Basic Drag and Drop in iOS

Question

I want to have a view in which there are vehicles driving around that the user can also drag and drop. What do you think is the best large-scale strategy for doing this? Is it best to get touch events from the views representing the vehicles, or from the larger view? Is there a simple paradigm you've used for drag and drop that you're satisfied with? What are the drawbacks of different strategies?

Answer 1

Assume you have a UIView scene with a background image and many vehicles, you may define each new vehicle as a UIButton (UIImageView will probably work too):

UIButton *button = [UIButton buttonWithType:UIButtonTypeCustom]; [button addTarget:self action:@selector(imageTouch:withEvent:) forControlEvents:UIControlEventTouchDown]; [button addTarget:self action:@selector(imageMoved:withEvent:) forControlEvents:UIControlEventTouchDragInside]; [button setImage:[UIImage imageNamed:@"vehicle.png"] forState:UIControlStateNormal]; [self.view addSubview:button]; 

Then you may move the vehicle wherever you want, by responding to the UIControlEventTouchDragInside event, e.g.:

- (IBAction) imageMoved:(id) sender withEvent:(UIEvent *) event {     CGPoint point = [[[event allTouches] anyObject] locationInView:self.view];     UIControl *control = sender;     control.center = point; } 

It's a lot easier for individual vehicle to handle its own drags, comparing to manage the scene as a whole.

Answer 2

In addition to ohho's answer I've tried similar implementation, but without problem of "fast" drag and centering to drag.

The button initialization is...

UIButton *button = [UIButton buttonWithType:UIButtonTypeCustom]; [button addTarget:self action:@selector(imageMoved:withEvent:) forControlEvents:UIControlEventTouchDragInside]; [button addTarget:self action:@selector(imageMoved:withEvent:) forControlEvents:UIControlEventTouchDragOutside]; [button setImage:[UIImage imageNamed:@"vehicle.png"] forState:UIControlStateNormal]; [self.view addSubview:button]; 

and method implementation:

- (IBAction) imageMoved:(id) sender withEvent:(UIEvent *) event {     UIControl *control = sender;      UITouch *t = [[event allTouches] anyObject];     CGPoint pPrev = [t previousLocationInView:control];     CGPoint p = [t locationInView:control];      CGPoint center = control.center;     center.x += p.x - pPrev.x;     center.y += p.y - pPrev.y;     control.center = center; } 

I don't say, that this is the perfect solution for the answer. Nevertheless I found this the easiest solution for dragging.