android view measure方法

最近项目需要，一位同事架构RecyclerView嵌套RecyclerView实现一个需求，而且哇啦啦啦写了一整天，结果显示一看，嵌套的RecyclerView不显示，不管怎么调试也不显示。。。这个问题其实和我们之前ListView嵌套ListView不显示一样，就是在建立的View嵌套树中，嵌套的ListView测量没有明确的测量依据，导致测量的大小直接返回0了，所以需要让RecyclerView重新测量一下，关键代码是：

int widthtSpecTitle=View.MeasureSpec.makeMeasureSpec(Interge.MAXVALUE >> 2,View.MeasureSpec.AT_MOST);
holder.itemView.measure(widthtSpecTitle,View.MeasureSpec.UNSPECIFIED);

意思就是告诉测量的View，对于宽width，尽量自己测量吧，对与长，我没有限制，你是多少就给多少吧。同时采用的测量是结合DataAdapter一起对itemView进行测量的，主要代码如下

private static int getItemHeight(Context context, ItemData data, int viewType) {
     RelativeLayout parent = new RelativeLayout(context);
     DataAdapter mAdapter = new DataAdapter();

     ItemHolder holder;
     holder = (ItemHolder) mAdapter.onCreateViewHolder(parent, viewType);
     mAdapter.onBindViewHolder(holder,data.get(position));
     int  measureHeight = 0;
      WindowManager wm=(WindowManager)context.getSystemService(Context.WINDOW_SERVICE);
     int width = wm.getDefaultDisplay().getWidth();
     int widthSpecTitle=View.MeasureSpec.makeMeasureSpec(width,View.MeasureSpec.AT_MOST);
     holder.itemView.measure(widthSpecTitle,View.MeasureSpec.UNSPECIFIED);
     measureHeight = holder.itemView.getMeasuredHeight();

     return measureHeight;
 }

调用View的measure会简介调用ViewGroup的onMeasure，View的measure的源码如下：

public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
          。。。。
          if (cacheIndex < 0 || sIgnoreMeasureCache) {
              // measure ourselves, this should set the measured dimension flag back
              // 重点onMeasure方法
              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
              setMeasuredDimensionRaw((int) (value >> 32), (int) value);
              mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
          }

         。。。。
  }

但是测量的结果总是不对，要不太大了，要不太小了，总结如下：
1 如果对于子视图的显示主要依赖负空间的大小然后按照比例计算的，Measure方法的调用需要添加适当的大小。
2 对于ImageView，如果没有设置默认的图片，等待网络加载在显示，默认的大小是为0的，源码如下，所以对于ImageVIew的测量需要特殊处理一下，单独做测量处理。

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
     。。。。
     resolveUri();
     int w;
     int h;
     
     // Desired aspect ratio of the view's contents (not including padding)
     float desiredAspect = 0.0f;
     
     // We are allowed to change the view's width
     boolean resizeWidth = false;
     
     // We are allowed to change the view's height
     boolean resizeHeight = false;
     
     final int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
     final int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);

     if (mDrawable == null) {
         // If no drawable, its intrinsic size is 0.
         mDrawableWidth = -1;
         mDrawableHeight = -1;
         w = h = 0;   //没有图片就直接返回大小为0
     } else {
         w = mDrawableWidth;
         h = mDrawableHeight;
         if (w <= 0) w = 1;
         if (h <= 0) h = 1;

         // We are supposed to adjust view bounds to match the aspect
         // ratio of our drawable. See if that is possible.
         if (mAdjustViewBounds) {
             resizeWidth = widthSpecMode != MeasureSpec.EXACTLY;
             resizeHeight = heightSpecMode != MeasureSpec.EXACTLY;
             
             desiredAspect = (float) w / (float) h;
         }
     }
     
    。。。。。
 }

～～～本人能力有限，难免有理解不透彻的地方，有什么问题欢迎指出，感谢～～～