85ViewTreeObserver.addOnGlobalLayoutListener(listener)

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:(我的项目中视频缩略图界面)：

// 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 ) {
 "white-space:pre" > 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