使用块保留`self`循环

Question

I'm afraid this question is pretty basic, but I think it's relevant to a lot of Objective-C programmers who are getting into blocks.

What I've heard is that since blocks capture local variables referenced within them as const copies, using self within a block can result in a retain cycle, should that block be copied. So, we are supposed to use __block to force the block to deal directly with self instead of having it copied.

__block typeof(self) bself = self;
[someObject messageWithBlock:^{ [bself doSomething]; }];

instead of just

[someObject messageWithBlock:^{ [self doSomething]; }];

What I'd like to know is the following: if this is true, is there a way that I can avoid the ugliness (aside from using GC)?

Answer 1

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.

Answer 2

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;

Answer 3

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.

Answer 4

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;
} 

Answer 5

Remember also that retain cycles can occur if your block refers to another object which then retains self .

I'm not sure that Garbage Collection can help in these retain cycles. If the object retaining the block (which I'll call the server object) outlives self (the client object), the reference to self inside the block will not be considered cyclic until the retaining object itself is released. If the server object far outlives its clients, you may have a significant memory leak.

Since there are no clean solutions, I would recommend the following workarounds. Feel free to choose one or more of them to fix your issue.

• Use blocks only for completion , and not for open-ended events. For example, use blocks for methods like doSomethingAndWhenDoneExecuteThisBlock: , and not methods like setNotificationHandlerBlock: . Blocks used for completion have definite ends of lives, and should be released by server objects after they are evaluated. This prevents the retain cycle from living for too long even if it occurs.
• Do that weak-reference dance you described.
• Provide a method to clean up your object before it's released, which "disconnects" the object from server objects that may hold references to it; and call this method before calling release on the object. While this method is perfectly fine if your object only has one client (or is a singleton within some context), but will break down if it has multiple clients. You're basically defeating the retain-counting mechanism here; this is akin to calling dealloc instead of release .

If you are writing a server object, take block arguments only for completion. Do not accept block arguments for callbacks, such as setEventHandlerBlock: . Instead, fall back to the classic delegate pattern: create a formal protocol, and advertise a setEventDelegate: method. Do not retain the delegate. If you don't even want to create a formal protocol, accept a selector as a delegate callback.

And lastly, this pattern should ring alarms:

- (void)dealloc {
    [myServerObject releaseCallbackBlocksForObject:self];
    ...
}

If you're trying to unhook blocks that may refer to self from inside dealloc , you're already in trouble. dealloc may never be called due to the retain cycle caused by references in the block, which means that your object is simply going to leak until the server object is deallocated.

Answer 6

__block __unsafe_unretained modifiers suggested in Kevin's post may cause to the bad access exception in case of block executed in a different thread. It's better use only __block modifier for the temp variable and make it nil after the usage.

__block SomeType* this = self;
[someObject messageWithBlock:^{
  [this doSomething]; // here would be BAD_ACCESS in case of __unsafe_unretained with
                      //  multithreading and self was already released
  this = nil;
}];

Answer 7

You can use libextobjc library. It is quite popular, it is used in ReactiveCocoa for example. https://github.com/jspahrsummers/libextobjc

It provides 2 macros @weakify and @strongify, so you can have:

@weakify(self)
[someObject messageWithBlock:^{
   @strongify(self)
   [self doSomething]; 
}];

This prevents a direct strong reference so we don't get into a retain cycle to self. And also, it prevents self from becoming nil half-way, but still properly decrements the retain count. More in this link: http://aceontech.com/objc/ios/2014/01/10/weakify-a-more-elegant-solution-to-weakself.html

Answer 8

How about this?

- (void) foo {
     __weak __block me = self;

     myBlock = ^ {
        [[me someProp] someMessage];
     }
     ...
 }

I don't get the the compiler warning anymore.

Answer 9

Block: a retain cycle will occur because it contains a block which is referenced in the block; If you make the block copy and use a member variable,self will retain.