Problems animating UIView alpha

问题:

I'm trying to apply a fade to an UIView I created programmatically on the top of another.

[UIView animateWithDuration:0.5 animations:^(void) {
    [self.view setAlpha:0];
}
                 completion:^(BOOL finished){
                     [self.view removeFromSuperview];
                 }];

The finished event is called properly after exactly 0.5 seconds, but I don't see any fade (I should see the UIView on the bottom).

If instead of using the alpha, I move away the UIView it works (I see the bottom UIView while the top UIView slides away), so it seems to be a problem related to alpha, but I can't figure out what's wrong!

[UIView animateWithDuration:0.5 animations:^(void) {
    CGRect o = self.view.frame;
    o.origin.x = 320;
    self.view.frame = o;
}
                 completion:^(BOOL finished){
                     [self.view removeFromSuperview];
                 }];

I used alpha animations previously and they works in this way usually...

回答1:

Try setting opaque to NO explicitly. I had the same problem and setting that solved my problem. Apparently, opaque views just don't seem to play well with alpha.

Credit goes to Hampus Nilsson's comment though.

回答2:

[UIView animateWithDuration:0.6 delay:0 options:UIViewAnimationOptionCurveEaseInOut 
animations:^{
    [self.view setAlpha:0];
} 
completion:^(BOOL finished) {
    [self.view removeFromSuperview];
}];

This will work right, and your fading will be nicer, because of options:UIViewAnimationOptionCurveEaseInOut.

回答3:

I ran into the exact problem. In my case, I wanted the view to be hidden at first, so I set hidden to true. I thought changing the alpha value changes hidden property automatically, but that wasn't the case.

So, if you want the view to be hidden at first, set its alpha to 0.

回答4:

I had exactly the same problem, and none of the suggestions worked for me. I overcame the problem by using the layer opacity instead. This is the code for showing the layer (using Xamarin, but you'll get the idea):

        //Enter the view fading
        zoomView.Layer.Opacity = 0;

        UIApplication.SharedApplication.KeyWindow.RootViewController.View.Add(zoomView);

        UIView.AnimateNotify(
            0.15, // duration
            () => { zoomView.Layer.Opacity = 1.0f },
            (finished) => { }
        );

And this is for fading out the same zoomView

UIView.AnimateNotify(
                0.15, // duration
                () => { zoomView.Layer.Opacity = 0; },
                (finished) =>
                {
                    if (finished)
                    {
                        zoomView.RemoveFromSuperview();
                    }
                }
            );

回答5:

I experience same issue in my case its because of thread So i end up with write animation block in main thread.

dispatch_async(dispatch_get_main_queue(), ^{        
    [UIView animateWithDuration:2 delay:0 options:UIViewAnimationOptionLayoutSubviews animations:^{
        View.alpha = 0.0f;
    } completion:^(BOOL finished) {
    }];
});

回答6:

One more piece of the puzzle. When you're animating constraints, you can set the new constraint constants outside of an animation block and then call layoutIfNeeded inside the animation.

With view alphas, these need to be set directly inside the animation block.

Change this:

//setup position changes
myConstraint.constant = 10
myOtherConstraint.constant = 10
//whoops, alpha animates immediately
view.alpha = 0.5

UIView.animate(withDuration: 0.25) {
   //views positions are animating, but alpha is not   
   self.view.layoutIfNeeded()
}

to

//setup position changes
myConstraint.constant = 10
myOtherConstraint.constant = 10

UIView.animate(withDuration: 0.25) {
   view.alpha = 0.5
   self.view.layoutIfNeeded()
}