onMeasure被执行两次原理解析

void measure(){
    boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT;
    boolean specChanged = widthMeasureSpec != mOldWidthMeasureSpec
        || heightMeasureSpec != mOldHeightMeasureSpec;
    boolean isSepcExactly = MeasureSpec.getMode(widthMeasureSpec) == MeasureSpec.EXACTLY
        && MeasureSpec.getMode(heightMeasureSpec) == MeasureSpec.EXACTLY;
    boolean matchesSpecSize = getMeasuredWidth() == MeasureSpec.getSize(widthMeasureSpec)
        && getMeasuredHeight() == MeasureSpec.getSize(heightMeasureSpec);
    final boolean needsLayout = specChanged
        && (sAlwaysRemeasureExactly || !isSpecExactly || !matchesSpecSize);
    if(forceLayout || needLayout){
        int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key);
        if (cacheIndex < 0 || sIgnoreMeasureCache) {
            onMeasure(widthMeasureSpec, heightMeasureSpec);
            mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
        } else {
            long value = mMeasureCache.valueAt(cacheIndex);
            setMeasuredDimensionRaw((int) (value >> 32), (int) value);
            mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
        }
   }
}

class View{
    LongSparseLongArray mMeasureCache;
    void measure(widthSpec,heightSpec){
        ---
        long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
        int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key);
        if(cacheIndex<0){
            onMeasure(widthSpec,heightSpec);
        }
        mOldWidthMeasureSpec = widthMeasureSpec;
        mOldHeightMeasureSpec = heightMeasureSpec;
        mMeasureCache.put(key,widhSpec|heightSpec);
        ---
    }
}

long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;

//不信你可以试下下面的代码
public class Test {
    public static void main(String[] args) {
        long widthMeasureSpec = makeMeasureSpec(10,0);
        long heightMeasureSpec =  makeMeasureSpec(20,0);
        long ss = widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
        System.out.println("=========="+ss);
    }
    private static final int MODE_MASK = 0x3 << 30;
    public static int makeMeasureSpec(int size,
                                      int mode) {
        return (size & ~MODE_MASK) | (mode & MODE_MASK);
    }
}
//42949672980
//42949672980
//42949672980

void measure(int widthMeasureSpec,int heightMeasureSpec){
    ----
    boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT; 
    if(forceLayout | needsLayout){
        onMeasure()
    }
    ----
}
public void layout(int l, int t, int r, int b){
    ---
    mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
    ---
}

class ViewRootImpl{
    void performTraversals(){
        if(layoutResuested){
        //标记1
            windowSizeMayChanged |= measureHierarchy(host,lp,res,desiredWindowWidth,desiredWindowHeight);
        }
        //标记2
        performMeasure()
        performLayout()
    }
    void measureHierarchy(){
        performMeasure()
    }
}