onMeasure被执行两次原理解析

什么情况下会onMeasure会执行?

进入Viewmeasure方法:

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:

  • 调用requestLayout
  • 调用forceRequestLayout

什么时候needsLayout=true:

  • 当长宽发生改变

什么时候调用了onMeasure>方法:

  • forceLayouy=true
  • 或者mMeasureCache没有当前的缓存

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

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oldWidthMeasureSpecmMeasureCache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:

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.

//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.

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ViewmProvateFlags赋值PFLAG_FORCE_LAYOUT.

为什么onMeasure会被执行两次?

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

分析外部是如何多次调用measure方法的

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WindowManagerGlobaladdView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.

setView的时候主动调用了一次ViewRootImpl.requestLayout,注意这个requestLayoutViewRootImpl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:

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

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.

作者:Librity

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