ARC: Getting EXC_BAD_ACCESS from inside block used

问题:

I must be doing something wrong, but the Automatic Reference Counting docs don't give me a hint on what it might be. What I'm doing is calling a method with a block callback from inside a delegate method. Accessing that same delegate from inside the block results in a bad access. The problem is the object I'm passing - loginController which is sending the message to its delegate - is clearly not released, when I don't access it inside the block I can call the method multiple times without an issue. Here's my code:

- (void)loginViewDidSubmit:(MyLoginViewController *)loginController
{
    NSString *user = loginController.usernameLabel.text;
    NSString *pass = loginController.passwordLabel.text;

    __block MyLoginViewController *theController = loginController;
    [self loginUser:user withPassword:pass callback:^(NSString *errorMessage) {
        DLog(@"error: %@", errorMessage);
        DLog(@"View Controller: %@", theController);    // omit this: all good
        theController = nil;
    }];
}

NSZombieEnabled does not log anything and there is no usable stack trace from gdb. What am I doing wrong here? Thanks for any pointers!

---

Edit:

I figured the problem has a bigger scope - the callback above is called from an NSURLConnectionDelegate method (the block itself is a strong property for that delegate so ARC should call Block_copy()). Do I need to take special measurements in this scenario?

Flow (the loginController stays visible all the time):

loginController

[delegate loginViewDidSubmit:self];

View Delegate

(method shown above calls the loginUser: method, which does something like:)
httpDelegate.currentCallback = callback;
httpDelegate.currentConnection = // linebreak for readability
    [[NSURLConnection alloc] initWithRequest:req
                                    delegate:httpDelegate
                            startImmediately:YES];

NSURLConnectionDelegate

- (void)connection:(NSURLConnection *)aConnection
  didFailWithError:(NSError *)error
{
    if (NULL != currentCallback) {
        currentCallback([error localizedDescription]);
        self.currentCallback = NULL;
    }
}

And this is where I get the bad access, but ONLY if I access that loginController variable...

回答1:

Set copy attribute to the property, or just call 'copy' method for the block.

- (void)loginUser:(NSString *)user withPassword:(NSString *)pass callback:(void (^callback)(NSString *))
{
    callback = [callback copy];

回答2:

The actual solution was that I had the block as a strong property, but it should have been a copy property! D'oh!

---

First "Solution":

I just found a way to prevent the bad access. As shown in my Edit above, the View Delegate forwards the block to the httpDelegate (an instance of another class), which in turn keeps a strong reference to the block. Assigning the block to a temporary variable and forwarding the temporary block variable solves the problem, for whatever reason. So:

This crashes on block execution, as described

httpDelegate.currentCallback = callback;

This works

MyCallbackType aCallback = callback;
httpDelegate.currentCallback = aCallback;

I'll accept this as the answer, if anybody has more insights I'm happy to revise my decision. :)

回答3:

I figure what is happening there is that the loginController is dead right after calling its delegate. Therefore a crash occurs. Without more information I can think of possible scenarios only:

1. The block do not retains the loginController object (__block type modifier). If the block is executed asynchronously, the loginController might no longer be available if it was killed elsewere. Therefore, no matter what you want to do with it, you wont be able to access it inside the block and the app will crash. This could happen if the controller is killed after sending loginViewDidSubmit.

2. I think most likely this could be your situation: The loginController calls its delegate object. The delegate method ends up synchronously invoking the callback block that kills the controller. The controller is expected to be alive after invoking the delegate method. Killing it inside the delegate method, most likely will cause crashes to happen. To make sure this is the problem, simply nil the loginController in the delegate method and put an NSLog statement in the controller after calling the delegate, never mind the block, you will get a crash there.

Perhaps if you paste some code we could help more.

My best.