在内存警告(Apple 文档缺陷)中卸载 iOS 6 中的视图的正确方法是什么?

问题描述

在 iOS 6 中，viewWillUnload 和 viewDidUnload 已被弃用，UIViewControllers 不再卸载在内存警告期间在屏幕上不可见的视图.视图控制器编程指南 有一个如何手动恢复此行为的示例.

In iOS 6, viewWillUnload and viewDidUnload are deprecated and UIViewControllers no longer unload views that are not visible on screen during a memory warning. The View Controller Programming Guide has an example of how to manually restore this behavior.

这是代码示例:

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Add code to clean up any of your own resources that are no longer necessary.
    if ([self.view window] == nil)
    {
        // Add code to preserve data stored in the views that might be
        // needed later.

        // Add code to clean up other strong references to the view in
        // the view hierarchy.
        self.view = nil;
    }
}

代码示例下方是以下注释:

Below the code sample is the following note:

下次访问视图属性时，重新加载视图和第一次一样.

这里有一个明显的缺陷.如果一个没有加载它的视图的视图控制器收到一个内存警告，它将在 if ([self.view window] == nil) 行加载它的视图，然后继续清理并释放它再次.充其量，这是低效的.最糟糕的是，如果加载了复杂的视图层次结构和支持数据，它会使内存状况变得更糟.我在 iOS 模拟器中验证了这种行为.

There is an obvious flaw here. If a view controller that has not loaded its view receives a memory warning it will load its view in the line if ([self.view window] == nil) and then proceed to clean up and release it again. At best, this is inefficient. At worst, it makes the memory conditions worse if a complex view hierarchy and supporting data are loaded. I verified this behavior in the iOS simulator.

我当然可以解决这个问题，但 Apple 文档出现这样的错误似乎很奇怪.我错过了什么吗?

I can certainly code around this but seems odd for Apple docs to have such an error. Am I missing something?

推荐答案

在视图控制器中正确检查正在加载和显示在屏幕上的视图是:

The correct check in a view controller for the view being loaded and on screen is:

if ([self isViewLoaded] && [self.view window] == nil)

我在 iOS 6 中使用类似于 iOS 5 的视图控制器卸载视图和清理的完整解决方案如下:

My full solution in iOS 6 to have a view controller unload views and cleanup similar to iOS 5 is the following:

// will not be called in iOS 6, see iOS docs
- (void)viewWillUnload
{
  [super viewWillUnload];
  [self my_viewWillUnload];
}

// will not be called in iOS 6, see iOS docs
- (void)viewDidUnload
{
  [super viewDidUnload];
  [self my_viewDidUnload];
}

// in iOS 6, view is no longer unloaded so do it manually
- (void)didReceiveMemoryWarning
{
  [super didReceiveMemoryWarning];
  if ([self isViewLoaded] && [self.view window] == nil) {
    [self my_viewWillUnload];
    self.view = nil;
    [self my_viewDidUnload];
  }
}

- (void)my_viewWillUnload
{
  // prepare to unload view
}

- (void)my_viewDidUnload
{
  // the view is unloaded, clean up as normal
}

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