iPhone - Wheel tracking finger jumps to same starting position? - iphone

Question

I am trying to implement a selection wheel in an iphone application. I have got the wheel to track the users finger but now for some reason when the user touches the screen the wheel jumps so the same section of the wheel tracks the finger each time.
How would I get it so the wheel rotates like the user is dragging it from any point?
-(void) touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
NSSet *allTouches = [event allTouches];
int tInput = [allTouches count]-1;
UITouch *touch =[[allTouches allObjects] objectAtIndex:tInput];
CGPoint location = [touch locationInView:self.view];
float theAngle = atan2( location.y-imageX.center.y, location.x-imageX.center.x );
CGAffineTransform cgaRotate = CGAffineTransformMakeRotation(theAngle);
imageX.transform = cgaRotate;
}
Any suggestion?

Answer 1

Right - that makes perfect sense - this is indeed what your code does.
What you need to add is the initial value where you started your drag as a relative rotation -- or track your last rotation.
A very elaborate way is shown below - but this should help get the point across.
#interface UntitledViewController : UIViewController {
CGPoint firstLoc;
UILabel * fred;
double angle;
}
#property (assign) CGPoint firstLoc;
#property (retain) UILabel * fred;
#implementation UntitledViewController
#synthesize fred,firstLoc;
- (void)viewDidLoad {
[super viewDidLoad];
self.fred = [[UILabel alloc] initWithFrame:CGRectMake(100,100,100,100)];
fred.text = #"Fred!"; fred.textAlignment = UITextAlignmentCenter;
[self.view addSubview:fred];
angle = 0; // we aint have rotated just yet...
};
// make sure we get them drag events.
- (BOOL)isFirstResponder { return YES; }
-(void)handleObject:(NSSet *)touches
withEvent:(UIEvent *)event
isLast:(BOOL)lst
{
UITouch *touch =[[[event allTouches] allObjects] lastObject];
CGPoint curLoc = [touch locationInView:self.view];
float fromAngle = atan2( firstLoc.y-fred.center.y,
firstLoc.x-fred.center.x );
float toAngle = atan2( curLoc.y-fred.center.y,
curLoc.x-fred.center.x );
// So the angle to rotate to is relative to our current angle and the
// angle through which our finger moved (to-from)
float newAngle = angle + (toAngle - fromAngle);
CGAffineTransform cgaRotate = CGAffineTransformMakeRotation(newAngle);
fred.transform = cgaRotate;
// we only 'save' the current angle when we're done with the drag.
//
if (lst)
angle = newAngle;
}
-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
UITouch *touch =[[[event allTouches] allObjects] lastObject];
// capture where we started - so we can later work out the
// rotation relative to this point.
//
firstLoc = [touch locationInView:self.view];
};
-(void) touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
[self handleObject:touches withEvent:event isLast:NO];
};
-(void) touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
[self handleObject:touches withEvent:event isLast:YES];
}
Obviously you can do this a lot more elegant - and above misses a bit of 0 .. 2xPI capping you need.

Answer 2

Because the view itself is still the old one. You may want to fetch the zip example file in:
http://bynomial.com/blog/?p=77
and in PuttyView.m change the:
- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
....
else {
self.center = CGPointMake(self.center.x + touchPoint.x - touchStart.x,
self.center.y + touchPoint.y - touchStart.y);
into
else {
NSSet *allTouches = [event allTouches];
int tInput = [allTouches count]-1;
UITouch *touch =[[allTouches allObjects] objectAtIndex:tInput];
CGPoint location = [touch locationInView:self];
float theAngle = atan2( location.y-self.center.y, location.x-self.center.x );
CGAffineTransform cgaRotate = CGAffineTransformMakeRotation(theAngle);
self.transform = cgaRotate;
}
where you thus stay within the view and its relative angle. Alternatively - in your code - keep the relative angle itself.