ViewTreeObserver对于获取组件宽高的帮助

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