避免在block中循环引用(Retain Cycle in Block)

让我们长话短说。请参阅如下代码:

- (IBAction)didTapUploadButton:(id)sender
{
  NSString *clientID = @"YOUR_CLIENT_ID_HERE";

  NSString *title = [[self titleTextField] text];
  NSString *description = [[self descriptionTextField] text];

  [[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:YES];

  __weak MLViewController *weakSelf = self;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    UIImage *image = [UIImage imageNamed:@"balloons.jpg"];
    NSData *imageData = UIImageJPEGRepresentation(image, 1.0f);

[MLIMGURUploader uploadPhoto:imageData 
                       title:title 
                 description:description 
               imgurClientID:clientID completionBlock:^(NSString *result) {
  dispatch_async(dispatch_get_main_queue(), ^{
    [[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:NO];
    [[weakSelf linkTextView] setText:result];
  });
} failureBlock:^(NSURLResponse *response, NSError *error, NSInteger status) {
  dispatch_async(dispatch_get_main_queue(), ^{
  [[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:NO];
  [[[UIAlertView alloc] initWithTitle:@"Upload Failed"
                              message:[NSString stringWithFormat:@"%@ (Status code %d)", 
                                               [error localizedDescription], status]
                             delegate:nil
                    cancelButtonTitle:nil
                    otherButtonTitles:@"OK", nil] show];
  });
}];

  });
}

这是一个上传IMAGE的方法,这一段里面我们可以看到代码:

__weak MLViewController *weakSelf = self;

并且这样使用了weakSelf

[[weakSelf linkTextView] setText:result];

所以我们有一个问题:为什么我们不直接的使用self, 而是用一个weak指针指向他.并且在block区间使用这个weak指针.

这就是我要说的。我们都知道,block的属性是copy、self的属性是retain。因此,如果我们使用self在block部分,我们将得到一个retain循环(互相retain)。他们的内存永远不会释放。出于这个原因,我们必须要按照上面的方法做。