IOS视图生命周期

以视图的4种状态为基础，我们来系统了解一下视图控制器的生命周期。在视图不同的生命周期中，视图控制器会回调不同的方法，具体如图所示

IOS视图生命周期

1.viewDidLoad方法开发文档中的解释：

- (void)viewDidLoad; 
// Called after the view has been loaded. For view controllers created in code, this is after -loadView. For view controllers unarchived from a nib, this is after the view is set.

在视图控制器已被实例化，视图被加载到内存中时，会调用viewDidLoad方法，这时视图并未出现。在该方法中，通常会对所控制的视图进行初始化处理。如设置导航栏的标题和返回按钮：

self.title = @"标题";
self.navigationItem.backBarButtonItem = [[UIBarButtonItem alloc] initWithTitle:@"返回" style:UIBarButtonItemStylePlain target:self action:@selector(function)];

2.viewWillAppear，viewDidAppear，viewWillDisappear，viewDidDisappear方法开发文档中的解释：

- (void)viewWillAppear:(BOOL)animated;    
// Called when the view is about to made visible. Default does nothing
- (void)viewDidAppear:(BOOL)animated;    
// Called when the view has been fully transitioned onto the screen. Default does nothing
- (void)viewWillDisappear:(BOOL)animated; 
// Called when the view is dismissed, covered or otherwise hidden. Default does nothing
- (void)viewDidDisappear:(BOOL)animated;  
// Called after the view was dismissed, covered or otherwise hidden. Default does nothing

视图可见前后会调用viewWillAppear:方法和viewDidAppear:方法，视图不可见前后会调用viewWillDisappear:方法和viewDidDisappear:方法。这4个方法调用父类相应的方法以实现其功能，编码时调用父类方法的位置可根据实际情况做调整，参见如下代码：

- (void) viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
}

viewDidLoad方法在应用运行的时候只调用一次，而上述这4个方法可以被反复调用多次，一些需要每次打开页面都执行的方法可以写在viewWillAppear方法中，如刷新界面，查询数据等。

- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    [self.tableView reloadData];
}

3.didReceiveMemoryWarning方法开发文档中的解释：

- (void)didReceiveMemoryWarning; 
// Called when the parent application receives a memory warning. On iOS 6.0 it will no longer clear the view by default.

在低内存的情况下，IOS会调用didReceiveMemoryWarning和viewDidUnload方法。在IOS6之后就不再调用viewDidUnload方法，而仅支持didReceiveMemoryWarning方法。didReceiveMemoryWarning方法的主要职能是释放内存，包括视图控制器中的一些成员变量和视图的释放。

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
}

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IOS视图生命周期

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