Question

My UIView structure:
I have a "master" UIView (actually UIScrollView, but it’s only purpose scrolling per pagination).
As a subView on this "master" I have my "pageView" (subclass of UIScrollView). This UIScrollView can have any content (e.g. a UIImageView).
The "master" has another subView: PaintView (subclass of UIView). With this view, I track the finger movements and draw it.

The structure looks like this:

[master.view addSubview: pageView];
[master.view addSubview: paintView];

I know when the user zooms in (pageView is responsible for this) and over delegate/method calls I change the frame of paintView according to the zoom change, during the zoom action.
After zooming (scrollViewDidEndZooming:withView:atScale) I call a custom redraw method of paintView.

Redraw method and drawRect:

-(void)redrawForScale:(float)scale {
    for (UIBezierPath *path in _pathArray) {
        //TransformMakeScale...
    }
    [self setNeedsDisplay];
} 

-(void)drawRect:(CGRect)rect {
    for (UIBezierPath *path in _pathArray) {
        [path strokeWithBlendMode:kCGBlendModeNormal alpha:1.0];
    } 
}

The problem:
When I zoom-in, I receive a memory warning and the app crashes.

In the Allocations profiler I can see that the app own a lot of memory, but I can’t see why.
When I don’t call 'setNeedDisplay' after my 'redrawForScale' method the app isn’t crashing.

When I log the rect in drawRect I see values like this: {{0, 0.299316}, {4688, 6630}}.

Answer 1

The problem is (re)drawing such a huge CGRect (correct me if this is wrong).
The solution for me is to avoid calling a custom drawRect method. In some stackoverflow answers (and I think somewhere in the Appls docs) it was mentioned that a custom drawRect: will reduce the performance (because iOS can’t do some magic on a custom drawRect method).

My solution:
Using a CAShapeLayer. For every UIBezierPath I create a CAShapeLayer instance, add the UIBezierPath to the path attribute of the layer and add the layer as a sublayer to my view.

CAShapeLayer *shapeLayer = [[CAShapeLayer alloc] initWithLayer:self.layer];
shapeLayer.lineWidth = 10.0;
shapeLayer.strokeColor = [UIColor blackColor].CGColor;
shapeLayer.fillColor = [UIColor clearColor].CGColor;
shapeLayer.path = myBezierPath.CGPath;
[self.layer addSublayer:shapeLayer];

With this solution I don’t need to implement drawRect. So the App won’t crash, even with a big maximumZoomScale.

After changing the UIBezierPath (translate, scale, change color) I need to set the changed bezierPath to the shapeLayer again (set the path attribute of shapeLayer again).