Question

I have enabled my Cocoa Touch app to be navigable by swiping left or right to alter positions in history. The animation is kind of done like Android’s “card” style. Where swiping to the left (<–) just moves the current screen image out of the way, while showing the previous view beneath it.

This works fine, but when I want to swipe the other way (–>), to go back, I need to get the previous image, and move that over the current view. Now, I had this working if I only store the previous image, but what if I go <– a few times, then I will not have enough images.

So, the solution is obvious, use an NSMutableArray and just throw the latest image at the front of the array, and when you swipe the other way, just use the first image in the array. However, the image never shows when I start the animation. It just shows nothing. Here’s the required methods you should see:

-(void)animateBack {
    CGRect screenRect = [self.view bounds];

    UIImage *screen = [self captureScreen];

    [imageArray insertObject:screen atIndex:0]; //Insert the screenshot at the first index
    imgView = [[UIImageView alloc] initWithFrame:screenRect];
    imgView.image = screen;
    [imgView setHidden:NO];
    NSLog(@"Center of imgView is x = %f, y = %f", imgView.center.x, imgView.center.y);
    CGFloat startPointX = imgView.center.x;
    CGFloat width = screenRect.size.width;
    NSLog(@"Width = %f", width);
    imgView.center = CGPointMake(imgView.center.x, imgView.center.y);

    [self.view addSubview: imgView];
    [self navigateBackInHistory];

    [UIView animateWithDuration:.3 animations:^{
        isSwiping = 1;
        imgView.center = CGPointMake(startPointX - width, imgView.center.y);


    } completion:^(BOOL finished){
        // Your animation is finished
        [self clearImage];
        isSwiping = 0;
    }];
}

-(void)animateForward {
    CGRect screenRect = [self.view bounds];

    //UIImage *screen = [self captureScreen];
    UIImage *screen = [imageArray objectAtIndex:0]; //Get the latest image
    imgView = [[UIImageView alloc] initWithFrame:screenRect];
    imgView.image = screen;
    [imgView setHidden:NO];
    NSLog(@"Center of imgView is x = %f, y = %f", imgView.center.x, imgView.center.y);
    CGFloat startPointX = imgView.center.x;
    CGFloat width = screenRect.size.width;
    NSLog(@"Width = %f", width);
    imgView.center = CGPointMake(imgView.center.x - width, imgView.center.y);

    [self.view addSubview: imgView];

    [UIView animateWithDuration:.3 animations:^{
        isSwiping = 1;
        imgView.center = CGPointMake(startPointX, imgView.center.y);


    } completion:^(BOOL finished){
        // Your animation is finished
        [self navigateForwardInHistory];
        [self clearImage];
        isSwiping = 0;
    }];

}



-(void)clearImage {
    [imgView setHidden:YES];
    imgView.image = nil;
}

-(void)navigateBackInHistory {
    [self saveItems:self];
    [self alterIndexBack];
    item = [[[LEItemStore sharedStore] allItems] objectAtIndex:currentHistoryIndex];
    [self loadItems:self];
}

-(void)navigateForwardInHistory {
    [imageArray removeObjectAtIndex:0]; //Remove the latest image, since we just finished swiping this way.
    [self saveItems:self];
    [self alterIndexForward];
    item = [[[LEItemStore sharedStore] allItems] objectAtIndex:currentHistoryIndex];
    [self loadItems:self];
}

Note that imgView is a class-level UIImageView and imageArray is a class level array.

Any ideas? Thanks.

Edit

Here’s the code at the top of my .m to initalize it. Still does not work.

.....
NSMutableArray *imageArray;

- (void)viewDidLoad
{
    [super viewDidLoad];
    imageArray = [imageArray initWithObjects:nil];

Answer 1

It looks like you forgot to create the array. Something like this at the appropriate time would do (assuming ARC):

imageArray = [NSMutableArray array];

Glad that worked out.