obj-c weak self in a block: why the 2nd one doesn't need a weak self inside in two similar cases

问题

I finally found my memory bug is caused by referring self strongly in a block. But I don't know why in a similar case, the weak is not needed:

I have a CameraCaptureManager class doing image capture tasks, and a CameraViewController has a strong property of this manager. The manager has weak delegate property pointing back to the controller.

This is where I must use weakSelf in the manager, otherwise -(void)dealloc won't be called:

    // in CameraCaptureManager
    __weak CameraCaptureManager *weakSelf = self;
    void (^deviceOrientationDidChangeBlock)(NSNotification *) = ^(NSNotification *notification) {
        UIDeviceOrientation deviceOrientation = [[UIDevice currentDevice] orientation];
        [weakSelf updateVideoOrientation:deviceOrientation];
    };
    self.deviceOrientationDidChangeObserver = [notificationCenter addObserverForName:UIDeviceOrientationDidChangeNotification
                                                                              object:nil
                                                                               queue:nil
                                                                          usingBlock:deviceOrientationDidChangeBlock];  

The manager holds the deviceOrientationDidChangeObserver strongly, so weakSelf is needed to break the memory retain cycle. That's fine, I got that... but I find I don't have use weakSelf in a similar case in the same class:

[self.stillImageOutput captureStillImageAsynchronouslyFromConnection:captureConnection
                                                   completionHandler:^(CMSampleBufferRef imageDataSampleBuffer, NSError *error){

                                                       UIImage *image = nil;

                                                       if (imageDataSampleBuffer != NULL) {
                                                           NSData *imageData = [AVCaptureStillImageOutput jpegStillImageNSDataRepresentation:imageDataSampleBuffer]; 
                                                           image = [[UIImage alloc] initWithData:imageData];
                                                       } 

                                                       if ([self.delegate respondsToSelector:@selector(captureManager:capturedStillImage:)]) {
                                                           [self.delegate captureManager:weakSelf capturedStillImage:image];
                                                       }
                                                   }]; 

The manager also holds the stillImageOutput strongly, but why I can use the strong "self" in the completion block? The manager object gets dealloc with this strong self inside the block. I'm confused, please shed some light.

Also do I need to use weakSelf in the 2nd case even when it won't cause any retain cycle?

回答1:

In your second code example you have a temporary retain cycle. When the completionHandler block has been called, the block is released and with it the captured self, so that the release cycle is broken.

来源：https://stackoverflow.com/questions/17910753/obj-c-weak-self-in-a-block-why-the-2nd-one-doesnt-need-a-weak-self-inside-in-t