void android.view.ViewTreeObserver.addOnGlobalLayoutListener(OnGlobalLayoutListener listener)

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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() {@Overridepublic 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

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