一个tabbarController,里面有1，2 两个ViewController,当从1切换到2时，执行的顺序是

2 viewWillAppear
1 viewWillDisappear
1 viewDidDisappear
2 viewDidAppear

2->1：

1 viewWillAppear
2 viewWillDisappear
2 viewDidDisappear
1 viewDidAppear

navController中，1进入2

1 viewWillDisappear
2 viewWillAppear
1 viewDidDisappear // 已经几乎完全显示后显示了2的画面之后（有些背景等可能还没加载）
2 viewDidAppear

2返回1：

2 viewWillDisappear
1 viewWillAppear
2 viewDidDisappear
1 viewDidAppear

modol窗口

1 viewWillDisappear
2 viewWillAppear
2 viewDidAppear
1 viewDidDisappear

2->1

2 viewWillDisappear
1 viewWillAppear
1 viewDidAppear
2 viewDidDisappear