iPhone - ignoring the second touch on an area

I have a view that the users are allowed to finger paint. The code is working perfectly if the area is touched with one finger. For example: I touch it with one finger and move the finger. Then, a line is drawn as I move the first finger. If I touch with a second finger the same view, the line that was being drawn by the first finger stops.

I would like to ignore any touch beyond the first, i.e., to track the first touch but ignore all others to the same view.

I am using touchesBegan/moved/ended.

I have used this to detect the touches

UITouch *touch = [[event allTouches] anyObject];
lastPoint = [touch locationInView:myView];

I have also tried this

lastPoint = [[touches anyObject] locationInView:myView];

but nothing changed.

How do I d开发者_StackOverflow中文版o that - track the first touch and ignore any subsequent touch to a view?

thanks.

NOTE: the view is NOT adjusted to detect multiple touches.

A given touch will maintain the same memory address as long as it is in contact with the screen. This means you can save the address as an instance variable and ignore any events from other objects. However, do not retain the touch. If you do, a different address will be used and your code won't work.

Example:

Add currentTouch to your interface:

@interface MyView : UIView {
    UITouch *currentTouch;
    ...
}
...
@end

Modify touchesBegan: to ignore the touch if one is already being tracked:

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    if(currentTouch) return;
    currentTouch = [touches anyObject];
    ...
}

Modify touchesMoved: to use currentTouch instead of getting a touch from the set:

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
    if(!currentTouch) return;
    CGPoint currentPoint = [currentTouch locationInView:myView];
    ...
}

Modify touchesEnded: and touchesCancelled: to clear currentTouch, but only if currentTouch has ended or been cancelled.

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
    if(currentTouch && currentTouch.phase == UITouchPhaseEnded) {
        ...
        currentTouch = nil;
    }
}
- (void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event {
    if(currentTouch && currentTouch.phase == UITouchPhaseCancelled) {
        ...
        currentTouch = nil;
    }
}

yourView.multipleTouchEnabled = NO;

From the reference documents on UIView

multipleTouchEnabled

A Boolean value that indicates whether the receiver handles multitouch events.

@property(nonatomic, getter=isMultipleTouchEnabled) BOOL multipleTouchEnabled Discussion

When set to YES, the receiver receives all touches associated with a multitouch sequence. When set to NO, the receiver receives only the first touch event in a multitouch sequence. The default value of this property is NO.

Other views in the same window can still receive touch events when this property is NO. If you want this view to handle multitouch events exclusively, set the values of both this property and the exclusiveTouch property to YES.