UIBezierPath drawing takes up 100% of CPU

Question

I have a UIBezierPath is drawn on the screen every time the user's touch is moved. This works fine on my simulator on a Mac Pro, but as soon as I moved it to a physical device, the drawing began to lag a lot. Using instruments, I checked the CPU usage, and it hits 100% while the user is drawing.

This becomes quite the problem because I have an NSTimer that is supposed to fire at 30fps, but when the CPU is overloaded by the drawing, the timer fires at only around 10fps.

How can I optimize the drawing so that it doesn't take 100% of the CPU?

UITouch* lastTouch;

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    if(lastTouch != nil)
        return;

    lastTouch = [touches anyObject];
    [path moveToPoint:[lastTouch locationInView:self]];
    [self drawDot];
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    if([touches anyObject] == lastTouch)
        [self drawDot];
}

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
    if([touches anyObject] == lastTouch)
        lastTouch = nil;
}

- (void)drawDot
{
    if(!self.canDraw)
        return;

    [path addLineToPoint:[lastTouch locationInView:self]];
    [self setNeedsDisplayInRect:CGRectMake([lastTouch locationInView:self].x-30, [lastTouch locationInView:self].y-30, 60, 60)];
}

- (void)drawRect:(CGRect)rect
{    
    CGColorRef green = [UIColor green].CGColor;
    CGContextRef context = UIGraphicsGetCurrentContext();

    CGColorRef gray = [UIColor gray].CGColor;
    CGContextSetStrokeColor(context, CGColorGetComponents(gray));
    [shape stroke];

    CGContextSetStrokeColor(context, CGColorGetComponents(green));
    [path stroke];
}

How big (how many line segments) does the path have when it uses 100%? — Wain, Jan 14 '15 at 22:13

score 3 · Answer 1 · answered Jan 14 '15 at 22:54

You shouldn't really be using -drawRect in modern code - it's 30 years old and designed for very old hardware (25Mhz CPU with 16MB of RAM and no GPU) and has performance bottlenecks on modern hardware.

Instead you should be using Core Animation or OpenGL for all drawing. Core Animation can be very similar to drawRect.

Add this to your view's init method:

self.layer.contentsScale = [UIScreen mainScreen].scale;

And implement:

- (void)drawLayer:(CALayer *)layer inContext:(CGContextRef)ctx
{
   // basically the same code as you've got inDrawRect, although I recommend
   // trying Core Graphics (CGContextMoveToPoint(), CGContextAddLineToPoint(), 
   // CGContextStrokePath(), etc)
}

And to make it re-draw (inside touchesMoved/etc):

[self.layer setNeedsDisplay];

I would also update your touch event code to just append to an NSMutableArray of points (perhaps encoded in in an [NSValue valueWithCGPoint:location]). That way you aren't creating a graphics path while responding to touch events.

@David I think all drawing is on the GPU in iOS. Your `[path stroke]` uses Core Animation internally, but it also does other things in order to maintain backwards compatibility. — Abhi Beckert, Jan 15 '15 at 01:14
Basically, `drawRect` is perfectly fine if it only ever executes once or twice. You should never allow it to execute multiple times in quick succession. drawRect is fine when triggered as your view first appears on the screen or perhaps in response to a single tap event. It should not do any drawing in response to a touch move event. — Abhi Beckert, Jan 15 '15 at 01:14

score 3 · Answer 2 · answered Jan 15 '15 at 01:17

A good place to look for efficient drawRect: is the wwdc video from a few years ago: https://developer.apple.com/videos/wwdc/2012/?id=238 . about 26m in he starts debugging a very simple painting app which is very similar to what you've described. At minute 30 he starts optimizing after the first optimization of setNeedsDisplayInRect: (which you're already doing).

short story: he uses an image as a backing store for what's already drawn so that each drawRect: call he only draws 1 image + very short additional line segments instead of drawing extremely long paths every time.

UIBezierPath drawing takes up 100% of CPU

2 Answers2