做一个实验，通过实验来分析viewController的生命周期。
和生命周期几个相关的方法
- (void)viewDidLoad {
[super viewDidLoad];
NSLog(@"FirstVC viewDidLoad");
}
-(void)viewWillAppear:(BOOL)animated
{
[super viewWillAppear];
NSLog(@"FirstVC viewWillAppear");
}
- (void)didReceiveMemoryWarning {
[super didReceiveMemoryWarning];
NSLog(@"FirstVC didReceiveMemoryWarning");
}
-(void)viewWillDisappear:(BOOL)animated
{
[super viewWillDisappear:YES];
NSLog(@"FirstVC viewWillDisappear");
}
-(void)loadView
{
[super loadView];
NSLog(@"FirstVC loadView");
}
-(void)viewDidLayoutSubviews
{
[super viewDidLayoutSubviews];
NSLog(@"FirstVC viewDidLayoutSubviews");
}
-(void)viewWillLayoutSubviews
{
[super viewWillLayoutSubviews];
NSLog(@"FirstVC viewWillLayoutSubviews");
}
-(void)viewDidAppear:(BOOL)animated
{
[super viewDidAppear:YES];
NSLog(@"FirstVC viewDidAppear");
}
-(void)viewDidDisappear:(BOOL)animated
{
[super viewDidDisappear:YES];
NSLog(@"FirstVC viewDidDisappear");
}
-(void)awakeFromNib
{
[super awakeFromNib];
NSLog(@"FirstVC awakeFromNib");
}
请注意:为了保证代码顺利执行，且保证模拟器顺利加载ViewController，请务必添加[super viewxxxxx]的代码来初始化。
我试过去掉所有的[super viewxxxx]代码，控制台打印如下
2016-03-24 10:31:28.328 SIMAlbum[33599:524075] FirstVC awakeFromNib
2016-03-24 10:31:28.333 SIMAlbum[33599:524075] SecondView awakeFromNib
2016-03-24 10:31:28.771 SIMAlbum[33599:524075] FirstVC loadView
2016-03-24 10:31:28.771 SIMAlbum[33599:524075] FirstVC viewDidLoad
2016-03-24 10:31:28.834 SIMAlbum[33599:524075] FirstVC loadView
2016-03-24 10:31:28.834 SIMAlbum[33599:524075] FirstVC viewDidLoad
2016-03-24 10:31:28.835 SIMAlbum[33599:524075] FirstVC loadView
2016-03-24 10:31:28.835 SIMAlbum[33599:524075] FirstVC viewDidLoad
2016-03-24 10:31:28.835 SIMAlbum[33599:524075] FirstVC loadView
2016-03-24 10:31:28.835 SIMAlbum[33599:524075] FirstVC viewDidLoad
2016-03-24 10:31:28.835 SIMAlbum[33599:524075] FirstVC viewWillAppear
2016-03-24 10:31:28.835 SIMAlbum[33599:524075] FirstVC loadView
2016-03-24 10:31:28.835 SIMAlbum[33599:524075] FirstVC viewDidLoad
2016-03-24 10:31:28.842 SIMAlbum[33599:524075] FirstVC loadView
2016-03-24 10:31:28.842 SIMAlbum[33599:524075] FirstVC viewDidLoad
2016-03-24 10:31:28.897 SIMAlbum[33599:524075] FirstVC viewDidAppear
但是你会发现模拟器加载的是一个黑色的没有任何内容的ViewController
相应的在第一个，即将push出来的ViewController也放入上面的代码。
以上便是与viewController生命周期相关的方法。
从StoryBoard加载
先放出进入第一个viewController时的控制台输出
2016-03-24 10:55:17.503 SIMAlbum[35103:546098] FirstVC awakeFromNib
2016-03-24 10:55:17.506 SIMAlbum[35103:546098] SecondView awakeFromNib
2016-03-24 10:55:17.625 SIMAlbum[35103:546098] FirstVC loadView
2016-03-24 10:55:17.626 SIMAlbum[35103:546098] FirstVC viewDidLoad
2016-03-24 10:55:17.658 SIMAlbum[35103:546098] FirstVC viewWillAppear
2016-03-24 10:55:17.676 SIMAlbum[35103:546098] FirstVC viewWillLayoutSubviews
2016-03-24 10:55:17.676 SIMAlbum[35103:546098] FirstVC viewDidLayoutSubviews
2016-03-24 10:55:17.678 SIMAlbum[35103:546098] FirstVC viewWillLayoutSubviews
2016-03-24 10:55:17.678 SIMAlbum[35103:546098] FirstVC viewDidLayoutSubviews
2016-03-24 10:55:17.769 SIMAlbum[35103:546098] FirstVC viewDidAppear
没错，和我们熟悉的生命周期大致内容是一致的。
接下来push进第二个viewController：
2016-03-24 10:55:38.848 SIMAlbum[35103:546098] SecondView awakeFromNib
2016-03-24 10:55:38.853 SIMAlbum[35103:546098] SecondView loadView
2016-03-24 10:55:38.865 SIMAlbum[35103:546098] SecondView viewDidLoad
2016-03-24 10:55:38.865 SIMAlbum[35103:546098] FirstVC viewWillDisappear
2016-03-24 10:55:38.867 SIMAlbum[35103:546098] SecondView viewWillAppear
2016-03-24 10:55:38.884 SIMAlbum[35103:546098] SecondView viewWillLayoutSubviews
2016-03-24 10:55:38.903 SIMAlbum[35103:546098] SecondView viewDidLayoutSubviews
2016-03-24 10:55:38.904 SIMAlbum[35103:546098] FirstVC viewWillLayoutSubviews
2016-03-24 10:55:38.904 SIMAlbum[35103:546098] FirstVC viewDidLayoutSubviews
2016-03-24 10:55:38.905 SIMAlbum[35103:546098] SecondView viewWillLayoutSubviews
2016-03-24 10:55:38.905 SIMAlbum[35103:546098] SecondView viewDidLayoutSubviews
2016-03-24 10:55:39.413 SIMAlbum[35103:546098] FirstVC viewDidDisappear
2016-03-24 10:55:39.413 SIMAlbum[35103:546098] SecondView viewDidAppear
接下来push进第三个ViewController，为了看到第二个viewController的过程，没有在第三个viewController添加任何代码，控制台输出如下：
2016-03-24 10:55:57.906 SIMAlbum[35103:546098] SecondView viewWillDisappear
2016-03-24 10:55:57.920 SIMAlbum[35103:546098] SecondView viewWillLayoutSubviews
2016-03-24 10:55:57.920 SIMAlbum[35103:546098] SecondView viewDidLayoutSubviews
2016-03-24 10:55:58.424 SIMAlbum[35103:546098] SecondView viewDidDisappear
返回第二个viewController:
2016-03-24 10:56:10.539 SIMAlbum[35103:546098] SecondView viewWillAppear
2016-03-24 10:56:10.541 SIMAlbum[35103:546098] SecondView viewWillLayoutSubviews
2016-03-24 10:56:10.552 SIMAlbum[35103:546098] SecondView viewDidLayoutSubviews
2016-03-24 10:56:11.055 SIMAlbum[35103:546098] SecondView viewDidAppear
返回第一个viewController
2016-03-24 10:56:48.577 SIMAlbum[35103:546098] SecondView viewWillDisappear
2016-03-24 10:56:48.577 SIMAlbum[35103:546098] FirstVC viewWillAppear
2016-03-24 10:56:48.587 SIMAlbum[35103:546098] FirstVC viewWillLayoutSubviews
2016-03-24 10:56:48.587 SIMAlbum[35103:546098] FirstVC viewDidLayoutSubviews
2016-03-24 10:56:49.091 SIMAlbum[35103:546098] SecondView viewDidDisappear
2016-03-24 10:56:49.091 SIMAlbum[35103:546098] FirstVC viewDidAppear
2016-03-24 10:56:49.093 SIMAlbum[35103:546098] FirstVC viewWillLayoutSubviews
2016-03-24 10:56:49.093 SIMAlbum[35103:546098] FirstVC viewDidLayoutSubviews
代码加载viewController
依旧做了个实验，进入代码生成的viewController时控制台输出如下：
2016-03-24 11:09:49.361 SimDraw[36310:564381] FirstVC loadView
2016-03-24 11:09:49.370 SimDraw[36310:564381] FirstVC viewDidLoad
2016-03-24 11:09:49.381 SimDraw[36310:564381] FirstVC viewWillAppear
2016-03-24 11:09:49.393 SimDraw[36310:564381] FirstVC viewWillLayoutSubviews
2016-03-24 11:09:49.393 SimDraw[36310:564381] FirstVC viewDidLayoutSubviews
2016-03-24 11:09:49.395 SimDraw[36310:564381] FirstVC viewWillLayoutSubviews
2016-03-24 11:09:49.395 SimDraw[36310:564381] FirstVC viewDidLayoutSubviews
2016-03-24 11:09:49.929 SimDraw[36310:564381] FirstVC viewDidAppear
退出时
2016-03-24 11:10:20.636 SimDraw[36310:564381] FirstVC viewWillDisappear
2016-03-24 11:10:21.166 SimDraw[36310:564381] FirstVC viewDidDisappear
分析与总结
以上的结果简单粗暴，虽然和印象中的一样，但是还是有些许出入，我系统的做了个viewControll的图：
注意到其中的viewWillLayoutSubviews和viewDidLayoutSubviews，调用情况视具体的viewDidLoad和viewWillAppear等方法中的代码而定。
viewWillLayoutSubviews调用情况分析
- init初始化不会触发layoutSubviews
- addSubview会触发layoutSubviews
- 设置view的Frame会触发layoutSubviews，当然前提是frame的值设置前后发生了变化
- 滚动一个UIScrollView会触发layoutSubviews
- 旋转Screen会触发父UIView上的layoutSubviews事件
- 改变一个UIView大小的时候也会触发父UIView上的layoutSubviews事件