既然要研究UIViewController的生命周期, 先寫一段代碼, 我們從代碼的執行順序來了解UIViewController的生命周期
首先 要知道一個UIViewController中都會執行那些方法
1. - (void)loadView
2. - (void)viewDidLoad
3. - (void)viewWillAppear:(BOOL)animated
4. - (void)viewDidAppear:(BOOL)animated
5. - (void)viewWillDisappear:(BOOL)animated
6. - (void)viewDidDisappear:(BOOL)animated
在一段程序執行過程中 按以上順序依次執行
下面 我們書寫代碼 運行程序 來看看 首先創建一個 ViewController
- (void)loadView {
[super loadView];
NSLog(@"%@------%s",[self class], FUNCTION);
}

- (void)viewDidLoad {
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
NSLog(@"%@------%s",[self class], __FUNCTION__);
self.view.backgroundColor = [UIColor redColor];
self.navigationItem.leftBarButtonItem = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemSave target:self action:@selector(barClick:)];
}

- (void)viewWillAppear:(BOOL)animated {
[super viewWillAppear:animated];
NSLog(@"%@------%s",[self class], __FUNCTION__);
}

- (void)viewDidAppear:(BOOL)animated {
[super viewDidAppear:animated];
NSLog(@"%@------%s",[self class], __FUNCTION__);
}

- (void)viewWillDisappear:(BOOL)animated {
[super viewWillDisappear:animated];
NSLog(@"%@------%s",[self class], __FUNCTION__);
}

- (void)viewDidDisappear:(BOOL)animated {
[super viewDidDisappear:animated];
NSLog(@"%@------%s",[self class], __FUNCTION__);
}

62114635-2C6A-435C-817F-F35C96D17FFA.png

由圖可知程序的運行順序是:

1. loadView

2. viewDidLoad

3. viewWillAppear

4. viewDidAppear
   這是只有一個頁面的情況 如果在這個頁面上放一個button push到下一個頁面 這是程序執行順序又會是怎么樣的呢
   下面讓我們再看看程序的運行結果

    - (void)loadView {
   [super loadView];
   NSLog(@"%@------%s",[self class], __FUNCTION__);
   }
   
   - (void)viewDidLoad {
   [super viewDidLoad];
   // Do any additional setup after loading the view, typically from a nib.
   NSLog(@"%@------%s",[self class], __FUNCTION__);
   self.view.backgroundColor = [UIColor redColor];
   self.navigationItem.leftBarButtonItem =   [[UIBarButtonItem alloc]   initWithBarButtonSystemItem:UIBarButtonSystemItemSave target:self action:@selector(barClick:)];
   }
   
   - (void)viewWillAppear:(BOOL)animated {
   [super viewWillAppear:animated];
   NSLog(@"%@------%s",[self class], __FUNCTION__);
    }
   
    - (void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];
    NSLog(@"%@------%s",[self class], __FUNCTION__);
    }
   
    - (void)viewWillDisappear:(BOOL)animated {
    [super viewWillDisappear:animated];
    NSLog(@"%@------%s",[self class], __FUNCTION__);
    }
   
    - (void)viewDidDisappear:(BOOL)animated {
    [super viewDidDisappear:animated];
    NSLog(@"%@------%s",[self class], __FUNCTION__);
    }  
   
    #pragma mark - buttonClick
    - (void)barClick:(UIBarButtonItem *)button {
    FirstViewController *VC = [[FirstViewController   alloc] init];
    [self.navigationController   pushViewController:VC animated:YES];
    }
   
   
   
    - (void)loadView {
    [super loadView];
    NSLog(@"%@------%s",[self class], __FUNCTION__);
    }
   
    - (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
   

   NSLog(@"%@------%s",[self class], FUNCTION);
   self.view.backgroundColor = [UIColor greenColor];

   }

   // 視圖將要出現時就會調用

   • (void)viewWillAppear:(BOOL)animated {
     [super viewWillAppear:animated];
     NSLog(@"%@------%s",[self class], FUNCTION);
     }
     // 視圖完全出現時就會調用
   • (void)viewDidAppear:(BOOL)animated {
     [super viewDidAppear:animated];
     NSLog(@"%@------%s",[self class], FUNCTION);
     }
     // 視圖將要消失會調用
   • (void)viewWillDisappear:(BOOL)animated {
     [super viewWillDisappear:animated];
     NSLog(@"%@------%s",[self class], FUNCTION);
     }
     // 視圖完全消失會調用
   • (void)viewDidDisappear:(BOOL)animated {
     [super viewDidDisappear:animated];
     NSLog(@"%@------%s",[self class], FUNCTION);
     }

4A3CCD27-6838-4F94-BEEF-511FCECB79FA.png

在ViewController頁面中有一個button 點擊button可以push到FirstViewController 由程序運行結果可知: 當剛運行程序時按順序依次執行:

1. loadView
2. viewDidLoad
3. viewWillAppear
4. viewDidAppear
5. viewWillDisappear
6. FirstViewController loadView
7. FirstViewController viewDidLoad
8. FirstViewController viewWillAppear
9. viewController viewDidDisappear
10. FirstViewController viewDidappear

當再一次點擊左上角的返回鍵時 從FirstViewController跳轉到viewController 由程序的運行結果可知 程序的運行順序為;

1. loadView
2. viewDidLoad
3. viewWillAppear
4. viewDidAppear
5. viewWillDisappear
6. FirstViewController loadView
7. FirstViewController viewDidLoad
8. FirstViewController viewWillAppear
9. viewController viewDidDisappear
10. FirstViewController viewDidappear
11. FirstViewController viewWillDisappear
12. viewController viewWillAppear
13. FirstViewController viewWillDisappear

14. viewController viewDidAppear

    
    
    8037D62C-4A2C-4CB3-9289-D705418B2839.png

從程序的執行順序可以看出來每個viewController只執行一次loadView和viewDidLoad