Creating a draggable image view

Question

I have been searching the net for a long time and I have not found a concrete way of making an image view draggable. Here is what I have so far:

tempViewController.h

#import <UIKit/UIKit.h>
#import "MyRect.h"
@class UIView;
@interface tempViewController : UIViewController

@property (nonatomic, strong) MyRect *rect1;
@end

tempViewController.m

#import "tempViewController.h"

@interface tempViewController ()

@end

@implementation tempViewController

@synthesize rect1 = _rect1;

- (void)viewDidLoad
{
    [super viewDidLoad];

    _rect1 = [[MyRect alloc]initWithFrame:CGRectMake(150.0, 100.0, 80, 80)];
    [_rect1 setImage:[UIImage imageNamed:@"cloud1.png"]];
    [_rect1 setUserInteractionEnabled:YES];
    [self.view addSubview:_rect1];

}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}


-(void) touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch *touch = [[event allTouches]anyObject];
    if([touch view] == _rect1)
    {
        CGPoint pt = [[touches anyObject] locationInView:_rect1];
        NSLog(@"%@",NSStringFromCGPoint(pt));
        _rect1.center = pt;
    }
}

-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch *touch = [[event allTouches]anyObject];
    if([touch view] == _rect1)
    {
        CGPoint pt = [[touches anyObject] locationInView:_rect1];
        NSLog(@"%@",NSStringFromCGPoint(pt));
        _rect1.center = pt;
    }
}


@end

---

MyRect right now is an empty UIImageView Class.

Dragging the image from a point such as [532,589] on micron moves it to a totally different part of the screen such as [144, 139]

Answer 1

Just attach a UIPanGestureRecognizer to your view. In the recognizer's action, update your view's center based on the “translation” (offset) of the recognizer, then reset the recognizer's translation to zero. Here's an example:

- (void)viewDidLoad {
    [super viewDidLoad];

    UIView *draggableView = [[UIView alloc] initWithFrame:CGRectMake(150, 100, 80, 80)];
    draggableView.userInteractionEnabled = YES;
    draggableView.backgroundColor = [UIColor redColor];
    [self.view addSubview:draggableView];

    UIPanGestureRecognizer *panner = [[UIPanGestureRecognizer alloc]
        initWithTarget:self action:@selector(panWasRecognized:)];
    [draggableView addGestureRecognizer:panner];
}

- (void)panWasRecognized:(UIPanGestureRecognizer *)panner {
    UIView *draggedView = panner.view;
    CGPoint offset = [panner translationInView:draggedView.superview];
    CGPoint center = draggedView.center;
    draggedView.center = CGPointMake(center.x + offset.x, center.y + offset.y);

    // Reset translation to zero so on the next `panWasRecognized:` message, the
    // translation will just be the additional movement of the touch since now.
    [panner setTranslation:CGPointZero inView:draggedView.superview];
}

Answer 2

Don't use -touchesBegan:withEvent: etc.

What you want to use for such "high-level" things are UIGestureRecognizers.

You would add a pan gesture recognizer to your view set a delegate (possibly the view itself), and in the callback move the view by the distance the recognizer moved.

You either remember the initial position and move by the -translationInView each time, or simply move by the translation and then use -setTranslation:inView: to reset the recognizers translation to zero, so on the next call of the delegate method, you will again get the movement since the last call.