round(totalDistance * coeffY);
        // Pin to mMinY <= mFinalY <= mMaxY
        mFinalY = Math.min(mFinalY, mMaxY);
        mFinalY = Math.max(mFinalY, mMinY);
    }
   
}

  public void startScroll(int startX, int startY, int dx, int dy, int duration) {
        mMode = SCROLL_MODE;
        mFinished = false;
        mDuration = duration;
        mStartTime = AnimationUtils.currentAnimationTimeMillis();
        mStartX = startX;
        mStartY = startY;
        mFinalX = startX + dx;
        mFinalY = startY + dy;
        mDeltaX = dx;
        mDeltaY = dy;
        mDurationReciprocal = 1.0f / (float) mDuration;
    }

  scroller.startScroll(getScrollX(), 0, distance, 0);

 invalidate(); // 刷新視圖 

       @Override
     public void computeScroll() {
          if (scroller.computeScrollOffset()) {
               scrollTo(scroller.getCurrX(), 0);
          }
     }

       @Override
     public void computeScroll() {
          if (scroller.computeScrollOffset()) {
               scrollTo(scroller.getCurrX(), 0);
               invalidate();
          }
     }

/**
*
*/
package com.kince.scrolldemo;

import android.content.Context;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Scroller;

/**
* @author kince
*
*
*/
public class CusScrollView extends ViewGroup {

     private int lastX = 0;
     private int currX = 0;
     private int offX = 0;
     private Scroller mScroller;

     /**
     * @param context
     */
     public CusScrollView(Context context) {
          this(context, null);
          // TODO Auto-generated constructor stub
     }

     /**
     * @param context
     * @param attrs
     */
     public CusScrollView(Context context, AttributeSet attrs) {
          this(context, attrs, 0);
          // TODO Auto-generated constructor stub
     }

     /**
     * @param context
     * @param attrs
     * @param defStyle
     */
     public CusScrollView(Context context, AttributeSet attrs, int defStyle) {
          super(context, attrs, defStyle);
          // TODO Auto-generated constructor stub
          mScroller = new Scroller(context);
     }

     /*
     * (non-Javadoc)
     *
     * @see android.view.ViewGroup#onLayout(boolean, int, int, int, int)
     */
     @Override
     protected void onLayout(boolean changed, int l, int t, int r, int b) {
          // TODO Auto-generated method stub

          for (int i = 0; i < getChildCount(); i++) {
               View v = getChildAt(i);
               v.layout(0 + i * getWidth(), 0, getWidth() + i * getWidth(),
                         getHeight());
          }
     }

     @Override
     public boolean onTouchEvent(MotionEvent event) {
          // TODO Auto-generated method stub
          switch (event.getAction()) {
          case MotionEvent.ACTION_DOWN:
               // 僅僅考慮水平方向
               lastX = (int) event.getX();
               return true;

          case MotionEvent.ACTION_MOVE:
               currX = (int) event.getX();
               offX = currX - lastX;
               // scrollBy(-offX, 0);
               mScroller.startScroll(getScrollX(), 0, -offX, 0);

               break;

          case MotionEvent.ACTION_UP:
//               scrollTo(0, 0);
               mScroller.startScroll(getScrollX(), 0, -100, 0);
               break;
          }
          invalidate();
          return super.onTouchEvent(event);
     }

     @Override
     public void computeScroll() {
          // TODO Auto-generated method stub
          if (mScroller.computeScrollOffset()) {
               scrollTo(mScroller.getCurrX(), 0);
               invalidate();
          }
     }

}