子视图是以栈的方式存放的,也就是说有层次的存放 addSubview: addsubview时都是在最上面面添加 eg:
UIView *v1 = [UIView new]; v1.frame = CGRectMake(100, 100, 100, 60); v1.backgroundColor = [UIColor redColor]; UIView *v2 = [UIView new]; v2.backgroundColor = [UIColor purpleColor]; v2.frame = CGRectMake(80, 120, 100, 60); [self.view addSubview:v1]; [self.view addSubview:v2];看到如下效果 
insertSubview insertSubView可以控制将view添加到指定的层 eg:
UIView *v1 = [UIView new]; v1.frame = CGRectMake(100, 100, 100, 60); v1.backgroundColor = [UIColor redColor]; UIView *v2 = [UIView new]; v2.backgroundColor = [UIColor purpleColor]; v2.frame = CGRectMake(80, 120, 100, 60); [self.view addSubview:v1]; [self.view addSubview:v2]; UIView *v3 = [UIView new]; v3.backgroundColor = [UIColor yellowColor]; v3.frame = CGRectMake(60, 140, 160, 60); [self.view insertSubview:v3 atIndex:3];看到如下效果： 
可以看到v3在v1跟v2之间，按照我们的打算，atIndex:3 应该是在第四层，而现在感觉不像，我们可以打印一下看下究竟：
打印如下：
2017-02-08 10:10:07.310 sa1[1058:39734] ( "<_UILayoutGuide: 0x7f8893602e80; frame = (0 0; 0 0); hidden = YES; layer = <CALayer: 0x60800003e220>>", "<_UILayoutGuide: 0x7f8893609080; frame = (0 0; 0 0); hidden = YES; layer = <CALayer: 0x60800003e080>>", "<UIView: 0x7f8893406ab0; frame = (100 100; 100 60); layer = <CALayer: 0x60000003de40>>", "<UIView: 0x7f8893407270; frame = (60 140; 160 60); layer = <CALayer: 0x60000003dc00>>", "<UIView: 0x7f8893407c60; frame = (80 120; 100 60); layer = <CALayer: 0x60000003de60>>")可以看到v1之前，也就是self.view有两层，v1 index就是2， v2的为4，v3 插在 index:3 ，所以在v1跟v2之间。
可以看到： [self.view addSubView:xx.view] 其实就等于[self.view insertSubView:xx.view atIndex:[self.view.subViews count]]; 即在最顶层添加view。
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