212

How can I avoid this warning in xcode. Here is the code snippet:

[player(AVPlayer object) addPeriodicTimeObserverForInterval:CMTimeMakeWithSeconds(0.1, 100)
queue:nil usingBlock:^(CMTime time) {
    current+=1;

    if(current==60)
    {
        min+=(current/60);
        current = 0;
    }

    [timerDisp(UILabel) setText:[NSString stringWithFormat:@"%02d:%02d",min,current]];///warning occurs in this line
}];
devios1
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user1845209
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7 Answers7

520

The capture of self here is coming in with your implicit property access of self.timerDisp - you can't refer to self or properties on self from within a block that will be strongly retained by self.

You can get around this by creating a weak reference to self before accessing timerDisp inside your block:

__weak typeof(self) weakSelf = self;
[player addPeriodicTimeObserverForInterval:CMTimeMakeWithSeconds(0.1, 100)
                                     queue:nil
                                usingBlock:^(CMTime time) {
                                                current+=1;

                                                if(current==60)
                                                {
                                                    min+=(current/60);
                                                    current = 0;
                                                }

                                                 [weakSelf.timerDisp setText:[NSString stringWithFormat:@"%02d:%02d",min,current]];
                                            }];
Tim
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  • __weak typeof(self) weakSelf = self;...is giving error "the current deployment target does not support automated __weak references"...my deployment target is 4.3 – user1845209 Jan 28 '13 at 06:46
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    Try using `__unsafe_unretained` instead. – Tim Jan 28 '13 at 06:51
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    Resolved. use this instead: __unsafe_unretained typeof(self) weakSelf = self; thanks fo the help @Tim – user1845209 Jan 28 '13 at 06:51
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    Good answer, but I take small issue with you saying: “you can't refer to self or properties on self from within a block that will be strongly retained by self.” This is not strictly true. Please see my answer below. Better to say, “you *must* take great care if you refer to self…” – Chris Suter Mar 25 '13 at 00:53
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    I don't see a retain cycle in the OP's code. The block is not strongly retained by `self`, it's retained by the main dispatch queue. Am I wrong? – erikprice Mar 28 '13 at 17:37
  • Is this necessary, even with ARC? – bobobobo Mar 29 '13 at 04:56
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    @erikprice: you're not wrong. I interpreted the question to be primarily about the error Xcode presents ("How can I avoid this warning in xcode"), rather than about the actual presence of a retain cycle. You're correct in saying no retain cycle is evident just from the snippet OP provided. – Tim Mar 29 '13 at 05:02
  • @bobobobo: yes, this is necessary in ARC. – Tim Mar 29 '13 at 05:02
  • I see, thanks. The net effect is similar, as long as the main dispatch queue retains the block then `self` cannot be deallocated. In the case of -addPeriodicTimeObserverWithTimeInterval:, it's possible for a block that retains `self` to be enqueued before the previous block that retains `self` is executed, creating not so much a retain cycle, but a permanent state of `self` being retained. – erikprice Mar 29 '13 at 14:58
  • If I use __weak typeof(self) weakSelf = self; then all the properties for this object will be NULL. – Bagusflyer Apr 12 '13 at 10:27
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    This has helped me http://stackoverflow.com/questions/7205128/fix-warning-capturing-an-object-strongly-in-this-block-is-likely-to-lead-to-a – iCoder86 Jun 19 '13 at 07:36
  • follow up question: http://stackoverflow.com/questions/18831460/how-to-keep-a-strong-reference-inside-of-a-block-without-having-a-retain-cycle – abbood Sep 16 '13 at 15:13
  • What if you did `typeof(self) strongSelf = self;` outside of the block (instead of __weak) then in the block said `strongSelf = nil;` after usage? – Matt Jun 21 '14 at 02:08
  • In my sample project, it does look like that will avoid this warning. Keep in mind, though, that you are still temporarily establishing a retain cycle - since your `strongSelf` is (as you say) strong, `self` and the block will retain each other. Setting `strongSelf` to `nil` should break the cycle, though. Whether this approach applies to your situation is left to individual discretion. – Tim Jun 21 '14 at 02:31
  • In most cases you will only use one or a couple of members of `self` in this block, most likely just to update a slider. Casting `self` is overkill, better be explicit and cast only the objects that you truly need in the block. For example, if it's an instance of `UISlider*`, say, `_timeSlider`, just do the following before the block declaration: `UISlider* __weak slider = _timeSlider;` Then just use `slider` inside the block. Technically this is more precise as it narrows down the potential retain cycle to only the object you need, not all the objects inside `self` – Luis Artola Dec 22 '15 at 18:39
  • You can use this as well: __weak __typeof(&*self)weakSelf = self; – Itai Spector Feb 29 '16 at 11:37
53
__weak MyClass *self_ = self; // that's enough
self.loadingDidFinishHandler = ^(NSArray *receivedItems, NSError *error){
    if (!error) {
       [self_ showAlertWithError:error];
    } else {
       self_.items = [NSArray arrayWithArray:receivedItems];
       [self_.tableView reloadData];
    }
};

And one very important thing to remember: do not use instance variables directly in block, use it as a properties of weak object, sample:

self.loadingDidFinishHandler = ^(NSArray *receivedItems, NSError *error){
        if (!error) {
           [self_ showAlertWithError:error];
        } else {
           self_.items = [NSArray arrayWithArray:receivedItems];
           [_tableView reloadData]; // BAD! IT ALSO WILL BRING YOU TO RETAIN LOOP
        }
 };

and don't forget to do:

- (void)dealloc {
    self.loadingCompletionHandler = NULL;
}

another issue can appear if you will pass weak copy of not retained by anybody object:

MyViewController *vcToGo = [[MyViewCOntroller alloc] init];
__weak MyViewController *vcToGo_ = vcToGo;
self.loadingCompletion = ^{
    [vcToGo_ doSomePrecessing];
};

if vcToGo will be deallocated and then this block fired I believe you will get crash with unrecognized selector to a trash which is contains vcToGo_ variable now. Try to control it.

iiFreeman
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46

Better version

__strong typeof(self) strongSelf = weakSelf;

Create a strong reference to that weak version as the first line in your block. If self still exists when the block starts to execute and hasn’t fallen back to nil, this line ensures it persists throughout the block’s execution lifetime.

So the whole thing would be like this:

// Establish the weak self reference
__weak typeof(self) weakSelf = self;

[player addPeriodicTimeObserverForInterval:CMTimeMakeWithSeconds(0.1, 100)
                                 queue:nil
                            usingBlock:^(CMTime time) {

    // Establish the strong self reference
    __strong typeof(self) strongSelf = weakSelf;

    if (strongSelf) {
        [strongSelf.timerDisp setText:[NSString stringWithFormat:@"%02d:%02d",min,current]];
    } else {
        // self doesn't exist
    }
}];

I have read this article many times. This is an excellent article by Erica Sadun on How To Avoid Issues When Using Blocks And NSNotificationCenter

Swift update:

For example, in swift a simple method with success block would be:

func doSomeThingWithSuccessBlock(success: () -> ()) {
    success()
}

When we call this method and need to use self in the success block. We'll be using the [weak self] and guard let features. 

    doSomeThingWithSuccessBlock { [weak self] () -> () in
        guard let strongSelf = self else { return }
        strongSelf.gridCollectionView.reloadData()
    }

This so-called strong-weak dance is used by popular open source project Alamofire.

For more info check out swift-style-guide

Warif Akhand Rishi
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  • What if you did `typeof(self) strongSelf = self;` outside of the block (instead of __weak) then in the block said `strongSelf = nil;` after usage? I don't see how your example ensures that weakSelf isn't nil by the time the block executes. – Matt Jun 21 '14 at 02:05
  • To avoid possible retain cycles, we establish a weak self reference outside any block that uses self in its code. In ur way, u have to ensure that the block is executed. Another block of ur code is now responsible for freeing ur previously retained memory. – Warif Akhand Rishi Jun 21 '14 at 08:40
  • @Matt the purpose of this example is not to make the weakSelf retained. The purpose is, if the weakSelf is not nil, make a strong reference inside the block. So once the block starts executing with self, self doesn't become nil inside the block. – Warif Akhand Rishi Jun 22 '14 at 08:04
15

In another answer, Tim said:

you can't refer to self or properties on self from within a block that will be strongly retained by self.

This isn’t quite true. It’s OK for you to do this so long as you break the cycle at some point. For example, let’s say you have a timer that fires that has a block that retains self and you also keep a strong reference to the timer in self. This is perfectly fine if you always know that you will destroy the timer at some point and break the cycle.

In my case just now, I had this warning for code that did:

[x setY:^{ [x doSomething]; }];

Now I happen to know that clang will only produce this warning if it detects the method starts with “set” (and one other special case that I won’t mention here). For me, I know there is no danger of there being a retain loop, so I changed the method name to “useY:” Of course, that might not be appropriate in all cases and usually you will want to use a weak reference, but I thought it worth noting my solution in case it helps others.

Chris Suter
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4

Many times, this is not actually a retain cycle.

If you know that it's not, you need not bring fruitless weakSelves into the world.

Apple even forces these warnings upon us with the API to their UIPageViewController, which includes a set method (which triggers these warnings–as mentioned elsewhere–thinking you are setting a value to an ivar that is a block) and a completion handler block (in which you'll undoubtedly refer to yourself).

Here's some compiler directives to remove the warning from that one line of code:

#pragma GCC diagnostic push
#pragma clang diagnostic ignored "-Warc-retain-cycles"
    [self.pageViewController setViewControllers:@[newViewController] direction:navigationDirection animated:YES completion:^(BOOL finished) {
        // this warning is caused because "setViewControllers" starts with "set…", it's not a problem
        [self doTheThingsIGottaDo:finished touchThePuppetHead:YES];
    }];
#pragma GCC diagnostic pop
bshirley
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1

Adding two cents on improving precision and style. In most cases you will only use one or a couple of members of self in this block, most likely just to update a slider. Casting self is overkill. Instead, it's better to be explicit and cast only the objects that you truly need inside the block. For example, if it's an instance of UISlider*, say, _timeSlider, just do the following before the block declaration: 

UISlider* __weak slider = _timeSlider;

Then just use slider inside the block. Technically this is more precise as it narrows down the potential retain cycle to only the object that you need, not all the objects inside self.

Full example:

UISlider* __weak slider = _timeSlider;
[_embeddedPlayer addPeriodicTimeObserverForInterval:CMTimeMake(1, 1)
     queue:nil
     usingBlock:^(CMTime time){
        slider.value = time.value/time.timescale;
     }
];

Additionally, most likely the object being cast to a weak pointer is already a weak pointer inside self as well minimizing or eliminating completely the likelihood of a retain cycle. In the example above, _timeSlider is actually a property stored as a weak reference, e.g:

@property (nonatomic, weak) IBOutlet UISlider* timeSlider;

In terms of coding style, as with C and C++, variable declarations are better read from right to left. Declaring SomeType* __weak variable in this order reads more naturally from right to left as: variable is a weak pointer to SomeType.

Luis Artola
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1

I ran into this warning recently and wanted to understand it a bit better. After a bit of trial and error, I discovered that it originates from having a method start with either "add" or "save". Objective C treats method names starting with "new", "alloc", etc as returning a retained object but doesn't mention (that I can find) anything about "add" or "save". However, if I use a method name in this way:

[self addItemWithCompletionBlock:^(NSError *error) {
            [self done]; }];

I will see the warning at the [self done] line. However, this will not:

[self itemWithCompletionBlock:^(NSError *error) {
    [self done]; }];

I will go ahead and use the "__weak __typeof(self) weakSelf = self" way to reference my object but really don't like having to do so since it will confuse a future me and/or other dev. Of course, I could also not use "add" (or "save") but that's worse since it takes away the meaning of the method.

Ray M.
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