Ios – Detect direction of UIScrollView scroll in scrollViewWillBeginDragging

cocoa-touchiosiphone

I did google enough, & I did check posts like these ( Finding the direction of scrolling in a UIScrollView? ) in stackoverflow before posting this. I have a dynamic number of photos in an iPhone App, that am displaying through UIScrollView. At any point in time, I have only 3 photos being displayed in the scroll-view. When I have, say 4 photos, in total:
1st photo : displayed at offset 0.0
2nd photo : displayed at offset 320.0
3rd photo : displayed at offset 640.0

Now, when the user scrolls to the 4th photo, the scroll-view resets to 0.0 offset. If the user tries to scroll 'beyond' the 4th photo, scrolling should stop in the right-direction only (so that user doesn't scroll 'beyond'). But currently, the user 'is able' to scroll beyond the last photo ; however, I detect this programmatically & reset the offset. But it doesn't look neat, as the user sees the black background momentarily. I want to detect that the user has started scrolling 'right' (remember, scrolling 'left' i.e. to the 'previous' photo is okay) in scrollViewWillBeginDragging, so that I can stop any further scrolling to the right.

What I tried:

Trying using self.scrollView.panGestureRecognizer's
translationInView isn't working, because there is no
panGestureRecognizer instance returned in the first place (!),
though the UIScrollView API claims so.
Detecting this in scrollViewDidEndDecelerating is possible, though
it'll not serve my purpose.

Best Answer

I had no issues determining direction in scrollViewWillBeginDragging when checking the scroll view's panGestureRecognizer:

- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView
{ 
    CGPoint translation = [scrollView.panGestureRecognizer translationInView:scrollView.superview];

    if(translation.y > 0)
    {
        // react to dragging down
    } else
    {
        // react to dragging up
    }
}

I found it very useful in canceling out of a scroll at the very first drag move when the user is dragging in a forbidden direction.

Related Solutions

Iphone – Photos app-like gap between pages in UIScrollView with pagingEnabled

Note that this answer is quite old. The basic concept still works but you should not be hard coding view sizes in iOS7 and 8. Even if you ignore that advice, you should not use 480 or 330.

Have you tried making the frame of the UIScrollView slightly larger than the screen (assuming that you want to display your images fullscreen and then arranging your subviews on the same slightly-larger-than-the-screen boundaries.

#define kViewFrameWidth 330; // i.e. more than 320

CGRect scrollFrame;
scrollFrame.origin.x = 0;
scrollFrame.origin.y = 0; 
scrollFrame.size.width = kViewFrameWidth;
scrollFrame.size.height = 480;

UIScrollView* myScrollView = [[UIScrollView alloc] initWithFrame:scrollFrame];
myScrollView.bounces = YES;
myScrollView.pagingEnabled = YES;
myScrollView.backgroundColor = [UIColor redColor];

UIImage* leftImage = [UIImage imageNamed:@"ScrollTestImageL.png"];
UIImageView* leftView = [[UIImageView alloc] initWithImage:leftImage];
leftView.backgroundColor = [UIColor whiteColor];
leftView.frame = CGRectMake(0,0,320,480);

UIImage* rightImage = [UIImage imageNamed:@"ScrollTestImageR.png"];
UIImageView* rightView = [[UIImageView alloc] initWithImage:rightImage];
rightView.backgroundColor = [UIColor blackColor];
rightView.frame = CGRectMake(kViewFrameWidth * 2,0,320,480);

UIImage* centerImage = [UIImage imageNamed:@"ScrollTestImageC.png"];
UIImageView* centerView = [[UIImageView alloc] initWithImage:centerImage];
centerView.backgroundColor = [UIColor grayColor];
centerView.frame = CGRectMake(kViewFrameWidth,0,320,480);

[myScrollView addSubview:leftView];
[myScrollView addSubview:rightView];
[myScrollView addSubview:centerView];

[myScrollView setContentSize:CGSizeMake(kViewFrameWidth * 3, 480)];
[myScrollView setContentOffset:CGPointMake(kViewFrameWidth, 0)];

[leftView release];
[rightView release];
[centerView release];

Apologies if this doesn't compile, I tested it in a landscape app and hand edited it back to portrait. I'm sure you get the idea though. It relies on the superview clipping which for a full screen view will always be the case.

Ios – How to detect when a UIScrollView has finished scrolling

The 320 implementations are so much better - here is a patch to get consistent start/ends of the scroll.

-(void)scrollViewDidScroll:(UIScrollView *)sender 
{   
[NSObject cancelPreviousPerformRequestsWithTarget:self];
    //ensure that the end of scroll is fired.
    [self performSelector:@selector(scrollViewDidEndScrollingAnimation:) withObject:sender afterDelay:0.3]; 

...
}

-(void)scrollViewDidEndScrollingAnimation:(UIScrollView *)scrollView
{
    [NSObject cancelPreviousPerformRequestsWithTarget:self];
...
}

Related Topic