View繪制的三部曲， 測量，布局，繪畫
今天我們分析測量過程

view的測量是從ViewRootImpl發起的，View需要重繪，都是發送請求給ViewRootImpl，然後他組織重繪
在重繪的過程中，有一步就是測量，通過代碼來分析測量過程

    private boolean measureHierarchy(final View host, final WindowManager.LayoutParams lp,
            final Resources res, final int desiredWindowWidth, final int desiredWindowHeight) {
        
 
int childWidthMeasureSpec;
        int childHeightMeasureSpec;
        boolean windowSizeMayChange = false;

        if (DEBUG_ORIENTATION || DEBUG_LAYOUT) Log.v(TAG,
                "Measuring " + host + " in display " + desiredWindowWidth
                + "x" + desiredWindowHeight + "...");
    
        
 
boolean goodMeasure = false;
        if (lp.width == ViewGroup.LayoutParams.WRAP_CONTENT) {
            // On large screens, we don‘t want to allow dialogs to just
            // stretch to fill the entire width of the screen to display
            // one line of text.  First try doing the layout at a smaller
            
 
// size to see if it will fit.
            final DisplayMetrics packageMetrics = res.getDisplayMetrics();
            res.getValue(com.android.internal.R.dimen.config_prefDialogWidth, mTmpValue, true);
            int baseSize = 0;
            if (mTmpValue.type == TypedValue.TYPE_DIMENSION) {
                baseSize = (int)mTmpValue.getDimension(packageMetrics);
            }
            if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": baseSize=" + baseSize);
            if (baseSize != 0 && desiredWindowWidth > baseSize) {
                //獲取測量的規格，是一個32位的二進制數值，前兩位標識mode，後30位表示view的長/寬
                childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width);
                childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
                //向DecorView發起重繪
                performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
                if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": measured ("
                        + host.getMeasuredWidth() + "," + host.getMeasuredHeight() + ")");
                if ((host.getMeasuredWidthAndState()&View.MEASURED_STATE_TOO_SMALL) == 0) {
                    goodMeasure = true;
                } else {
                    // Didn‘t fit in that size... try expanding a bit.
                    baseSize = (baseSize+desiredWindowWidth)/2;
                    if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": next baseSize="
                            + baseSize);
                    childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width);
                    performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
                    if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": measured ("
                            + host.getMeasuredWidth() + "," + host.getMeasuredHeight() + ")");
                    if ((host.getMeasuredWidthAndState()&View.MEASURED_STATE_TOO_SMALL) == 0) {
                        if (DEBUG_DIALOG) Log.v(TAG, "Good!");
                        goodMeasure = true;
                    }
                }
            }
        }

        if (!goodMeasure) {
            childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);
            childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
            performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
            if (mWidth != host.getMeasuredWidth() || mHeight != host.getMeasuredHeight()) {
                windowSizeMayChange = true;
            }
        }

        if (DBG) {
            System.out.println("======================================");
            System.out.println("performTraversals -- after measure");
            host.debug();
        }

        return windowSizeMayChange;
    }

這個函數通過getRootMeasureSpec方法，獲取測量規格，然後調用performMeasure方法開始分發給整個的view樹。

    private static int getRootMeasureSpec(int windowSize, int rootDimension) {
        int measureSpec;
        switch (rootDimension) {

        case ViewGroup.LayoutParams.MATCH_PARENT:
            // Window can‘t resize. Force root view to be windowSize.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
            break;
        case ViewGroup.LayoutParams.WRAP_CONTENT:
            // Window can resize. Set max size for root view.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
            break;
        default:
            // Window wants to be an exact size. Force root view to be that size.
            measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
            break;
        }
        return measureSpec;
    }

通過MeasureSpec的makeMeasureSpec方法來生成測量規格，先判斷出布局是 match_parent 或者是 wrap_content，或者是確定的數值
然後把windowSize傳遞下去。

    public static int makeMeasureSpec(int size, int mode) {
            if (sUseBrokenMakeMeasureSpec) {
                return size + mode;
            } else {
                return (size & ~MODE_MASK) | (mode & MODE_MASK);
            }
    }

API大於17的都會走else判斷，這塊我分析一下計算結果。有助於理解後邊的運算

makeMeasureSpec的運算結果是一個32位的二進制數值，前2位表示測量的規格 EXACTLY/AT_most 後30位表示 windowSize，舉個運算例子
size=320,mode=EXACTLY,換算成二進制就是下邊的兩串值
size=0000 0000 0000 0000 0000 0001 0100 0000
mode=0100 0000 0000 0000 0000 0000 0000 0000
mask=1100 0000 0000 0000 0000 0000 0000 0000
最後用與運算整合size和mode
0000 0000 0000 0000 0000 0001 0100 0000 |
0100 0000 0000 0000 0000 0000 0000 0000 =
0100 0000 0000 0000 0000 0001 0100 0000

    private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
        try {
            mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
    }

這個mView就是我們在window類中組合出來的DecorView，這個方法調用了view的measure方法，measure會調用OnMeasure方法，然後就實現了整個view樹的測量工作

    public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
        boolean optical = isLayoutModeOptical(this);
        if (optical != isLayoutModeOptical(mParent)) {
            Insets insets = getOpticalInsets();
            int oWidth  = insets.left + insets.right;
            int oHeight = insets.top  + insets.bottom;
            widthMeasureSpec  = MeasureSpec.adjust(widthMeasureSpec,  optical ? -oWidth  : oWidth);
            heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
        }

        // 這塊又把寬的測量規格和高的測量規格拼接在了一起，作為緩存中的key
        long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
        if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);
        if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
                widthMeasureSpec != mOldWidthMeasureSpec ||
                heightMeasureSpec != mOldHeightMeasureSpec) {

            // first clears the measured dimension flag
            mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;

            resolveRtlPropertiesIfNeeded();
            //判斷是否強制測量，如果強制就重新調用onMeasure，整個view樹重新測量，否則就從緩存中得到上次的測量規格，
            //因為DecorView是FrameLayout的子類,所以onMeasure就是調用FrameLayout的onMeasure方法
            int cacheIndex = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ? -1 :
                    mMeasureCache.indexOfKey(key);
            if (cacheIndex < 0 || sIgnoreMeasureCache) {
                // measure ourselves, this should set the measured dimension flag back
                onMeasure(widthMeasureSpec, heightMeasureSpec);
                mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
            } else {
                long value = mMeasureCache.valueAt(cacheIndex);
                // Casting a long to int drops the high 32 bits, no mask needed
                //調用完這個方法之後，getMeasuredWidth和getMeasuredHeight就可以取到值了
                setMeasuredDimensionRaw((int) (value >> 32), (int) value);
                mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
            }

            // flag not set, setMeasuredDimension() was not invoked, we raise
            // an exception to warn the developer
            if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {
                throw new IllegalStateException("View with id " + getId() + ": "
                        + getClass().getName() + "#onMeasure() did not set the"
                        + " measured dimension by calling"
                        + " setMeasuredDimension()");
            }

            mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
        }

        mOldWidthMeasureSpec = widthMeasureSpec;
        mOldHeightMeasureSpec = heightMeasureSpec;
        //在按照固定的格式，把本次的測量規格put到集合中
        mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
                (long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
    }

這個方法做的功能大概就是這樣，先判斷是否有緩存，沒緩存就測量一下整個樹，然後按照固定的格式在存入緩存中

從這裏開始，我們的整個測量過程就開始跑起來了

     protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int count = getChildCount();
        //只有match_parent和固定寬高 的mode才是EXACTLY，wrap_content是AT_MOST
        final boolean measureMatchParentChildren =
                MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
                MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
        mMatchParentChildren.clear();

        int maxHeight = 0;  //包含padding的高度
        int maxWidth = 0;   //包含width的寬度
        int childState = 0; //前16位是寬的測量mode，後16位是高的測量mode
        //輪尋測量所有的child
        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            //只要是不為GONE，就會進入測量範圍
            if (mMeasureAllChildren || child.getVisibility() != GONE) {
                //測量child的時候，減去padding加上margin的距離，得到child的規格後回調用child的measure，然後在重新走上邊分析的流程，調用child的onMeasure
                measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
                //賦值maxWidth和maxHeight，之後就可以用getHeight和getWidth方法了
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                maxWidth = Math.max(maxWidth,
                        child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
                maxHeight = Math.max(maxHeight,
                        child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
                childState = combineMeasuredStates(childState, child.getMeasuredState());
                if (measureMatchParentChildren) {
                    if (lp.width == LayoutParams.MATCH_PARENT ||
                            lp.height == LayoutParams.MATCH_PARENT) {
                        mMatchParentChildren.add(child);
                    }
                }
            }
        }

        // Account for padding too
        maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
        maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();

        // Check against our minimum height and width
        maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
        maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());

        // Check against our foreground‘s minimum height and width
        final Drawable drawable = getForeground();
        if (drawable != null) {
            maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
            maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
        }
        //設置子空間的測量結果
        setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
                resolveSizeAndState(maxHeight, heightMeasureSpec,
                        childState << MEASURED_HEIGHT_STATE_SHIFT));
        //開始測量寬高都是MATCH_PARENT的布局
        count = mMatchParentChildren.size();
        if (count > 1) {
            for (int i = 0; i < count; i++) {
                final View child = mMatchParentChildren.get(i);
                final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

                final int childWidthMeasureSpec;
                if (lp.width == LayoutParams.MATCH_PARENT) {
                    final int width = Math.max(0, getMeasuredWidth()
                            - getPaddingLeftWithForeground() - getPaddingRightWithForeground()
                            - lp.leftMargin - lp.rightMargin);
                    childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
                            width, MeasureSpec.EXACTLY);
                } else {
                    childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
                            getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
                            lp.leftMargin + lp.rightMargin,
                            lp.width);
                }

                final int childHeightMeasureSpec;
                if (lp.height == LayoutParams.MATCH_PARENT) {
                    final int height = Math.max(0, getMeasuredHeight()
                            - getPaddingTopWithForeground() - getPaddingBottomWithForeground()
                            - lp.topMargin - lp.bottomMargin);
                    childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
                            height, MeasureSpec.EXACTLY);
                } else {
                    childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
                            getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
                            lp.topMargin + lp.bottomMargin,
                            lp.height);
                }

                child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
            }
        }
    }

[Android FrameWork 6.0源碼學習] View的重繪過程