On Mar 2, 2011, at 5:12 AM, Roland King wrote:
> I was trying to animate the backgroundColor of one of my UIView subclasses
> yesterday using
>
> [ UIView animateWithDuration:2.0 animations:^{ [ myView
> setBackgroundColor:someColor ]; } ];
>
> but no matter what the duration set, the background color was changing
> instantly. Whilst looking to see if there were two places I called
> setBackgroundColor: (there weren't) I realized that my UIView subclass has
> its own drawRect: method (this view is basically a piece of paper with some
> lines drawn on it, so it needs a custom drawRect: method). Commenting that
> out, the view background animated properly, but of course I had no content.
> Even just adding an empty drawRect: method was enough to defeat the
> animation.
When you implement -drawRect:, the background color of your view is then drawn
into the associated CALayer, rather than just being set on the CALayer as a
style property. Now Core Animation can animate both transitions, but UIView
disables the animation of a layer's contents, which thus prevents you from
getting a contents crossfade (as in the vast majority of cases this wouldn't be
what you want).
The crossfade may be exactly what you want in this case. Likely the best way to
defeat UIView's override in this case may be to create a subclass of CALayer
that overrides -actionForKey:. In that override, you would check for the
@"contents" key and return a CABasicAnimation ([CABasicAnimation animation]
should do it) and in all other cases return what the default implementation
does. This has the downside that you will *always* cross fade and you won't be
able to integrate with UIKit's animation APIs (you just get animation for
contents on all the time).
Another solution would be to use a super view that contains just the background
color and have your lines composited over it (assuming they are simple lines
this could be done with simple subviews that are sized for each line). This
would allow you to avoid -drawRect: entirely, which would allow you to animate
the background color and may offer some memory usage benefits (if you can use
subviews).
--
David Duncan
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