使用KVO跟踪NSProgress

Question

I am using KVO to track the progress of a file that is being received:

-(void)session:(MCSession *)session didStartReceivingResourceWithName:(NSString *)resourceName fromPeer:(MCPeerID *)peerID withProgress:(NSProgress *)progress

{
NSLog(@"RECEIVING... %@ from peer: %@", progress, peerID);
dispatch_sync(dispatch_get_main_queue(), ^{
    [progress addObserver:self
                forKeyPath:@"fractionCompleted"
                   options:NSKeyValueObservingOptionNew
                   context:NULL];
});}

and

- (void)observeValueForKeyPath:(NSString *)keyPath
                  ofObject:(id)object
                    change:(NSDictionary *)change
                   context:(void *)context
{
if (object == progress) {
    // Handle new fractionCompleted value
[progressBar setProgress:progress.fractionCompleted animated:YES];
    NSLog(@"Fraction Complete: %@", [NSNumber numberWithDouble:progress.fractionCompleted]);
    return;
}

[super observeValueForKeyPath:keyPath
                     ofObject:object
                       change:change
                      context:context];
}

I would like to use this to update a UIprogressView... But the code is crashing at this line and I don't get why:

[super observeValueForKeyPath:keyPath
                     ofObject:object
                       change:change
                      context:context];

EDIT:

This is the error

*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason:     '<ViewController1: 0x15b57000>: An -observeValueForKeyPath:ofObject:change:context: message was     received but not handled.

EDIT:

If I remove the super observeValueForKeyPath:keyPath..., the app doesn't crash, but the NSLog(@"Fraction Complete: %@ always reports the value "0".

Answer 1

Does your superclass have observeValueForKeyPath:ofObject:change:context: implemented? You can do an if check on that:

if ([super respondsToSelector:@selector(observeValueForKeyPath:ofObject:change:context:){
    NSLog(@"Yeah, I'm here matey!");
} else {
    NSLog(@"Uh oh said the selector ghost");
}