未使用 xib 时，内存警告后不调用 -viewDidUnload

Question

当我使用 xib 时，在内存警告后调用 -viewDidUnload。
但是当我以编程方式而不是使用 xib 创建视图时，不会调用 -viewDidUnload。

（在这两种情况下，都会调用 -didReceiveMemoryWarning。）

当我不使用 xib 文件时，为什么不调用 -viewDidUnload？
如果我不使用 xib，我是否不必为 -viewDidUnload 编写代码？

以下是测试代码和结果：
（我正在使用 ARC）

@implementation ViewControllerA
- (void)viewDidLoad
{
    [super viewDidLoad];

    self.title = [NSString stringWithFormat:@"%d", gNavigationController.viewControllers.count];

    // button to pop view controller to navigation controller
    UIButton *button = [UIButton buttonWithType:UIButtonTypeRoundedRect];
    button.frame = CGRectMake(100, 100, 200, 50);
    [button setTitle:@"push" forState:UIControlStateNormal];
    [button addTarget:self action:@selector(push) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:button];

    // button to generate memory warning
    button = [UIButton buttonWithType:UIButtonTypeRoundedRect];
    button.frame = CGRectMake(100, 200, 200, 50);
    [button setTitle:@"memory warning" forState:UIControlStateNormal];
    [button addTarget:self action:@selector(generateMemoryWarning) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:button];

    NSLog(@"view did load %@", self.title);
}

- (void)generateMemoryWarning {
    [[UIApplication sharedApplication] performSelector:@selector(_performMemoryWarning)];
}

- (void)push {
    UIViewController *viewController = [[ViewControllerA alloc] init];
    [gNavigationController pushViewController:viewController animated:YES];
}

- (void)viewDidUnload
{
    [super viewDidUnload];
    NSLog(@"view did unload %@", self.title);
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    NSLog(@"view did receive memory warning %@", self.title);
}

@end

结果

如果 ViewControllerA 没有 xib：

view did load 1
view did load 2
view did receive memory warning 1
view did receive memory warning 2

如果 ViewControllerA 有 xib：

view did load 1
view did load 2
view did unload 1
view did receive memory warning 1
view did receive memory warning 2

Answer 1

如果您使用UIViewContoller 而不从NIB 初始化它，您需要继承-loadView 方法。否则 iOS 假定视图不能被卸载/重新加载。

只需将以下内容添加到您的实现中就足够了：

- (void)loadView {
    [super loadView];
    self.view = yourCreatedView;
}

很遗憾，documentation 对此不是很清楚。