一个 forceLayout() 和 requestLayout() 的测试

两个view：

一个是系统默认的FrameLayout,  A

一个是自己自定义的MyView extends View，重载了onMeasure函数(): B

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    // TODO Auto-generated method stub
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    // Log.e(TAG, "onMeasure " + MeasureSpec.getSize(widthMeasureSpec)
    //      + " " + MeasureSpec.getSize(heightMeasureSpec));
    setMeasuredDimension(MeasureSpec.makeMeasureSpec(mW-=10, MeasureSpec.EXACTLY),
            MeasureSpec.makeMeasureSpec(mH-=10, MeasureSpec.EXACTLY));
}

	@Override
	protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
		// TODO Auto-generated method stub
		super.onMeasure(widthMeasureSpec, heightMeasureSpec);
		// Log.e(TAG, "onMeasure " + MeasureSpec.getSize(widthMeasureSpec)
		// 		+ " " + MeasureSpec.getSize(heightMeasureSpec));
		setMeasuredDimension(MeasureSpec.makeMeasureSpec(mW-=10, MeasureSpec.EXACTLY),
				MeasureSpec.makeMeasureSpec(mH-=10, MeasureSpec.EXACTLY));
	}

逻辑很简单，每次只要B的onMeasure被触发，那么B的尺寸就原来小10.

A包含B,

几个测试case: (measure() 是final的，无法覆盖，不过B的measure()应该是每次被调到的)

1. A.requestLayout();                                    不会调用到B.onMeasure() 和 B.onLayout(), 会调用B.layout() (注意连draw()都没有被调到，这意味着B被A认为完全没变，不需要重绘)

2. B.forceLayout(); A.requestLayout()           会调用到B的.onMeasure() 和 onLayout() 以及 layout(), draw(), onDraw()

3. B.requestLayout()                                      会调用到B的.onMeasure() 和 onLayout() 以及 layout(),draw(), onDraw()

4. B.invalidate()                                               B的layout(), onLayout() 和 onMeasure() 没有被调到，只有 draw() 和 onDraw() 被调到.

5  A.invalidate()                                               B的所有函数都不会被调到.

这个结果也符合code的逻辑，forceLayout()如果注释说的一样，是在下一次layout bypass 的过程(自己不会发起一次layout bypass)中，会强制的重新onMeasure和onLayout()

而requestLayout() 除了干forceLayout()的事情将自己的FORCE_LAYOUT标志位设上外，还会发起一次layout pass，

在layout bypass 从 A 传到 B时，虽然 A 的 onMeasure() 和 onLayout() 会调用 B 的 measure() 和 layout(), 但是，因为 B的layout状态没有什么改变，因此

onMeasure()和 onLayout()不会被调用.

在当前4.4的code:

public void forceLayout() {
     ................................................
        mPrivateFlags |= PFLAG_FORCE_LAYOUT;
        mPrivateFlags |= PFLAG_INVALIDATED;
    }

public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
       ...............................................................
        if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
               .......................................................
            int cacheIndex = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ? -1 :
                    mMeasureCache.indexOfKey(key);
            if (cacheIndex < 0 || sIgnoreMeasureCache) {
                ..................................................
                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
                setMeasuredDimension((int) (value >> 32), (int) value);
                mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
            }
......................................................................................
            mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
        }
.............................................................................
}


public void layout(int l, int t, int r, int b) {
        .....................................................
        if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
            onLayout(changed, l, t, r, b);
            mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;

           ...................................................................
        }

        mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
        mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
    }

public void forceLayout() {
     ................................................
        mPrivateFlags |= PFLAG_FORCE_LAYOUT;
        mPrivateFlags |= PFLAG_INVALIDATED;
    }

public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
       ...............................................................
        if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
               .......................................................
            int cacheIndex = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ? -1 :
                    mMeasureCache.indexOfKey(key);
            if (cacheIndex < 0 || sIgnoreMeasureCache) {
                ..................................................
                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
                setMeasuredDimension((int) (value >> 32), (int) value);
                mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
            }
......................................................................................
            mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
        }
.............................................................................
}


public void layout(int l, int t, int r, int b) {
        .....................................................
        if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
            onLayout(changed, l, t, r, b);
            mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;

           ...................................................................
        }

        mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
        mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
    }

可见forceLayout()可以导致onMeasure() 和 onLayout(). 而requestLayout() 干的事情比forceLayout()只多不少.

View的forceLayout后面一般会紧跟着View的measure(), 这样可以把view的measuredSize 通过 setMeasuredDimension 设上.

而PFLAG_INVALIDATED 这个flag 应该是标记 重绘的。

而requestLayout()/forceLayout() 设置的另一个flag PFLAG_FORCE_LAYOUT，也一定可以在measure()中 将PFLAG_LAYOUT_REQUIRED flag给打上，

而PFLAG_LAYOUT_REQUIRED 则是可以保证了在调用 layout()函数时，onLayout() 函数会被调用到.

还有一点，之前没有仔细看， View的measure(A, B) 函数:

           如果有PFLAG_FORCE_LAYOUT 或者 本次的measure的尺寸 A, B 和 之前的尺寸不一样
               if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
                widthMeasureSpec != mOldWidthMeasureSpec ||
                heightMeasureSpec != mOldHeightMeasureSpec) {

            ........................................................

            这一步其实是从之前的MeasureCache里找是否存在 和 本次的measure尺寸一致的 cache
                        int cacheIndex = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ? -1 :
                    mMeasureCache.indexOfKey(key);
            如果没有找到，那么就调用onMeasure, 一般来说，调用了onMeasure才能真正的setMeasuredDimension
                        if (cacheIndex < 0 || sIgnoreMeasureCache) {
                // measure ourselves, this should set the measured dimension flag back
                onMeasure(widthMeasureSpec, heightMeasureSpec);
                                mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
                        } else {
                               如果找到有cache，那么就将cache的值 设为 setMeasuredDimension
                               long value = mMeasureCache.valueAt(cacheIndex);
                               // Casting a long to int drops the high 32 bits, no mask needed
                               setMeasuredDimension((int) (value >> 32), (int) value);
                               mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
                        }
                 }

                ..............................................
                mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;

                ..............................................
                mOldWidthMeasureSpec = widthMeasureSpec;
                mOldHeightMeasureSpec = heightMeasureSpec;

                mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
                      (long) mMeasuredHeight & 0xffffffffL); // suppress sign extension

	       如果有PFLAG_FORCE_LAYOUT 或者 本次的measure的尺寸 A, B 和 之前的尺寸不一样
               if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
				widthMeasureSpec != mOldWidthMeasureSpec ||
				heightMeasureSpec != mOldHeightMeasureSpec) {

			........................................................

			这一步其实是从之前的MeasureCache里找是否存在 和 本次的measure尺寸一致的 cache
                        int cacheIndex = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ? -1 :
					mMeasureCache.indexOfKey(key);
			如果没有找到，那么就调用onMeasure, 一般来说，调用了onMeasure才能真正的setMeasuredDimension
                        if (cacheIndex < 0 || sIgnoreMeasureCache) {
				// measure ourselves, this should set the measured dimension flag back
				onMeasure(widthMeasureSpec, heightMeasureSpec);
                                mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
                        } else {
                               如果找到有cache，那么就将cache的值 设为 setMeasuredDimension
                               long value = mMeasureCache.valueAt(cacheIndex);
                               // Casting a long to int drops the high 32 bits, no mask needed
                               setMeasuredDimension((int) (value >> 32), (int) value);
                               mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
                        }
                 }
                
                ..............................................
                mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;

                ..............................................
                mOldWidthMeasureSpec = widthMeasureSpec;
                mOldHeightMeasureSpec = heightMeasureSpec;

                mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
                      (long) mMeasuredHeight & 0xffffffffL); // suppress sign extension 

这说明调用measure(A, B)， 只要尺寸变化，都会影响 MeasuredHeight/Width

借鉴：http://blog.csdn.net/fyfcauc/article/details/41895509