iPhone/iOS: How to implement Drag and Drop for subviews of a Scrollview?

Question

I am having an horizontal scrollview in an UIViewController, where i have many images in small sizes. I am keeping the images in scrollview because the images a

Answer 1

For a similar problem, I made UIScrollView subclass like the following: PoliteScrollView passes touch messages to it's subviews when it determines that they are being dragged.

@interface PoliteScrollView : UIScrollView

// number of pixels perpendicular to the scroll views orientation to make a drag start counting as a subview drag
// defaults to 1/10 of the scroll view's width or height, whichever is smaller
@property(nonatomic,assign) CGFloat subviewDraggingThreshold;
// yes when a subview is being dragged
@property(nonatomic,assign) BOOL isDraggingSubview;
// the subview being dragged
@property(nonatomic,strong) UIView *draggingSubview;

@end

@protocol PoliteScrollViewDelegate 
@optional
- (void)scrollView:(PoliteScrollView *)scrollView subviewTouchesBegan:(NSSet *)touches;
- (void)scrollView:(PoliteScrollView *)scrollView subviewTouchesMoved:(NSSet *)touches;
- (void)scrollView:(PoliteScrollView *)scrollView subviewTouchesEnded:(NSSet *)touches;
@end

The key design idea is that dragging in a scroll view is ambiguous. Is the user scrolling or dragging a subview? PoliteScroll view handles this by providing two things: (1) a notion of orientation (horizontal if it's longer than it is wide, vertical otherwise), and (2) a threshold distance for what constitutes a drag in the direction perpendicular to it's orientation. (defaults to 1/10 the width or height).

I pasted this and several other files are in a paste bin, containing the following:

• PoliteScrollView .h and .m
• DraggableImageView.h and .m - that changes it's position when it gets touch messages.
• ViewController.m - that demonstrates thetwo in combination.

To combine these in a project, paste the paste bin into files appropriately named, add a storyboard with a PoliteScrollView (be sure to set it's delegate), add in some images (the ViewController tries to add puppy0.jpeg to puppy4.jpeg.