What is the proper way to avoid Retain Cycle while using blocks

Question

What is the proper way to add objects in NSMutableArray which is strongly defined by property.

[tapBlockView setTapBlock:^(UIImage* image) {
   [self.myImageArray addObject:image]; // self retain cycle
}

If I will create weak reference something like

__weak NSMutableArray *array = self.myImageArray;
[tapBlockView setTapBlock:^(UIImage* image) {
    [array addObject:image]; // If I will do this then how will I update original Array ?
}

I have also tried

__weak id weakSelf = self;
[tapBlockView setTapBlock:^(UIImage* image) {
    [weakSelf storeImageInaNewMethod:image]; // Calling SToreImageInaNewMethod
}

and

-(void)storeImageInaNewMethod:(UIImage*)image {
   [self.myImageArray addObject:image]; // This again retaining cycle
}

What is the proper way to update original object defined by property ?

Blocks are like function pointers.Can't you just set the block to nil when you don't need it anymore? — Ramy Al Zuhouri, Nov 06 '12 at 19:23

Kaan Dedeoglu · Answer 1 · 2015-01-23T16:13:40.523

15

After maddy's answer - this is from 2012 WWDC lecture on GCD and asynchronous programming:

__weak MyClass *weakSelf = self;

[tapBlockView setTapBlock:^(UIImage* image) {
    __strong MyClass *strongSelf = weakSelf;
    if(strongSelf) {
        [strongSelf.myImageArray addObject:image];
    }
}];

edited Jan 23 '15 at 16:13

answered Nov 06 '12 at 19:21

Kaan Dedeoglu

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I am not getting why you are changing into __strong again ? I guess only this code is sufficient ? [weakSelf.myImageArray addObject:image]; – Tariq Nov 06 '12 at 19:25
6

The reason for the cast back to `__strong` is to guarantee that if `weakSelf` is still alive when the first line of the block is executed, it will continue to live for the remainder of the execution of the block. – Carl Veazey Feb 25 '13 at 18:53
If you turned on warnings then you would see why the __strong is needed: A weak variable can disappear at any point, including in the middle of the call weakSelf.myImageArray which surely will cause trouble. – gnasher729 Apr 28 '15 at 12:08

score 9 · Accepted Answer · answered Nov 06 '12 at 19:17

9

Try a combination of the 2nd and 3rd.

__weak id weakSelf = self;
[tapBlockView setTapBlock:^(UIImage* image) {
    [weakSelf.myImageArray addObject:image];
}

answered Nov 06 '12 at 19:17

rmaddy

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Note this is only valid if you use weakSelf on the first line of the block, if you do anything else first then you need Kaan's answer. – malhal Jul 24 '16 at 00:28

score 1 · Answer 3 · answered Nov 06 '12 at 19:34

In your case you only need to reference an array which is referenced by self, so:

NSMutableArray *array = self.myImageArray;
[tapBlockView setTapBlock:^(UIImage* image)
                          {
                             [array addObject:image]; // No cycle
                          }];

Works fine provided that self.myImageArray does not return different array references at different times. There is no cycle: the current object references the array and the block, and in turn the block references the array.

If self.myImageArray does return different array references as different times then use a weak reference to self, your case 3.

score 0 · Answer 4 · answered Nov 06 '12 at 19:16

0

Your second and third ones appear correct. The second one works because you did not create a copy of the array, so that still points to the original one. The third one works because the reference to self is weak.

answered Nov 06 '12 at 19:16

Catfish_Man

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The 2nd one only works if `self.myImageArray` returns a direct reference to the ivar. That may not be a safe assumption. – rmaddy Nov 06 '12 at 19:18

What is the proper way to avoid Retain Cycle while using blocks

4 Answers4

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