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.
My assumption is that if there is no custom drawing, the view can smoothly
animate its layer between the two colors, and animate any other simple
properties, but if there is custom drawing, it doesn't want to call that
constantly as it animates the color change, so animation doesn't happen.
Is there anything I missed which will make this work,? I was thinking that the
other transition animation methods ( [ UIView transitionWithView: .. ] ) take a
'before' and 'after' snapshot and then, if you are flipping, animate one away,
and one in. I perhaps expected that UIView to do basically that, and crossfade
the old view out and the new one in, but I see no option for that, just flips
and curls, I want fade. This would be more generally useful than just this
example.
Obviously I have a few other options including building an entirely new UIView
on a color change, adding them both and fading their alphas in and out, or
splitting my 'paper' into a background view and an otherwise-clear lines view.
That may well be the right way to go with this, however I was interested to
know if there was some part of UIView animation which would automatically
handle a smooth transition of one single UIView between two drawn states.
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