Question

Programming for iOS, I have a composite custom view consisting of many UIViews. Some UIViews in this composites are responsible for drawing shadow and others for some custom shading. The shadow and shading need to be redrawn upon rotation recognized by UIRotationGestureRecognizer. However the speed of the rotation is far from satisfactory. When I commented out setNeedDisplay, the rotational speed is fine. However, if I do call setNeedDisplay, even when I commented out everything in all drawRects for the shadow and shading views, the rotation still lags significantly.

Are there any recommendations to speed things up?

I can think of one possible solution: make sure the system calls drawRect less often while in rotation. But I do not know how to do this, nor do I know if this is the best solution. Any suggestion appreciated. Thanks.

Answer 1

Calling setNeedsDisplay: too often, especially every frame will always be slow. setNeedsDisplay runs on the CPU, not the GPU. Don’t redraw views during rotation and zooming. Wait until the end of the animation, then call setNeedsDisplay: to “render” the final position.

Take a look at how various UIKit views handle large animations:

• While MapKit zooms in, the map image scales and looks blurry. Once the zoom gesture stops it renders a new image at that scale. (In this case the image is downloaded from the internet, but it still illustrates the concept.)

• ZoomingPDF Sample code (see apple developer docs) shows how zooming on PDFs doesn’t render in realtime, but after the zooming finishes.

Hope this helps.