How do I avoid capturing self in blocks when implementing an API?

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How do I avoid capturing self in blocks when implementing an API?

ghost丶桃子 2016-05-18 18:15:35 浏览712 评论0

摘要: Short answer Instead of accessing self directly, you should access it indirectly, from a reference that will not be retained.

Short answer

Instead of accessing self directly, you should access it indirectly, from a reference that will not be retained. If you're not using Automatic Reference Counting (ARC), you can do this:

__block MyDataProcessor*dp = self;
self.progressBlock =^(CGFloat percentComplete){[dp.delegate myAPI:dp isProcessingWithProgress:percentComplete];}

The __block keyword marks variables that can be modified inside the block (we're not doing that) but also they are not automatically retained when the block is retained (unless you are using ARC). If you do this, you must be sure that nothing else is going to try to execute the block after the MyDataProcessor instance is released. (Given the structure of your code, that shouldn't be a problem.) Read more about__block.

If you are using ARC, the semantics of __block changes and the reference will be retained, in which case you should declare it __weak instead.

Long answer

Let's say you had code like this:

self.progressBlock =^(CGFloat percentComplete){[self.delegate processingWithProgress:percentComplete];}

The problem here is that self is retaining a reference to the block; meanwhile the block must retain a reference to self in order to fetch its delegate property and send the delegate a method. If everything else in your app releases its reference to this object, its retain count won't be zero (because the block is pointing to it) and the block isn't doing anything wrong (because the object is pointing to it) and so the pair of objects will leak into the heap, occupying memory but forever unreachable without a debugger. Tragic, really.

That case could be easily fixed by doing this instead:

id progressDelegate = self.delegate;
self.progressBlock =^(CGFloat percentComplete){[progressDelegate processingWithProgress:percentComplete];}

In this code, self is retaining the block, the block is retaining the delegate, and there are no cycles (visible from here; the delegate may retain our object but that's out of our hands right now). This code won't risk a leak in the same way, because the value of the delegate property is captured when the block is created, instead of looked up when it executes. A side effect is that, if you change the delegate after this block is created, the block will still send update messages to the old delegate. Whether that is likely to happen or not depends on your application.

Even if you were cool with that behavior, you still can't use that trick in your case:

self.dataProcessor.progress =^(CGFloat percentComplete){[self.delegate myAPI:self isProcessingWithProgress:percentComplete];};

Here you are passing self directly to the delegate in the method call, so you have to get it in there somewhere. If you have control over the definition of the block type, the best thing would be to pass the delegate into the block as a parameter:

self.dataProcessor.progress =^(MyDataProcessor*dp,CGFloat percentComplete){[dp.delegate myAPI:dp isProcessingWithProgress:percentComplete];};

This solution avoids the retain cycle and always calls the current delegate.

If you can't change the block, you could deal with it. The reason a retain cycle is a warning, not an error, is that they don't necessarily spell doom for your application. If MyDataProcessor is able to release the blocks when the operation is complete, before its parent would try to release it, the cycle will be broken and everything will be cleaned up properly. If you could be sure of this, then the right thing to do would be to use a #pragma to suppress the warnings for that block of code. (Or use a per-file compiler flag. But don't disable the warning for the whole project.)

You could also look into using a similar trick above, declaring a reference weak or unretained and using that in the block. For example:

__weak MyDataProcessor*dp = self;// OK for iOS 5 only
__unsafe_unretained MyDataProcessor*dp = self;// OK for iOS 4.x and up
__block MyDataProcessor*dp = self;// OK if you aren't using ARC
self.progressBlock =^(CGFloat percentComplete){[dp.delegate myAPI:dp isProcessingWithProgress:percentComplete];}

All three of the above will give you a reference without retaining the result, though they all behave a little bit differently: __weak will try to zero the reference when the object is released;__unsafe_unretained will leave you with an invalid pointer; __block will actually add another level of indirection and allow you to change the value of the reference from within the block (irrelevant in this case, since dp isn't used anywhere else).

What's best will depend on what code you are able to change and what you cannot. But hopefully this has given you some ideas on how to proceed.

Awesome answer! Thanks, I have a much better understanding about what is going on and how this all works. In this case, I have control over everything so I'll re-architect some of the objects as needed. – XJones Oct 21 '11 at 21:18
Good answer. I've been using __weak myself in an iOS project. It gives you more safety than __block or __unsafe_unretained, but I imagine most people would be calling this in a situation where self is guaranteed to exist. I also like __weak as a keyword because it is more "explicit" about what's being accomplished, whereas the intentions of __block can be ambiguous. –  mikelikespie Nov 2 '11 at 1:15
O_O I was just passing by with a slightly different problem, got stuck reading, and now leave this page feeling all knowledgeable and cool. Thanks! –  Orc JMR Dec 19 '12 at 8:07
@OrcJMR You're welcome! –  benzado Dec 19 '12 at 22:12
is is correct, that if for some reason on the moment of block execution dp will be released (for example if it was a view controller and it was poped), then line [dp.delegate ... will cause EXC_BADACCESS? – peetonn Jan 30 '13 at 1:50
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There’s also the option to suppress the warning when you are positive that the cycle will get broken in the future:

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-retain-cycles"

self.progressBlock =^(CGFloat percentComplete){[self.delegate processingWithProgress:percentComplete];}#pragma clang diagnostic pop

That way you don’t have to monkey around with __weakself aliasing and explicit ivar prefixing.

Sounds like a very bad practice that takes more than 3 lines of code that can be replaced with __weak id weakSelf = self; –  Andy Sep 3 '13 at 14:05 
There’s often a larger block of code that can benefit from the suppressed warnings. –  zoul Sep 3 '13 at 17:56
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I believe the solution without ARC also works with ARC, using the __block keyword:

EDIT: Per the Transitioning to ARC Release Notes, an object declared with __block storage is still retained. Use __weak (preferred) or __unsafe_unretained (for backwards compatibility).

// code sample
self.delegate= aDelegate;

self.dataProcessor =[[MyDataProcessor alloc] init];// Use this inside blocks
__block id myself = self;

self.dataProcessor.progress =^(CGFloat percentComplete){[myself.delegate myAPI:myself isProcessingWithProgress:percentComplete];};

self.dataProcessor.completion =^{[myself.delegate myAPIDidFinish:myself];
    myself.dataProcessor = nil;};// start the processor - processing happens asynchronously and the processor is released in the completion block[self.dataProcessor startProcessing];
Didn't realize that the __block keyword avoided retaining it's referent. Thanks! I updated my monolithic answer. :-) –  benzado Oct 21 '11 at 21:41
According to Apple docs "In manual reference counting mode, __block id x; has the effect of not retaining x. In ARC mode, __block id x; defaults to retaining x (just like all other values)." –  XJones Oct 22 '11 at 2:08 
@XJones Thanks for the clarification, I've edited my answer. –  Tony Oct 24 '11 at 15:09
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For a common solution, I have these define in the precompile header. Avoids capturing and still enables compiler help by avoiding to use id

#defineBlockWeakObject(o) __typeof(o) __weak

Then in code you can do:

BlockWeakSelf weakSelf = self;
self.dataProcessor.completion =^{[weakSelf.delegate myAPIDidFinish:weakSelf];
    weakSelf.dataProcessor = nil;};
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