Sudden memory spike with CGContextFillRect - ios

Question

I see a sharp increase in memory usage (from 39 MB to 186 MB on iPad) with "CGContextFillRect" statement execution in my below code. Is there something wrong here.
My application eventually crashes.
PS: Surprisingly the memory spike is seen on 3rd and 4th gen iPads and not on 2nd Gen iPad.
- (id)initWithFrame:(CGRect)frame
{
self = [super initWithFrame:frame];
if (self) {
[self setBackgroundColor:[UIColor clearColor]];
}
return self;
}
- (id)initWithFrame:(CGRect)iFrame andHollowCircles:(NSArray *)iCircles {
self = [super initWithFrame:iFrame];
if (self) {
[self setBackgroundColor:[UIColor clearColor]];
self.circleViews = iCircles;
}
return self;
}
- (void)drawHollowPoint:(CGPoint)iHollowPoint withRadius:(NSNumber *)iRadius {
CGContextRef currentContext = UIGraphicsGetCurrentContext();
CGContextSetLineWidth(currentContext, self.circleRadius.floatValue);
[[UIColor whiteColor] setFill];
CGContextAddArc(currentContext, iHollowPoint.x, iHollowPoint.y, iRadius.floatValue, 0, M_PI * 2, YES);
CGContextFillPath(currentContext);
}
- (void)drawRect:(CGRect)rect {
CGContextRef currentContext = UIGraphicsGetCurrentContext();
CGContextSaveGState(currentContext);
CGRect aRect = [self.superview bounds];
[[UIColor whiteColor]setFill];
CGContextFillRect(currentContext, aRect);
CGContextSaveGState(currentContext);
[[UIColor blackColor]setFill];
CGContextFillRect(currentContext, aRect);
CGContextRestoreGState(currentContext);
for (MyCircleView *circleView in self.circleViews) {
[self drawHollowPoint:circleView.center withRadius:circleView.circleRadius];
}
CGContextTranslateCTM(currentContext, 0, self.bounds.size.height);
CGContextScaleCTM(currentContext, 1.0, -1.0);
CGContextSaveGState(currentContext);
}

Answer 1

This code doesn't quite make sense; I assume you've removed parts of it? You create a blank alpha mask and then throw it away.
If the above code is really what you're doing, you don't really need to draw anything. You could just create a 12MB memory area and fill it with repeating 1 0 0 0 (opaque black in ARGB) and then create an image off of that. But I assume you're actually doing more than that.
Likely you have this view configured with contentScaleFactor set to match the scale from UIScreen, and this view is very large. 3rd and 4th gen iPads have a Retina display, so the scale is 2, and the memory required to draw a view is 4x as large.
That said, you should only expect about 12MB to hold a full screen image (2048*1536*4). The fact that you're seeing 10x that suggests something more is going on, but I suspect that it's still related to perhaps drawing too many copies.
If possible, you can step the scale down to 1 to make retina and non-retina behave the same.
EDIT:
Your edited code is very different from your original code. There's no attempt to make an image in this code. I've tested it out as best I can, and I don't see any surprising memory spike. But there are several oddities:
You're not correctly balancing CGContextSaveGState with CGContextRestoreGState. That actually might cause a memory problem.
Why are you drawing the rect all in white and then all in black?
Your rect is [self.superview bounds]. That's in the wrong coordinate space. You should almost certainly mean [self bounds].
Why do you flip the context right before returning from drawRect and then save the context? This doesn't make sense at all.
I would assume your drawRect: would look like this:
- (void)drawRect:(CGRect)rect {
[[UIColor blackColor] setFill];
UIRectFill(rect); // You're only responsible for drawing the area given in `rect`
for (CircleView *circleView in self.circleViews) {
[self drawHollowPoint:circleView.center withRadius:circleView.circleRadius];
}
}