View的绘制完成通知

有时候需要在onCreate方法中知道某个View组件的宽度和高度等信息，而直接调用View组件的getWidth()、getHeight()、getMeasuredWidth()、getMeasuredHeight()、getTop()、getLeft()等方法是无法获取到真实值的，只会得到0。这是因为View组件布局要在onResume回调后完成。下面提供实现方法，onGlobalLayout回调会在view布局完成时自动调用:

类似：

 // This listener is used to get the final width of the GridView and then calculate the
 // number of columns and the width of each column. The width of each column is variable
 // as the GridView has stretchMode=columnWidth. The column width is used to set the height
 // of each view so we get nice square thumbnails.
 mGridView.getViewTreeObserver().addOnGlobalLayoutListener( //view 布局完成时调用，每次view改变时都会调用
         new ViewTreeObserver.OnGlobalLayoutListener() {
             @Override
             public void onGlobalLayout() {
                 if (mAdapter.getNumColumns() == 0) {
                         final int numColumns = (int) Math.floor(
                                  mGridView.getWidth() / (mImageThumbSize + mImageThumbSpacing));
                     if (numColumns > 0) {
                     <span style="white-space:pre">  </span>final int columnWidth =
                             (mGridView.getWidth() / numColumns) - mImageThumbSpacing;
                         mAdapter.setNumColumns(numColumns);   //设置 列数
                             mAdapter.setItemHeight(columnWidth);  //设置 高度
                     }
                 }
             }
         });

在gridview布局完成后，根据girdview的宽和高设置adapter列数和每个item高度