ios - Best way to transition between two images in a UIImageView

Question

I have implemented a pretty simple picture viewer that will allow the user to browse through a collection of images. They are loaded from the Internet, and displayed on the device through a UIImageView object. Something like this:

UIImage *image = [[UIImage alloc] initWithData:imageData];
[img setImage:image];

imageData is an instance of NSData that I use to load the contents of the image from an URL, and img is the UIImageView instance.

It all works well, but the new image replaces the one being displayed before without any transitions, and I was wondering if there is an easy way to do a good animation transition to improve the user experience.

Any idea how to do this? Code samples would be very appreciated.

Answer 1

I was just going through your post and had exactly the same requirement. The problem with all above solutions is, you will have to incorporate the logic of transition into your controller. In the sense the approach is not modular. Instead I wrote this subclass of UIImageView:

TransitionImageView.h file:

#import <UIKit/UIKit.h>


@interface TransitionImageView : UIImageView 
{
    UIImageView *mOriginalImageViewContainerView;
    UIImageView *mIntermediateTransitionView;
}
@property (nonatomic, retain) UIImageView *originalImageViewContainerView;
@property (nonatomic, retain) UIImageView *intermediateTransitionView;

#pragma mark -
#pragma mark Animation methods
-(void)setImage:(UIImage *)inNewImage withTransitionAnimation:(BOOL)inAnimation;

@end

TransitionImageView.m file:

#import "TransitionImageView.h"

#define TRANSITION_DURATION 1.0

@implementation TransitionImageView
@synthesize intermediateTransitionView = mIntermediateTransitionView;
@synthesize originalImageViewContainerView = mOriginalImageViewContainerView;

- (id)initWithFrame:(CGRect)frame {
    if ((self = [super initWithFrame:frame])) {
        // Initialization code
    }
    return self;
}

/*
// Only override drawRect: if you perform custom drawing.
// An empty implementation adversely affects performance during animation.
- (void)drawRect:(CGRect)rect {
    // Drawing code
}
*/

- (void)dealloc 
{
    [self setOriginalImageViewContainerView:nil];
    [self setIntermediateTransitionView:nil];
    [super dealloc];
}

#pragma mark -
#pragma mark Animation methods
-(void)setImage:(UIImage *)inNewImage withTransitionAnimation:(BOOL)inAnimation
{
    if (!inAnimation)
    {
        [self setImage:inNewImage];
    }
    else
    {
        // Create a transparent imageView which will display the transition image.
        CGRect rectForNewView = [self frame];
        rectForNewView.origin = CGPointZero;
        UIImageView *intermediateView = [[UIImageView alloc] initWithFrame:rectForNewView];
        [intermediateView setBackgroundColor:[UIColor clearColor]];
        [intermediateView setContentMode:[self contentMode]];
        [intermediateView setClipsToBounds:[self clipsToBounds]];
        [intermediateView setImage:inNewImage];

        // Create the image view which will contain original imageView's contents:
        UIImageView *originalView = [[UIImageView alloc] initWithFrame:rectForNewView];
        [originalView setBackgroundColor:[UIColor clearColor]];
        [originalView setContentMode:[self contentMode]];
        [originalView setClipsToBounds:[self clipsToBounds]];
        [originalView setImage:[self image]];

        // Remove image from the main imageView and add the originalView as subView to mainView:
        [self setImage:nil];
        [self addSubview:originalView];

        // Add the transparent imageView as subview whose dimensions are same as the view which holds it.
        [self addSubview:intermediateView];

        // Set alpha value to 0 initially:
        [intermediateView setAlpha:0.0];
        [originalView setAlpha:1.0];
        [self setIntermediateTransitionView:intermediateView];
        [self setOriginalImageViewContainerView:originalView];
        [intermediateView release];
        [originalView release];

        // Begin animations:
        [UIView beginAnimations:@"ImageViewTransitions" context:nil];
        [UIView setAnimationDuration:(double)TRANSITION_DURATION];
        [UIView setAnimationDelegate:self];
        [UIView setAnimationCurve:UIViewAnimationCurveEaseOut];
        [UIView setAnimationDidStopSelector:@selector(animationDidStop:finished:context:)];
        [[self intermediateTransitionView] setAlpha:1.0];
        [[self originalImageViewContainerView] setAlpha:0.0];
        [UIView commitAnimations];
    }
}

-(void)animationDidStop:(NSString *)animationID finished:(NSNumber *)finished context:(void *)context
{
    // Reset the alpha of the main imageView
    [self setAlpha:1.0];

    // Set the image to the main imageView:
    [self setImage:[[self intermediateTransitionView] image]];

    [[self intermediateTransitionView] removeFromSuperview];
    [self setIntermediateTransitionView:nil];

    [[self originalImageViewContainerView] removeFromSuperview];
    [self setOriginalImageViewContainerView:nil];
}

@end

You can even override the -setImage method of UIImageView and call my -setImage:withTransitionAnimation: method. If it is done like this make sure that you call [super setImage:] instead of [self setImage:] in the method -setImage:withTransitionAnimation: so that it wont end up in infinite recursive call!

-Raj