Drawing the layout is a two pass process: a measure pass and a layoutpass.
布局绘画涉及两个过程：测量过程和布局过程。
2
Your application will perform better by reducing as much as possible any unnecessary invocations of the layoutpass.
应用程序应尽可能减少不必要的布局处理过程调用，从而改善性能。
3
The layoutpass is a mathematically-intensive process—the larger the number of children in the collection, the greater the number of calculations required.