为什么onMeasure会被执行两次

发表于2023-02-07更新于2023-04-26

什么情况下会onMeasure会执行?

进入View的measure方法:

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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;
        }
   }
}

什么时候forceLayout=true:

  1. 调用requestLayout
  2. 调用forceRequestLayout

什么时候needsLayout=true:

  1. 当长宽发生改变

什么时候调用了onMeasure>方法:

  1. forceLayouy=true
  2. 或者mMeasureCache没有当前的缓存

所以总结:当调用了requestLayout一定会测发重测过程.当forceLayout=false的时候会去判断mMeasureCache值.现在研究下这个mMeasureCache

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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);
        ---
    }
}

这里可以看到oldWidthMeasureSpec和mMeasureCache都是缓存上一次的值,那他们有什么不同呢?不同点就是,oldWidthMeasureSpec>不仅仅缓存了测量的spec模式而且缓存了size.但是mMeasureCache只缓存了size.从这行代码可以看出:

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

这里一同运算就为了排除掉spec造成的影响.

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//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

什么时候mPrivateFlags会被赋值PFLAG_FORCE_LAYOUT.

在view viewGrouup的构造函数里面会主动赋值一次,然后在ViewGroup.addView时候会给当前View的mProvateFlags赋值PFLAG_FORCE_LAYOUT.

为什么onMeasure会被执行两次?

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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;
    ---
}

在第一次触发到measure方法时,forceLayoyt=true needsLayout=true,但是layout方法还没触发到.
在第二次触发到measure>方法时,forceLayout=true needsLayout=false,所以还是会进入onMeasure方法.这次会执行layout方法.然后我们在下次的时候forceLayout就等于false了.上面的这一段分析是分析的measure内部如何防止多次调用onMeasure.

现在分析外部是如何多次调用measure方法的:
在Activity执行到onResume生命周期的时候,会执行WindowManager.addView操作,WindowManager的具体实现类是WindowManagerImpl然后addView操作交给了代理类WindowManagerGlobal,然后在WindowManagerGlobal的addView里面执行了ViewRootImpl.setView操作(ViewRootImpl对象也是在这个时候创建的),在ViewRootImpl会主动调用一次requestLayout,也就开启了第一次的视图 测量 布局 绘制.

在setView的时候主动调用了一次ViewRootImpl.requestLayout,注意这个requestLayout是ViewRootImpl的内部方法,和view viewGroup那些requestLayout不一样.在ViewRootImpl.requestLayout内部调用了performTraversals方法:

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

从ViewRootImpl的执行逻辑你可以看出,在执行performLayout之前,他自己就已经调用了两次performMeasure方法.所以你现在就知道为啥了.