CODEWITHSUNDEEP
iphoneiosuiscrollviewquartz-graphicsquartz-2d
Imran
Imran

Reputation: 1498

UIScrollView pinch zoom into CGContextStrokePath without changing line width

I have a UIScrollView that contains a custom UIView. In my UIView I am overriding drawRect to draw a path using CGContextStrokePath. I would like to slightly alter the way the zoom works. Pinch zooming out will show more of the paths on the screen. This is what I want but i want the line width to stay the same not shrink as you zoom out so that they are still clear to the user.

I thought I would just do this (myUIView zoom target is called _lineView)

-(void)scrollViewDidZoom:(UIScrollView *)pScrollView
    {
      _lineView.zoomScale = _scrollView.zoomScale;
      [_lineView setNeedsDisplay];
    }

and then just calculate an appropriate line stroke size in my _lineView drawRect method to give the effect of constant line width as you zoom out.

This is really slow and I have read that this is expected as drawrect is not optimised to be called many times a second.

I then started looking at using a GLKView instead and just rendering the whole thing in opengl directly. The problem with this is I will have to implement all of the zooming and panning myself (with all the lovely zoom and pan bounce effects you get for free in UiScrollView). I will also have to implement all the controls I want to use in opengl, buttons etc.

Is there a way to do this whilst still using Quartz2d? I feel like opengl will give me lots of power but it will take me much longer to get the rest of my app done if I go down that route.

Upvotes: 2

Views: 1302

Answers (2)

Imran
Imran

Reputation: 1498

I figured this out. I found a simple way to do this that does not slow down the zoom / bouncing animations at all and is very fast and fluid.

Quartz has a class called CAShapeLayer that lets you do some pretty cool stuff. Among these is being able to set a CGPath property and specify a linewidth. Changes are reflected in the view.

So i basically call shapeLayer.lineWidth from my scrollViewDidZoom method and it does exactly what i need.

Upvotes: 3

fzwo
fzwo

Reputation: 9902

What I would do if I were you is to use default zooming behavior while zooming (which does not redraw, but instead just applies a transform to the zoomed view, which can be done by the GPU very, very quickly, but as you have noticed can lead to inferior quality).

Then, when the user finished zooming, redraw the whole view as you do now. The appropriate delegate method is scrollViewDidEndZooming:withView:atScale:.

This way, you have fast (but slightly ugly) zooming, and nice (but slightly, probably unnoticeably, slower) display after the zoom is finished.

Upvotes: 1

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