Capturing an Objective-C object weakly within C block without declaring an explicit __weak or __block variable

问题

I am capturing a method-scoped object in a C block and I want to avoid retain cycles. Here is my code: (balloon is a custom view created within my current method)

balloon.onAddedToViewHierarchy = ^{
        CGRect globalRect = [[UIApplication sharedApplication].keyWindow convertRect:self.frame fromView:self.superview];
        CGRect targetFrame = CGRectMake(20, 0, SCREEN_WIDTH - 40, 60);
        targetFrame.origin.y = below ? globalRect.origin.y + globalRect.size.height + 10 : globalRect.origin.y - 10 - 60/*height*/;
        balloon.frame = globalRect;//targetFrame;
    };

at the last line (balloon.frame = globalRect;) I am told that capturing balloon in block will lead to a retain cycle. I know about ARC, ownerships, retain counts etc and I know the reason. I am just looking for a way to implicitly get rid of retaining balloon (it's already guaranteed to be retained by being referenced from other places) and using a __weak pointer, or __block if appropriate. I know I can do this:

__weak UIView *weakBalloon = balloon;
balloon.onAddedToViewHierarchy = ^{
        CGRect globalRect = [[UIApplication sharedApplication].keyWindow convertRect:self.frame fromView:self.superview];
        CGRect targetFrame = CGRectMake(20, 0, SCREEN_WIDTH - 40, 60);
        targetFrame.origin.y = below ? globalRect.origin.y + globalRect.size.height + 10 : globalRect.origin.y - 10 - 60/*height*/;
        weakBalloon.frame = globalRect;//targetFrame;
    };

But actually I'm exploring ways to implement the same behavior without explicitly creating a new pointer. I am looking for something like this: (it won't compile, just a demo)

balloon.onAddedToViewHierarchy = ^{
        CGRect globalRect = [[UIApplication sharedApplication].keyWindow convertRect:self.frame fromView:self.superview];
        CGRect targetFrame = CGRectMake(20, 0, SCREEN_WIDTH - 40, 60);
        targetFrame.origin.y = below ? globalRect.origin.y + globalRect.size.height + 10 : globalRect.origin.y - 10 - 60/*height*/;
        ((__weak UIView*)balloon).frame = globalRect;//targetFrame;
    };

Is something similar possible? I know there's nothing wrong with declaring a __weak variable, it will just work perfectly. I'm just curious that if such an implicit behavior is possible or not.

回答1:

Consider using the @weakify and @strongify macros, included in Reactive Cocoa or the Extended Objective-C Library.

For demonstration of this, see the Avoiding Retain Cycles discussion in Ray Wenderlich's part 2 of 2 discussion about Reactive Cocoa.

That article provides the following example:

__weak RWSearchFormViewController *bself = self; // Capture the weak reference

[[self.searchText.rac_textSignal map:^id(NSString *text) {
    return [self isValidSearchText:text] ? [UIColor whiteColor] : [UIColor yellowColor];
  }]
  subscribeNext:^(UIColor *color) {
    bself.searchText.backgroundColor = color;
  }];

Which they go on to suggest could be replaced with:

@weakify(self)
[[self.searchText.rac_textSignal map:^id(NSString *text) {
    return [self isValidSearchText:text] ? [UIColor whiteColor] : [UIColor yellowColor];
  }]
  subscribeNext:^(UIColor *color) {
    @strongify(self)
    self.searchText.backgroundColor = color;
  }];

来源：https://stackoverflow.com/questions/27203931/capturing-an-objective-c-object-weakly-within-c-block-without-declaring-an-expli