-(void)loadView;

• loadView方法在UIVIewController的view被访问且为空的时候调用。

• loadView方法不应该直接被调用，而是由系统调用。它会加载或创建一个View并把它赋值给UIViewController的view属性。
  以下代码会导致死循环
  ```
  -(void)loadView{
  NSLog(@"ViewController loadView");
  }

• (void)viewDidLoad {
  [super viewDidLoad];
  // Do any additional setup after loading the view, typically from a nib.
  NSLog(@"ViewController viewDidLoad");
  self.view.backgroundColor = [UIColor whiteColor];
  }
  ```

  - (void)viewDidLoad;

• 当loadView将view加载完成，会调用viewDidLoad方法，该方法在UIVIewController初始化过程中只会执行一次，在该方法中可以进行页面相关的初始化。

  -(void)viewWillAppear:(BOOL)animated；

• 页面将要出现时调用。页面每次即将出现时都会调用。

• 当app有多级ViewController，pop操作后都会调用即将出现的ViewController的该方法，如果每次ViewController的出现都需要刷新页面，可在该方法中实现。

  -(void)viewWillLayoutSubviews；

• ViewController的view即将对subviews进行布局。

  -(void)viewDidLayoutSubviews；

• ViewController的view对subviews的布局完成。

  -(void)viewDidAppear:(BOOL)animated；

• 页面已经出现时调用。页面每次出现时都会调用。

• 在view被添加到视图层级中以及多视图，上下级视图切换时调用这个方法，在这里可以对正在显示的视图做进一步的设置。

  -(void)viewWillDisappear:(BOOL)animated；

• 页面将要消失时调用。可能会被多次调用。

• 页面之间push或者pop时都会调用。

  -(void)viewDidDisappear:(BOOL)animated；

• 页面已经消失时调用。可能会被多次调用。

• 页面之间push或者pop时都会调用。

  -(void)dealloc;

• 页面被销毁时调用。
• 该方法可以用来调试页面是否存在内存泄漏，如果存在，在页面出栈时将不会调用该方法。

• 在该方法中可以处理需要手动释放的东西，比如观察者的移除。

  常用周期方法执行顺序

  由于我编码习惯是纯代码布局，所以对xib布局和storyboard布局的相关方法未加研究，代码如下

• (void)viewDidLoad {
  [super viewDidLoad];
  // Do any additional setup after loading the view, typically from a nib.
  NSLog(@"ViewController viewDidLoad");
  self.view.backgroundColor = [UIColor whiteColor];
  }

-(void)viewWillAppear:(BOOL)animated{
[super viewWillAppear:animated];
NSLog(@"ViewController viewWillAppear");
}

-(void)viewDidAppear:(BOOL)animated{
[super viewDidAppear:animated];
NSLog(@"ViewController viewDidAppear");
}

-(void)viewWillDisappear:(BOOL)animated{
[super viewWillDisappear:animated];
NSLog(@"ViewController viewWillDisappear");
}

-(void)viewDidDisappear:(BOOL)animated{
[super viewDidDisappear:animated];
NSLog(@"ViewController viewDidDisappear");
}

-(void)viewWillLayoutSubviews{
[super viewWillLayoutSubviews];
NSLog(@"ViewController viewWillLayoutSubviews");
}

-(void)viewDidLayoutSubviews{
[super viewDidLayoutSubviews];
NSLog(@"ViewController viewDidLayoutSubviews");
}

• (void)didReceiveMemoryWarning {
  [super didReceiveMemoryWarning];
  // Dispose of any resources that can be recreated.
  }

• 打印结果：

ViewController viewDidLoad
ViewController viewWillAppear
ViewController viewWillLayoutSubviews
ViewController viewDidLayoutSubviews
ViewController viewDidAppear

以上只是显示一个页面的代码和打印结果，下面我们来看一下页面间的跳转时，生命周期执行的顺序。
从A页面调转到B页面，A页面代码：

• (void)viewDidLoad {
  [super viewDidLoad];
  // Do any additional setup after loading the view, typically from a nib.
  NSLog(@"A viewDidLoad");
  self.view.backgroundColor = [UIColor whiteColor];
  }

-(void)viewWillAppear:(BOOL)animated{
[super viewWillAppear:animated];
NSLog(@"A viewWillAppear");
}

-(void)viewDidAppear:(BOOL)animated{
[super viewDidAppear:animated];
NSLog(@"A viewDidAppear");
}

-(void)viewWillDisappear:(BOOL)animated{
[super viewWillDisappear:animated];
NSLog(@"A viewWillDisappear");
}

-(void)viewDidDisappear:(BOOL)animated{
[super viewDidDisappear:animated];
NSLog(@"A viewDidDisappear");
}

-(void)viewWillLayoutSubviews{
[super viewWillLayoutSubviews];
NSLog(@"A viewWillLayoutSubviews");
}

-(void)viewDidLayoutSubviews{
[super viewDidLayoutSubviews];
NSLog(@"A viewDidLayoutSubviews");
}

• (void)didReceiveMemoryWarning {
  [super didReceiveMemoryWarning];
  // Dispose of any resources that can be recreated.
  }

-(void)touchesBegan:(NSSet

B页面代码：

• (void)viewDidLoad {
  [super viewDidLoad];
  // Do any additional setup after loading the view, typically from a nib.
  NSLog(@"B viewDidLoad");
  self.view.backgroundColor = [UIColor whiteColor];
  }

-(void)dealloc{
NSLog(@"B dealloc");
}

-(void)viewWillAppear:(BOOL)animated{
[super viewWillAppear:animated];
NSLog(@"B viewWillAppear");
}

-(void)viewDidAppear:(BOOL)animated{
[super viewDidAppear:animated];
NSLog(@"B viewDidAppear");
}

-(void)viewWillDisappear:(BOOL)animated{
[super viewWillDisappear:animated];
NSLog(@"B viewWillDisappear");
}

-(void)viewDidDisappear:(BOOL)animated{
[super viewDidDisappear:animated];
NSLog(@"B viewDidDisappear");
}

-(void)viewWillLayoutSubviews{
[super viewWillLayoutSubviews];
NSLog(@"B viewWillLayoutSubviews");
}

-(void)viewDidLayoutSubviews{
[super viewDidLayoutSubviews];
NSLog(@"B viewDidLayoutSubviews");
}

• (void)didReceiveMemoryWarning {
  [super didReceiveMemoryWarning];
  // Dispose of any resources that can be recreated.
  }

-(void)touchesBegan:(NSSet

}

从A页面跳转到B页面，再从B页面回到A页面，打印结果：

A viewDidLoad
A viewWillAppear
A viewWillLayoutSubviews
A viewDidLayoutSubviews
A viewDidAppear
************************跳转啦****************************
B viewDidLoad
A viewWillDisappear
B viewWillAppear
B viewWillLayoutSubviews
B viewDidLayoutSubviews
A viewDidDisappear
B viewDidAppear
************************回去啦****************************
B viewWillDisappear
A viewWillAppear
B viewDidDisappear
A viewDidAppear
B dealloc