CAShapeLayer path animation has serious delay

Question

I'm attempting to create a loading screen that draws a circle while my app makes network requests. The amount drawn of the circle is meant to represent how close the request is to finishing. However there is a serious delay (~8 seconds) between the network request and the animation. After A LOT of searching I haven't found anyone that has had this problem before so I'm very desperate.

My setup right now is that an NSProgress object will be updated as a request is being made and will trigger a KVO Notification with the NSProgress object in the userInfo. This is the same method found here.

#Client.m
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    if ([keyPath isEqualToString:@"fractionCompleted"] && [object isKindOfClass:[NSProgress class]]) {
        NSProgress *progress = (NSProgress *)object;
        NSDictionary *userInfo = @{@"progress":progress};
        [[NSNotificationCenter defaultCenter] postNotificationName:@"FractionCompleted" object:self userInfo:userInfo];
    }
}

Then the view controller that is listening for the notification will update the LoadingProgressView with the fractionCompleted of the NSProgress object that it receives.

#MainViewController.m
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(updateProgress:) name:@"FractionCompleted" object:nil];
...
...
- (void)updateProgress:(NSNotification*)note {
    NSProgress* prog = note.userInfo[@"progress"];
    [self.progressIndicatorView updateProgress:(CGFloat)prog.fractionCompleted];
}

Now within the LoadingProgressView, the CAShapeLayer's strokeEnd property is set as the fractionCompleted value. I am using the same idea from the tutorial found here.

#LoadingProgressView.m
- (void)updateProgress:(CGFloat)frac {
    _circlePathLayer.strokeEnd = frac;
}

When I actually make a request, nothing happens until about 5 seconds AFTER the request is finished. At that point the entire circle is animated at once.

I have ABSOLUTELY NO IDEA why this is happening and it's driving me crazy. I can clearly see using the debugger that the strokeEnd property is being updated in real-time and yet the LoadingProgressView refuses to re-render until much later. Any help is very much appreciated. Thanks.

EDIT: OK so a temporary solution was to fork a new thread with a delay of 0 to update the progress view for each notification. However this seems like poor thread management since I could be creating over a hundred different threads to do the same task. I'm wondering if there's anything else I can do.

- (void)updateProgress:(NSNotification*)note {
    NSProgress* prog = note.userInfo[@"progress"];
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        [self.progressIndicatorView updateProgress:(CGFloat)prog.fractionCompleted];
    });
}

Answer 1

I ran into this same problem. It seems to be caused by setting the path from a background thread. Ensuring this happens on the main thread solved the problem. Now updating the shape draws immediately.