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I'm working on an drawing app for iPhone. It works fine for like 5 seconds in the iPhone simulator but as more I draw it gets more laggy. When I test it on the device it gets even more laggy and I can't even draw a simple tree. When I check how high percent the processor is running at in xcode it usually go between 97-100%. Is there any way to fix this?

(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event { UITouch *touch = [touches anyObject]; currentPoint = [touch locationInView:self.view]; UIGraphicsBeginImageContext(CGSizeMake(320, 568)); [drawImage.image drawInRect:CGRectMake(0, 0, 320, 568)]; CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound); CGContextSetLineWidth(UIGraphicsGetCurrentContext(), 5.0); CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), 0, 1, 0, 1); CGContextBeginPath(UIGraphicsGetCurrentContext()); CGPathMoveToPoint(path, NULL, lastPoint.x, lastPoint.y); CGPathAddLineToPoint(path, NULL, currentPoint.x, currentPoint.y); CGContextAddPath(UIGraphicsGetCurrentContext(),path); CGContextStrokePath(UIGraphicsGetCurrentContext()); [drawImage setFrame:CGRectMake(0, 0, 320, 568)]; drawImage.image = UIGraphicsGetImageFromCurrentImageContext(); UIGraphicsEndImageContext(); lastPoint = currentPoint; [self.view addSubview:drawImage]; }
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    You should externalize your context (define it at a higher level, perhaps as an ivar of your class) and re-use it on every touch. You wouldn't need recreating it over and over again. Commented Feb 25, 2014 at 10:20
  • Also, avoid calling UIGraphicsGetCurrentContext() so frequently. Just call it once and assign the result: CGContextRef ctx = UIGraphicsGetCurrentContext(); Commented Feb 25, 2014 at 10:21
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    Also, here you're re-adding drawImage on each frame. That might be a big performance hit. Commented Feb 25, 2014 at 10:22
  • could you tell me more in detail. Im pretty new to objective C. Commented Feb 25, 2014 at 10:23
  • "Also you are re-adding draw image for each frame" Yeah thats what i thought too. Commented Feb 25, 2014 at 10:24

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When running instruments on your method, I got the following results:

enter image description here

What that tells me:

  • Setting up a new context every time you want to draw something is
    waste of time. consider only setting it up once, store it somewhere, and you save almost a third of the time, the method currently needs
  • drawImage is the other most-consuming part. It will be enough to set that only once!
  • all other calls are almost negligible
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2 Comments

+1 for your answer. Could you show me how to fix it?
As cyrille said, make a property for your context and reuse it (just as you do with your drawImage image view). And I´m not entirely sure why you need to call the "drawImage" method at all. That should happen automatically when you set the drawImage´s image. Also rethink whether you really have to add the drawImage to your viewHierarchy every time. Setting the image on it is enough for your UI to display the image!
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There's a great talk about graphics performance including a demo and code walkthrough of a drawing app here:

https://developer.apple.com/videos/wwdc/2012/

The iOS App Performance: Graphics and Animations video

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Thanks, though my codes is pretty different to the demo example.
Perhaps you can take some inspiration from the methods used by the Apple performance engineers.

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