Question

How can I get touch offset when using UITouch? I am doing a project like a remote control. I have a view set to multi-touched and I want the view to act like a touch pad for Mac, so I need the touch offsets when people move to control the mouse. Any ideas?

Answer 1

This can be accomplished with a UIPanGestureRecognizer by measuring the diagonal distance from the center of the screen to the current touch location.

#import <QuartzCore/QuartzCore.h>

Declare the gesture and hook it to self.view so that the entire screen responds to touch events.

UIPanGestureRecognizer *pan = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(myPanRecognizerMethod:)];
[pan setDelegate:self];
[pan setMaximumNumberOfTouches:2];
[pan setMinimumNumberOfTouches:1];
[self.view setUserInteractionEnabled:YES];
[self.view addGestureRecognizer:pan];

Then in this method, we use the gesture recognizers state: UIGestureRecognizerStateChanged to update and integer that measures the diagonal distance between touch location and screen center as the touch location changes.

-(void)myPanRecognizerMethod:(id)sender
{
    [[[(UITapGestureRecognizer*)sender view] layer] removeAllAnimations];
    if ([(UIPanGestureRecognizer*)sender state] == UIGestureRecognizerStateChanged) {
        CGPoint touchLocation = [sender locationOfTouch:0 inView:self.view];
        NSNumber *distanceToTouchLocation = @(sqrtf(fabsf(powf(self.view.center.x - touchLocation.x, 2) + powf(self.view.center.y - touchLocation.y, 2))));
        NSLog(@"Distance from center screen to touch location is == %@",distanceToTouchLocation);
    }
}