【iOS】UIViewController的生命週期

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-(void)loadView;

• loadView方法在UIVIewController的view被訪問且為空白的時候調用。
• loadView方法不應該直接被調用，而是由系統調用。它會載入或建立一個View並把它賦值給UIViewController的view屬性。
  以下代碼會導致死迴圈

-(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;

• 當loadView將view載入完成，會調用viewDidLoad方法，該方法在UIVIewController初始化過程中只會執行一次，在該方法中可以進行頁面相關的初始化。

  -(void)viewWillAppear:(BOOL)animated；
• 頁面將要出現時調用。頁面每次即將出現時都會調用。

• 當app有多級ViewController，pop操作後都會調用即將出現的ViewController的該方法，如果每次ViewController的出現都需要重新整理頁面，可在該方法中實現。

  -(void)viewWillLayoutSubviews；

• ViewController的view即將對subviews進行布局。

  -(void)viewDidLayoutSubviews；

• ViewController的view對subviews的布局完成。

  -(void)viewDidAppear:(BOOL)animated；
• 頁面已經出現時調用。頁面每次出現時都會調用。

• 在view被添加到視圖層級中以及多視圖，上下級視圖切換時調用這個方法，在這裡可以對正在顯示的視圖做進一步的設定。

  -(void)viewWillDisappear:(BOOL)animated；
• 頁面將要消失時調用。可能會被多次調用。

• 頁面之間push或者pop時都會調用。

  -(void)viewDidDisappear:(BOOL)animated；
• 頁面已經消失時調用。可能會被多次調用。

• 頁面之間push或者pop時都會調用。

  -(void)dealloc;
• 頁面被銷毀時調用。
• 該方法可以用來調試頁面是否存在記憶體流失，如果存在，在頁面出棧時將不會調用該方法。

• 在該方法中可以處理需要手動釋放的東西，比如觀察者的移除。

  常用周期方法執行順序

  由於我編碼習慣是純程式碼布局，所以對xib布局和storyboard布局的相關方法未加研究，代碼如下

• (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.
  }

• 列印結果：

ViewController viewDidLoad
ViewController viewWillAppear
ViewController viewWillLayoutSubviews
ViewController viewDidLayoutSubviews
ViewController viewDidAppear

以上只是顯示一個頁面的代碼和列印結果，下面我們來看一下頁面間的跳轉時，生命週期執行的順序。
從A頁面調轉到B頁面，A頁面代碼：

• (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頁面代碼：

• (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

}

從A頁面跳轉到B頁面，再從B頁面回到A頁面，列印結果：

A viewDidLoad
A viewWillAppear
A viewWillLayoutSubviews
A viewDidLayoutSubviews
A viewDidAppear
************************跳轉啦****************************
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

【iOS】UIViewController的生命週期