How to crossfade between 2 images on iPhone using

问题:

I'm doing this to learn how to work with Core Animation animatable properties on iPhone (not to learn how to crossfade images, per se).

Reading similar questions on SO leads me to believe it can be done by animating the .contents property of the UIImageView's layer like so:

UIImage *image1 = [UIImage imageNamed:@"someImage1.png"];
UIImage *image2 = [UIImage imageNamed:@"someImage2.png"];

self.imageView.image = image1;
[self.view addSubview:self.imageView];

CABasicAnimation *crossFade = [CABasicAnimation animationWithKeyPath:@"contents"];
crossFade.duration = 5.0;
self.imageView.layer.contents = image2;
[self.imageView.layer addAnimation:crossFade forKey:@"animateContents"];

Did I get a detail wrong or is this not possible.

Update: the above code produces a blank UIImageView. When I change this line:

self.imageView.layer.contents = image2.CGImage;

...I can see the image now but it does not fade in, it just appears instantly.

回答1:

You were almost there.

CABasicAnimation *crossFade = [CABasicAnimation animationWithKeyPath:@"contents"];
crossFade.duration = 5.0;
crossFade.fromValue = image1.CGImage;
crossFade.toValue = image2.CGImage;
[self.imageView.layer addAnimation:crossFade forKey:@"animateContents"];

Note that the animation is independent of the actual values/contents of the UIImageView. Therefore you'll need to

self.imageView.image = image2;

... to set the final result for your image.

回答2:

I found this somewhere - it works great.

Swift

UIView.transition(with: imageView, duration: 0.33, options: .transitionCrossDissolve, animations: {
      imageView.image = image
}, completion: nil)

Objc

UIImage * toImage = [UIImage imageNamed:@"image.png"];
[UIView transitionWithView:self.view
                  duration:0.33f
                   options:UIViewAnimationOptionTransitionCrossDissolve
                animations:^{
                    self.imageview.image = toImage;
                } completion:NULL];

回答3:

extension UIImageView
{
    func mySetImage(image:UIImage)
    {
        guard let currentImage = self.image where currentImage != image else { return }
        let animation = CATransition()
        animation.duration = 0.3
        animation.type = kCATransitionFade
        layer.add(animation, forKey: "ImageFade")
        self.image = image
    }
}

回答4:

- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view from its nib.
self.navigationController.navigationBarHidden=false;
UIImage *img=[UIImage imageNamed:@"images.jpeg"];
imgview=[[UIImageView alloc]init];
imgview.frame=CGRectMake(30,90, img.size.width, img.size.height);
[self.view addSubview:imgview];
[self animateImages];
}
- (void)animateImages
{
static int count = 0;
NSArray *animationImages = @[[UIImage imageNamed:@"images.jpeg"], [UIImage imageNamed:@"images (1).jpeg"]];
UIImage *image = [animationImages objectAtIndex:(count % [animationImages count])];

[UIView transitionWithView:imgview
                  duration:2.0f // animation duration
                   options:UIViewAnimationOptionTransitionCrossDissolve
                animations:^{
                    imgview.image = image;
                } completion:^(BOOL finished) {
                    [self animateImages];
                    count++;
                }];

}

回答5:

Solution in swift

let image1:UIImage = UIImage(named: "someImage1")!;
let image2:UIImage = UIImage(named: "someImage2")!;
let crossFade:CABasicAnimation = CABasicAnimation(keyPath: "contents");
crossFade.duration = 5.0;
crossFade.fromValue = image1.CGImage;
crossFade.toValue = image2.CGImage;
imageView.layer.addAnimation(crossFade, forKey:"animateContents");

回答6:

My suggestion would be to just use two UIImageViews on top of each other. The one on top has the image1, and on bottom is image2:

- (void) crossfade {
    [UIView beginAnimations:nil context:nil];
    [UIView setAnimationDuration:.5];
    imageView1.alpha = !imageView1.alpha;
    [UIView commitAnimations];
}

That will fade between the two images whenever you call it. If you want to just do a single fade and remove the first image after you're done:

- (void) crossfade {
    [UIView beginAnimations:nil context:nil];
    [UIView setAnimationDuration:.5];
    [UIView setAnimationDelegate:imageView1];
    [UIView setAnimationDidStopSelector:@selector(removeFromSuperview)];
    imageView1.alpha = 0
    [UIView commitAnimations];
}

Edit:

If you're looking for a reason why your code doesn't work, it's because you're just setting the property to the new image, then adding a useless animation. CABasicAnimation has the fromValue and toValue that need to be set, then add that animation to the layer, and it should do the animation. Don't set the contents manually.