Stuttering animation whilst using CADisplayLink

Question

I am trying to animate some UIImageViews to 'fall' down the page using CADisplayLink to call a MoveObjects function. However, despite the function calls being at consistent 0.0155-0.017s intervals, the animation appears to stutter every few frames, in the same way as it did when I tried using a NSTimer to call the function. Is there anything that i'm doing wrong, which is making it stutter?

in viewDidLoad:

displayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(gameLoop)];
displayLink.frameInterval=1;
[displayLink addToRunLoop:[NSRunLoop mainRunLoop] forMode:NSDefaultRunLoopMode];

currently gameloop pnly calls UpdateObjects:

-(void)updateObjects{
    timeStamp=[displayLink timestamp];
    double frameTime=timeStamp-oldTime;
    oldTime= timeStamp;
    if (frameTime>10){
        frameTime=0;
    }
    NSLog(@"%f",frameTime);

    for (int i=0; i<objectsArray.count; i+=1) {
        UIImageView *currentObject=[objectsArray objectAtIndex:i];
        CGRect currentObjectFrame=[currentObject frame];
        [currentObject setFrame:CGRectMake(CGRectGetMinX(currentObjectFrame), CGRectGetMinY(currentObjectFrame)+200*frameTime,CGRectGetWidth(currentObjectFrame) , CGRectGetHeight(currentObjectFrame))];
    }
}

Any reason for not using a `CAAnimation` or `UIKitDynamics` for the fall movement? — David Rönnqvist, Feb 27 '14 at 20:46

Rob · Accepted Answer · 2014-03-08T03:53:54.193

A couple of thoughts:

I'm surprised that you're seeing stuttering, that you're really not seeing any deviation from the 0.0155-0.017s intervals. I'd double check that. I'd expect that for each stutter, you'd see a sub-optimal interval.
When scheduling something on a run loop, you might consider using NSRunLoopCommonModes, which is a little more expansive than than NSDefaultRunLoopMode. Depending upon what else you have going on, this can affect the performance.
I don't believe it's a problem here, but you should be wary about including integer expressions (e.g. 200, rather than 200.0f) when calculating floating point values, because sometimes the implicit casting can cause weird rounding issues, which if calculating screen coordinates for a manual animation can result in strange discontinuities in the animation.
In terms of general image animation issues, any of the following can cause suboptimal response times: using images with transparency values; images that need to be resized to display in the image view; using any layer effects, notably bezier-based shadows; excessive numbers of images; etc.
Obviously, I hope you're testing on physical device and not simulator, as simulator does not accurately reflect actual device graphical performance.

Nonetheless, I have a hard time reconciling the fact that you're seeing a stuttering with the claim that you're never seeing the interval greater than 0.017 seconds. If that was the case, that would lead me to suspect some rounding related issue (introducing some discontinuity in the CGRect values frame to frame), but I don't see any such issue here.

Thanks for the advice, it was very useful. The intervals called (intervals of 'moveObjects' being called) i've found do go up to 0.0176s at a very max, but the answer is probably in no.5 (sorry). I don't think that I have the means to run it on a Physical device yet, so I'll mark this answer as answered, and I'll just have to hope that this is a simulator issue. Thanks again, five excellent points. — user3361405, Feb 27 '14 at 19:40
It's not a solution but the point number 3 helped me improve several other aspects in my project. — Blazej SLEBODA, Mar 30 '16 at 16:00

Stuttering animation whilst using CADisplayLink

1 Answers1