iphone开发中切换两个sub view的层次关系方法
假设这两个子视图的tag分别为1000,和1001，则代码如下：
NSInteger index1 = [[parentView subviews] indexOfObject:[parentView viewWithTag:1000]];
NSInteger index2 = [[parentView subviews] indexOfObject:[parentView viewWithTag:1001]];
[parentView exchangeSubviewAtIndex:index1 withSubviewAtIndex:index2];