Retain cycle on `self` with blocks

Strictly speaking, the fact that it's a const copy has nothing to do with this problem. Blocks will retain any obj-c values that are captured when they are created. It just so happens that the workaround for the const-copy issue is identical to the workaround for the retain issue; namely, using the __block storage class for the variable.

In any case, to answer your question, there's no real alternative here. If you're designing your own block-based API, and it makes sense to do so, you could have the block get passed the value of self in as an argument. Unfortunately, this doesn't make sense for most APIs.

Please note that referencing an ivar has the exact same issue. If you need to reference an ivar in your block, either use a property instead or use bself->ivar.

Addendum: When compiling as ARC, __block no longer breaks retain cycles. If you're compiling for ARC, you need to use __weak or __unsafe_unretained instead.

Just use:

__weak id weakSelf = self;

[someObject someMethodWithBlock:^{
    [weakSelf someOtherMethod];
}];

For more information: WWDC 2011 - Blocks and Grand Central Dispatch in Practice.

https://developer.apple.com/videos/wwdc/2011/?id=308

Note: if that doesn't work you can try

__weak typeof(self)weakSelf = self;

This might be obvious, but you only have to do the ugly self alias when you know you’ll get a retain cycle. If the block is just a one-shot thing then I think you can safely ignore the retain on self. The bad case is when you have the block as a callback interface, for example. Like here:

typedef void (^BufferCallback)(FullBuffer* buffer);

@interface AudioProcessor : NSObject {…}
@property(copy) BufferCallback bufferHandler;
@end

@implementation AudioProcessor

- (id) init {
    …
    [self setBufferCallback:^(FullBuffer* buffer) {
        [self whatever];
    }];
    …
}

Here the API does not make much sense, but it would make sense when communicating with a superclass, for example. We retain the buffer handler, the buffer handler retains us. Compare with something like this:

typedef void (^Callback)(void);

@interface VideoEncoder : NSObject {…}
- (void) encodeVideoAndCall: (Callback) block;
@end

@interface Foo : NSObject {…}
@property(retain) VideoEncoder *encoder;
@end

@implementation Foo
- (void) somewhere {
    [encoder encodeVideoAndCall:^{
        [self doSomething];
    }];
}

In these situations I don’t do the self aliasing. You do get a retain cycle, but the operation is short-lived and the block will get out of memory eventually, breaking the cycle. But my experience with blocks is very small and it might be that self aliasing comes out as a best practice in the long run.

Posting another answer because this was a problem for me too. I originally thought I had to use blockSelf anywhere there was a self reference inside a block. This is not the case, it is only when the object itself has a block in it. And in fact, if you use blockSelf in these cases the object can get dealloc'd before you get the result back from the block and then it will crash when it tries to call it, so clearly you want self to be retained until the response comes back.

First case demonstrates when a retain cycle will occur because it contains a block which is referenced in the block:

#import <Foundation/Foundation.h>

typedef void (^MyBlock)(void);

@interface ContainsBlock : NSObject 

@property (nonatomic, copy) MyBlock block;

- (void)callblock;

@end 

@implementation ContainsBlock
@synthesize block = _block;

- (id)init {
    if ((self = [super init])) {

        //__block ContainsBlock *blockSelf = self; // to fix use this.
        self.block = ^{
                NSLog(@"object is %@", self); // self retain cycle
            };
    }
    return self;
}

- (void)dealloc {
    self.block = nil;
    NSLog (@"ContainsBlock"); // never called.
    [super dealloc];
} 

- (void)callblock {
    self.block();
} 

@end 

 int main() {
    ContainsBlock *leaks = [[ContainsBlock alloc] init];
    [leaks callblock];
    [leaks release];
}

You don't need blockSelf in the second case because the calling object does not have a block in it that will cause a retain cycle when you reference self:

#import <Foundation/Foundation.h>

typedef void (^MyBlock)(void);

@interface BlockCallingObject : NSObject 
@property (copy, nonatomic) MyBlock block;
@end

@implementation BlockCallingObject 
@synthesize block = _block;

- (void)dealloc {
    self.block = nil;
    NSLog(@"BlockCallingObject dealloc");
    [super dealloc];
} 

- (void)callblock {
    self.block();
} 
@end

@interface ObjectCallingBlockCallingObject : NSObject 
@end

@implementation ObjectCallingBlockCallingObject 

- (void)doneblock {
    NSLog(@"block call complete");
}

- (void)dealloc {
    NSLog(@"ObjectCallingBlockCallingObject dealloc");
    [super dealloc];
} 

- (id)init {
    if ((self = [super init])) {

        BlockCallingObject *myobj = [[BlockCallingObject alloc] init];
        myobj.block = ^() {
            [self doneblock]; // block in different object than this object, no retain cycle
        };
        [myobj callblock];
        [myobj release];
    }
    return self;
}
@end

int main() {

    ObjectCallingBlockCallingObject *myObj = [[ObjectCallingBlockCallingObject alloc] init];
    [myObj release];

    return 0;
}