UIImage detecting touch and dragging - iphone

Question

Fairly common question this, to which I have a few answers and I'm nearly there. I have a button which when pressed, will create an image (code as follows)
(numImages is set on load to ZERO and is used as a count up for the tag numbers of all images created)
UIImage *tmpImage = [[UIImage imageNamed:[NSString stringWithFormat:#"%i.png", sender.tag]] retain];
UIImageView *myImage = [[UIImageView alloc] initWithImage:tmpImage];
numImages += 1;
myImage.userInteractionEnabled = YES;
myImage.tag = numImages;
myImage.opaque = YES;
[self.view addSubview:myImage];
[myImage release];
I then have a touchesBegan method which will detect what's touched. What I need it to do is to allow the user to drag the newly created image. It's nearly working, but the image flickers all over the place when you drag it. I can access the image which you click on as I can get it's TAG, but I just cannot drag it nicely.
- (void) touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
UITouch *touch = [[event allTouches] anyObject];
CGPoint location = [touch locationInView:touch.view];
if (touch.view.tag > 0) {
touch.view.center = location;
}
NSLog(#"tag=%#", [NSString stringWithFormat:#"%i", touch.view.tag]);
}
- (void) touchesMoved:(NSSet *)touches withEvent: (UIEvent *)event {
[self touchesBegan:touches withEvent:event];
}
It works, in that I get an output of the tag for each image as I click on them. But when I drag, it flashes... any ideas?

Answer 1

In answer to my own question - I decided to create a class for handling the images I place on the view.
Code if anyone's interested....
Draggable.h
#import <Foundation/Foundation.h>
#interface Draggable : UIImageView {
CGPoint startLocation;
}
#end
Draggable.m
#import "Draggable.h"
#implementation Draggable
- (void) touchesBegan:(NSSet*)touches withEvent:(UIEvent*)event {
// Retrieve the touch point
CGPoint pt = [[touches anyObject] locationInView:self];
startLocation = pt;
[[self superview] bringSubviewToFront:self];
}
- (void) touchesMoved:(NSSet*)touches withEvent:(UIEvent*)event {
// Move relative to the original touch point
CGPoint pt = [[touches anyObject] locationInView:self];
CGRect frame = [self frame];
frame.origin.x += pt.x - startLocation.x;
frame.origin.y += pt.y - startLocation.y;
[self setFrame:frame];
}
#end
and to call it
UIImage *tmpImage = [[UIImage imageNamed:"test.png"] retain];
UIImageView *imageView = [[UIImageView alloc] initWithImage:tmpImage];
CGRect cellRectangle;
cellRectangle = CGRectMake(0,0,tmpImage.size.width ,tmpImage.size.height );
UIImageView *dragger = [[Draggable alloc] initWithFrame:cellRectangle];
[dragger setImage:tmpImage];
[dragger setUserInteractionEnabled:YES];
[self.view addSubview:dragger];
[imageView release];
[tmpImage release];

Answer 2

Usually you get an implicit animation when you change center. Are you messing with -contentMode or calling -setNeedsDisplay by any chance?
You can explicitly request animation to avoid the delete and re-draw this way:
if (touch.view.tag > 0) {
[UIView beginAnimations:#"viewMove" context:touch.view];
touch.view.center = location;
[UIView commitAnimations];
}
Do note that NSLog() can be very slow (much slower than you'd expect; it's much more complicated than a simple printf), and that can cause trouble in something called as often as touchesMoved:withEvent:.
BTW, you're leaking tmpImage.