How to restore animation when UIView reappears? - ios

Question

I have a custom UIView subclass (ValveStatusView). It is a subview placed in a custom 'UITableViewCellsubclass (ValveCell) . Upon receivingawakeFromNibinValveStatusView`, I set up a number of sublayers, one of which is animated. The animation setup code looks like this:
CABasicAnimation *animation = [CABasicAnimation animationWithKeyPath:#"position"];
animation.toValue = [NSValue valueWithCGPoint:CGPointMake(32, 64 + 6)];
animation.duration = 0.75;
animation.repeatCount = HUGE_VALF;
animation.timingFunction = [CAMediaTimingFunction functionWithName: kCAMediaTimingFunctionEaseIn];
[drop addAnimation:animation forKey:#"position"];
It makes a "drop" image repeatedly fall from a faucet, forever. These cells are embedded in a UITableView and then in a tabbed view. When I navigate to the other tab, and then back, my animation isn't running anymore. I presume when the view stack containing my view's is no longer mapped to the window, it shuts down the animation. Is that a correct assumption? Assuming it is, what's the idiomatic way to get it running again when the view is mapped again?
I looked through the UIView methods, but nothing jumped out as the obvious "hook" to get it running again. I admit to skimming through them, not exhaustively reading it all.

Answer 1

You should have mentioned in your question that the view containing the animated layer is a subclass of UITableViewCell. :) Because, that is actually the cause of your animation being stopped. UITableViewCell removes the animations from its sublayers to avoid unnecessary computations when a cell is removed from tableview for later reuse. To solve your problem you can override the prepareForReuse method of your cell and restart your animation there. So you should do something like this:
- (id)initWithStyle:(UITableViewCellStyle)style reuseIdentifier:(NSString *)reuseIdentifier
{
self = [super initWithStyle:style reuseIdentifier:reuseIdentifier];
if (self) {
// Initialization code
// some initial setup here...
[self addAnimation];
}
return self;
}
- (void)prepareForReuse {
[super prepareForReuse];
[self addAnimation];
}
- (void)addAnimation {
// remove previous animations
[drop removeAllAnimations];
CABasicAnimation *animation = [CABasicAnimation animationWithKeyPath:#"position"];
animation.toValue = [NSValue valueWithCGPoint:CGPointMake(32, 64 + 6)];
animation.duration = 0.75;
animation.repeatCount = HUGE_VALF;
animation.timingFunction = [CAMediaTimingFunction functionWithName: kCAMediaTimingFunctionEaseIn];
[drop addAnimation:animation forKey:#"position"];
}
Hope this helps.

Answer 2

Try putting your code in - (void) viewWillAppear: method instead of awakeFromNib, this way it will be triggered whenever the view is shown.
EDIT:
OK, I see now, you can do that by using NSNotificationCenter:
In the awakeFromNib of your customCell add an observer:
- (void) awakeFromNib
{
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(startAnimation:)
name:#"addAnimation"
object:nil];
}
in the startAnimation: method you will put of course your animation code.
And in the viewWillAppear of your UITableViewController you post the notification to restart the animations.
[[NSNotificationCenter defaultCenter]
postNotificationName:#"addAnimation"
object:self];
Somewhere in the CustomCell you must remove the observer, or the cell will never be unloaded.

Answer 3

I worded the original question poorly apparently. I've tried with edits to clarify the situation. Regardless of UITableViewCell containment or not, my desire was to find a solution that was indifferent to that. Encapsulation and all that.
The following code, added to ValveStatusView (my subclass of UIView).
- (void) didMoveToWindow {
if (self.window != nil) {
[self startDropAnimation];
}
}
does the trick. startDropAnimation is inspired by the refactoring suggested by #Davlat above:
- (void)startDropAnimation {
CALayer* drop = self.activeFaucet.sublayers.firstObject;
[drop removeAllAnimations];
CABasicAnimation *animation = [CABasicAnimation animationWithKeyPath:#"position"];
animation.toValue = [NSValue valueWithCGPoint:CGPointMake(32, 64 + 6)];
animation.duration = 0.75;
animation.repeatCount = HUGE_VALF;
animation.timingFunction = [CAMediaTimingFunction functionWithName: kCAMediaTimingFunctionEaseIn];
[drop addAnimation:animation forKey:#"position"];
}
didMoveToWindow was the hook I was looking for (I think).