未调用完成块。 如何检查有效性？

Question

I have this method which takes a block, but that block isn't always called. See the method:

- (void)updateWithCompletion:(void (^)(void))completion {
    [MYObject myMethodWithCompletion:^(NSArray *array, NSError *error) {
        if (error) {
            NSLog(@"%s, ERROR not nil", __FUNCTION__);
            completion();
            return;
        }

        NSLog(@"%s, calling completion %d", __FUNCTION__, &completion);
        completion();
        NSLog(@"%s, finished completion", __FUNCTION__);
    }];
}

I have some more NSLogs inside completion. Sometimes this program counter just blows right past the call to completion() in the code above. I don't see why this would be as the calling code always passes a literal block of code as input.

If you're curious of the output of the line containing the addressof operator, it's always something different, but never 0 or nil.

What would cause completion not to be executed?

Answer 1

It turns out that completion was pointing to a different code block than I was expecting. updateWithCompletion was called from viewWillAppear with an empty code block. This is why I wasn't seeing the NSLog statements. Once I saw that I had an empty code block it was obvious how to fix the problem.

Lessons learned: If a code block is nil, the program will crash. This wasn't happening, so that meant that it was calling something other than what I was expecting.

Hope this helps someone.

Answer 2

What do you mean, the program counter blows past the block? Completion blocks are nearly always executed asynchronously. This means that, yes, the program counter will jump from just before the call to -myMethodWithCompletion: directly to the end of the method where it returns back to the caller. And then at some point in the future, the completion block will be called.