objective-c - 类和 block 的生命周期

Question

当一个由 UIViewController 创建的类作为另一个类的完成 block 时，当 UIViewController 得到一个 dealloc 调用时，内存管理生命周期如何工作？

假设 UIViewController 继承类实例化了一个类，ControllerMediatorClass。

ControllerMediatorClass 反过来调用另一个类 ClassThatDealsWithNetworking，它的工作需要一些时间才能完成。

如果 UIViewController 在 ClassThatDealsWithNetworking 完成之前得到它的 dealloc，那么它所拥有的类什么时候会被清理掉？

当 MyUIViewController 将它设置为 nil 时，'ControllerMediatorClass' 的实例是否不会立即被释放，因为 ControllerMediatorClass 仍然充当 ClassThatDealsWithNetworking 实例的完成 block ？

我的UIViewController:

@property (nonatomic, strong) ControllerMediatorClass *mediatorClass;

- (IBAction)userTappedSomething
{
    [mediatorClass makeANetworkCall];
}

- (void)dealloc
{
   self.mediatorClass = nil;
}

Controller 中介类:

- (void)makeANetworkCall
{
    ClassThatDealsWithNetworking *networkCommand;

    [networkCommand execute:^(NSDictionary *data) 
        {          
            // handling completion that 
        } error:^(MyError *error) 
        {        
            // handling completion
        }
    ];
}

(使用 ARC)

Answer 1

Would the instance of 'ControllerMediatorClass' not get deallocated immediately when MyUIViewController sets it to nil because ControllerMediatorClass still serves as a completion block to the instance of ClassThatDealsWithNetworking?

是的。因为 block 会自动捕获它使用的对象，并在保留 block 时保留它们。

所以，作为 [ClassThatDealsWithNetworking execute:]方法可能会保留传递给它的完成 block ，然后在后台执行网络调用，然后在完成后调用该 block 并释放该 block ，当该 block 被保留时，该 block 中使用的每个变量也被保留。并且会在 block 被释放时被释放。

所以想象一下你的 ClassThatDealsWithNetworking 的这个伪代码类(class):

typedef void(^NetworkingCompletionBlock)(NSDictionary* data)
@interface ClassThatDealsWithNetworking
@property(nonatomic, copy) NetworkingCompletionBlock completionBlock;
-(void)execute:(NetworkingCompletionBlock)block;
@end

@implementation ClassThatDealsWithNetworking
-(void)execute:(NetworkingCompletionBlock)block {
  // make a copy of the block passed as a parameter
  // so that we keep the completionBlock around inside
  // the ClassThatDealsWithNetworking instance
  // until the network request has finished
  // ==> THIS WILL implicitly RETAIN every object used in the completionBlock
  self.completionBlock = block; 
  ...
  // Then perform the network request
  [NSURLConnection connectionWithRequest:... delegate:self];
  ...
}

-(void)connection:(NSURLConnection*)cxn didFailWithError:(NSError*)error
{
  // call the completion block here
  self.completionBlock(nil,error);
  // then release the completionBlock
  // ==> THIS WILL RELEASE every object previously implicitly retained by the completionBlock
  self.completionBlock = nil;
}
-(void)connectionDidFinishLoading:(NSURLConnection*)cxn
{
  NSDictionary* data = ...
  // call the completion block here
  self.completionBlock(data,nil);
  // then release the completionBlock
  // ==> THIS WILL RELEASE every object previously implicitly retained by the completionBlock
  self.completionBlock = nil;
}
@end

然后，如果你这样做:

[networkCommand execute:^(NSDictionary *data) 
{
    self.someProperty = data;
} error:^(MyError *error) 
{
    NSLog(@"%@", error);
    self.someProperty : nil;
}];

然后 self (您的 ControllerMediatorClass 在您的示例中) 将被 block 隐式保留本身，只要 block 存在，因为您的引用 self在 block 体的某处。所以编译器知道在执行 block 时它会需要它并保留它。它 释放 block 时会隐式释放 .

这确保了您在 block 体中使用的所有对象在 block 执行时仍然存在，避免崩溃。

请注意，如果您不小心，这可能会导致保留周期。例如，如果您忘记自我 self.completionBlock = nil在 ClassThatDealsWithNetworking类(在委托(delegate)方法或 dealloc 方法中)， block 将永远不会被 ClassThatDealsWithNetworking 释放实例，并将继续保留 self .

欲了解更多信息，请阅读 Blocks Programming Guide在 Apple 文档中。