ypresto / WeakSelfTester

Sample objective-c project to explain why use-weakSelf-for-every-block pattern is dangerous.

WeakSelfTester

Sample objective-c project to explain why use-weakSelf-for-every-block pattern is dangerous.

weakSelf Pattern

In Objective-C ARC, block retains all variables referenced in its code. This sometimes causes "retain cycle" i.e. memory leak.

self.callbackBlock = ^(id obj){
  [self render:obj];
}

Above code will make retain cycle among self and callbackBlock, because they have strong references for each other.

To prevent this, weakSelf pattern is often used among many developpers.

__weak weakSelf = self
self.callbackBlock = ^(id obj){
  [weakSelf render:obj];
}

Above code will make strong reference from self to callbackBlock, but not to self.

Why weakSelf can be dangerous?

For example, imagine model implementation which fetches data structure from RESTful API. It perhaps accepts block, uses async fetch methods of HTTP client and returns immediately. Uses weakSelf to call parseResponse: method.

// in FoobarManager

- (void)fetchAsyncWeakWithBlock:(void (^)(NSArray *foobars))callback {
    __weak typeof(self) weakSelf = self;

    // mimicking async fetch from server
    dispatch_async(dispatch_get_main_queue(), ^{
        __strong typeof(weakSelf) strongSelf = weakSelf;
        NSArray *foobars = [strongSelf parseResponse:@{@"foo": @"bar"}];
        NSLog(@"weakSelf: %@", weakSelf);
        callback(foobars);
    });
}

Then, think about caller. You can throw away model instance after calling some methods. Instance of FoobarManager in below code will not be retained after getInBackgroundWithWeakSelf returns.

// in ViewController

- (void)getInBackgroundWithWeakSelf {
    [[FoobarManager new] fetchAsyncWeakWithBlock:^(NSArray *foobars) {
        [self renderFoobarsForWeakSelf:foobars];
    }];
}

Now, something bad has been happened... No one retaines FoobarManager instance, because FoobarManager uses weakSelf.

__strong typeof(weakSelf) strongSelf = weakSelf; // nil!!

http://stackoverflow.com/a/20032131/1474113

What should I do then?

TODO