UIScrollView that allows subviews to be dragged

// PoliteScrollView.h

#import <UIKit/UIKit.h>

@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 <NSObject>
@optional
- (void)scrollView:(PoliteScrollView *)scrollView subviewTouchesBegan:(NSSet *)touches;
- (void)scrollView:(PoliteScrollView *)scrollView subviewTouchesMoved:(NSSet *)touches;
- (void)scrollView:(PoliteScrollView *)scrollView subviewTouchesEnded:(NSSet *)touches;
@end

// PoliteScrollView.m

#import "PoliteScrollView.h"

@interface PoliteScrollView ()
@property(nonatomic,assign) CGPoint startLocation;
@property(nonatomic,strong) UIView *hitView;
@end

@implementation PoliteScrollView

@synthesize startLocation=_startLocation;
@synthesize hitView=_hitView;

@synthesize subviewDraggingThreshold=_subviewDraggingThreshold;
@synthesize isDraggingSubview=_isDraggingSubview;
@synthesize draggingSubview=_draggingSubview;

- (BOOL)isHorizontal {
    return self.frame.size.width > self.frame.size.height;
}

- (CGFloat)subviewDraggingThreshold {
    if (_subviewDraggingThreshold == 0.0) {
        CGFloat orthoSize = ([self isHorizontal])? self.frame.size.height : self.frame.size.width;
        _subviewDraggingThreshold = orthoSize / 10.0;
    }
    return _subviewDraggingThreshold;
}

- (BOOL)isOrthoganalDrag:(CGPoint)delta {
    if ([self isHorizontal]) {
        return fabsf(delta.y) > self.subviewDraggingThreshold;
    } else {
        return fabsf(delta.x) > self.subviewDraggingThreshold;
    }
}


- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    [super touchesBegan:touches withEvent:event];
    CGPoint location = [[touches anyObject] locationInView:self];

    for (UIView *view in self.subviews) {
        if (CGRectContainsPoint(view.frame, location)) {
            self.hitView = view;
            self.startLocation = location;
            break;
        }
    }
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {

    [super touchesMoved:touches withEvent:event];

    if (self.isDraggingSubview) {

        if (self.delegate && [self.delegate respondsToSelector:@selector(scrollView:subviewTouchesMoved:)]) {
            [(id <PoliteScrollViewDelegate>)self.delegate scrollView:self subviewTouchesMoved:touches];
        }
        [self.draggingSubview touchesMoved:touches withEvent:event];

    } else {

        CGPoint location = [[touches anyObject] locationInView:self];
        CGPoint delta = CGPointMake(location.x-self.startLocation.x, location.y-self.startLocation.y);
        if ([self isOrthoganalDrag:delta]) {
            self.draggingSubview = self.hitView;
            self.isDraggingSubview = YES;
            self.scrollEnabled = NO;

            if (self.delegate && [self.delegate respondsToSelector:@selector(scrollView:subviewTouchesBegan:)]) {
                [(id <PoliteScrollViewDelegate>)self.delegate scrollView:self subviewTouchesBegan:touches];
            }
            [self.draggingSubview touchesBegan:touches withEvent:event];
        }
    }
}

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
    [super touchesEnded:touches withEvent:event];

    if (self.isDraggingSubview) {

        if (self.delegate && [self.delegate respondsToSelector:@selector(scrollView:subviewTouchesEnded:)]) {
            [(id <PoliteScrollViewDelegate>)self.delegate scrollView:self subviewTouchesEnded:touches];
        }
        [self.draggingSubview touchesEnded:touches withEvent:event];

        self.isDraggingSubview = NO;
        self.scrollEnabled = YES;
        self.draggingSubview = nil;
    }
}
@end

// DraggableImageView.h

#import <Foundation/Foundation.h>

@interface DraggableImageView : UIImageView
- (id)initWithImageView:(UIImageView *)imageView;
@end

// DraggableImageView.m
#import "DraggableImageView.h"

@interface DraggableImageView ()
@property(assign,nonatomic) CGPoint startLocation;
@end

@implementation DraggableImageView

@synthesize startLocation=_startLocation;

- (id)initWithImageView:(UIImageView *)imageView {

    self = [super initWithImage:imageView.image];
    if (self) {
    }
    return self;
}

- (void) touchesBegan:(NSSet*)touches withEvent:(UIEvent*)event {

    CGPoint location = [[touches anyObject] locationInView:self];
    self.startLocation = location;
}

- (void) touchesMoved:(NSSet*)touches withEvent:(UIEvent*)event {

    CGPoint location = [[touches anyObject] locationInView:self];
    CGPoint prevlocation = [[touches anyObject] previousLocationInView:self];
    self.frame = CGRectOffset(self.frame, location.x-prevlocation.x, location.y-prevlocation.y);
}

@end

// ViewController.m


#import "ViewController.h"
#import "PoliteScrollView.h"
#import "DraggableImageView.h"

#define kPUPPY_COUNT  5

#define kPUPPY_WIDTH  100
#define kPUPPY_HEIGHT 100


@interface ViewController ()
@property (weak, nonatomic) IBOutlet UIScrollView *scrollView;
@property (strong,nonatomic) DraggableImageView *dragImage;
@end

@implementation ViewController

@synthesize scrollView=_scrollView;
@synthesize dragImage=_dragImage;

- (void)initScrollView {

    for(UIView *subview in [self.scrollView subviews]) [subview removeFromSuperview];

    for (int i=0; i<kPUPPY_COUNT; i++) {
        UIImage *image = [UIImage imageNamed:[NSString stringWithFormat:@"puppy%d.jpeg", i]];
        UIImageView *imageView = [[UIImageView alloc] initWithImage:image];
        imageView.frame = CGRectMake(i*kPUPPY_WIDTH, 0.0, kPUPPY_WIDTH, kPUPPY_HEIGHT);
        [self.scrollView addSubview:imageView];
    }
    self.scrollView.clipsToBounds = NO;
    self.scrollView.contentSize = CGSizeMake(kPUPPY_COUNT*kPUPPY_WIDTH, kPUPPY_HEIGHT);
}

- (void)viewDidLoad
{
    [super viewDidLoad];
    [self initScrollView];
}

- (void)viewDidUnload
{
    [self setScrollView:nil];
    [super viewDidUnload];
    // Release any retained subviews of the main view.
}

- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
{
    return (interfaceOrientation != UIInterfaceOrientationPortraitUpsideDown);
}

- (IBAction)startOver:(id)sender {

    for (UIView *view in [self.view subviews]) {
        if ([view isKindOfClass:[DraggableImageView self]]) {
            [view removeFromSuperview];
        }
    }
}

- (void)scrollView:(PoliteScrollView *)scrollView subviewTouchesBegan:(NSSet *)touches {

    self.dragImage = [[DraggableImageView alloc] initWithImageView:(UIImageView *)scrollView.draggingSubview];
    self.dragImage.frame = [scrollView convertRect:scrollView.draggingSubview.frame toView:self.view];
    [self.view addSubview:self.dragImage];
    [self.dragImage touchesBegan:touches withEvent:nil];
}

- (void)scrollView:(PoliteScrollView *)scrollView subviewTouchesMoved:(NSSet *)touches {

    [self.dragImage touchesMoved:touches withEvent:nil];
}

- (void)scrollView:(PoliteScrollView *)scrollView subviewTouchesEnded:(NSSet *)touches {

    [self.dragImage touchesEnded:touches withEvent:nil];
}

@end