I am a little confused about block usage in Objective-C. I currently use ARC and I have quite a lot of blocks in my app, currently always referring to self
instead of its weak reference. May that be the cause of these blocks retaining self
and keeping it from being dealloced ? The question is, should I always use a weak
reference of self
in a block ?
-(void)handleNewerData:(NSArray *)arr
{
ProcessOperation *operation =
[[ProcessOperation alloc] initWithDataToProcess:arr
completion:^(NSMutableArray *rows) {
dispatch_async(dispatch_get_main_queue(), ^{
[self updateFeed:arr rows:rows];
});
}];
[dataProcessQueue addOperation:operation];
}
ProcessOperation.h
@interface ProcessOperation : NSOperation
{
NSMutableArray *dataArr;
NSMutableArray *rowHeightsArr;
void (^callback)(NSMutableArray *rows);
}
ProcessOperation.m
-(id)initWithDataToProcess:(NSArray *)data completion:(void (^)(NSMutableArray *rows))cb{
if(self =[super init]){
dataArr = [NSMutableArray arrayWithArray:data];
rowHeightsArr = [NSMutableArray new];
callback = cb;
}
return self;
}
- (void)main {
@autoreleasepool {
...
callback(rowHeightsArr);
}
}
This question is related to
ios
iphone
objective-c
automatic-ref-counting
weak-references
This is how you can use the self inside the block:
//calling of the block
NSString *returnedText= checkIfOutsideMethodIsCalled(self);
NSString* (^checkIfOutsideMethodIsCalled)(*)=^NSString*(id obj)
{
[obj MethodNameYouWantToCall]; // this is how it will call the object
return @"Called";
};
Some explanation ignore a condition about the retain cycle [If a group of objects is connected by a circle of strong relationships, they keep each other alive even if there are no strong references from outside the group.] For more information, read the document
As Leo points out, the code you added to your question would not suggest a strong reference cycle (a.k.a., retain cycle). One operation-related issue that could cause a strong reference cycle would be if the operation is not getting released. While your code snippet suggests that you have not defined your operation to be concurrent, but if you have, it wouldn't be released if you never posted isFinished
, or if you had circular dependencies, or something like that. And if the operation isn't released, the view controller wouldn't be released either. I would suggest adding a breakpoint or NSLog
in your operation's dealloc
method and confirm that's getting called.
You said:
I understand the notion of retain cycles, but I am not quite sure what happens in blocks, so that confuses me a little bit
The retain cycle (strong reference cycle) issues that occur with blocks are just like the retain cycle issues you're familiar with. A block will maintain strong references to any objects that appear within the block, and it will not release those strong references until the block itself is released. Thus, if block references self
, or even just references an instance variable of self
, that will maintain strong reference to self, that is not resolved until the block is released (or in this case, until the NSOperation
subclass is released.
For more information, see the Avoid Strong Reference Cycles when Capturing self section of the Programming with Objective-C: Working with Blocks document.
If your view controller is still not getting released, you simply have to identify where the unresolved strong reference resides (assuming you confirmed the NSOperation
is getting deallocated). A common example is the use of a repeating NSTimer
. Or some custom delegate
or other object that is erroneously maintaining a strong
reference. You can often use Instruments to track down where objects are getting their strong references, e.g.:
Or in Xcode 5:
You don't have to always use a weak reference. If your block is not retained, but executed and then discarded, you can capture self strongly, as it will not create a retain cycle. In some cases, you even want the block to hold the self until the completion of the block so it does not deallocate prematurely. If, however, you capture the block strongly, and inside capture self, it will create a retain cycle.
I totally agree with @jemmons:
But this should not be the default pattern you follow when dealing with blocks that call self! This should only be used to break what would otherwise be a retain cycle between self and the block. If you were to adopt this pattern everywhere, you'd run the risk of passing a block to something that got executed after self was deallocated.
//SUSPICIOUS EXAMPLE: __weak MyObject *weakSelf = self; [[SomeOtherObject alloc] initWithCompletion:^{ //By the time this gets called, "weakSelf" might be nil because it's not retained! [weakSelf doSomething]; }];
To overcome this problem one can define a strong reference over the weakSelf
inside the block:
__weak MyObject *weakSelf = self;
[[SomeOtherObject alloc] initWithCompletion:^{
MyObject *strongSelf = weakSelf;
[strongSelf doSomething];
}];
Source: Stackoverflow.com