iOS Loses Touches?

Question

I can't find anything to explain lost UITouch events. If you smash your full hand on the screen enough times, the number of touchesBegan will be different than the number of touchesEnded! I think the only way to actually know about these orphaned touches will be to reference them myself and keep track of how long they haven't moved.

Sample code:

int touchesStarted = 0;
int touchesFinished = 0;

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    touchesStarted += touches.count;
}

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
    touchesFinished += touches.count;
    NSLog(@"%d / %d", touchesStarted, touchesFinished);
}

- (void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event
{
    [self touchesEnded:touches withEvent:event];
}

Answer 1

Don't forget about touchesCancelled: UIResponder reference

Editing in response to the poster's update:

Each touch object provides what phase it is in:

typedef enum {
    UITouchPhaseBegan,
    UITouchPhaseMoved,
    UITouchPhaseStationary,
    UITouchPhaseEnded,
    UITouchPhaseCancelled,
} UITouchPhase;

I believe that if a touch starts and ends in the same touch event set, -touchesBegan:withEvent: will be called but will contain touches which have ended or cancelled.

You should change your counting code, then, to look like this:

int touchesStarted = 0;
int touchesFinished = 0;

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    [self customTouchHandler:touches];
}

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
    [self customTouchHandler:touches];
}

- (void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event
{
    [self customTouchHandler:touches];
}
- (void)customTouchHandler:(NSSet *)touches
{
    for(UITouch* touch in touches){
        if(touch.phase == UITouchPhaseBegan)
            touchesStarted++;
        if(touch.phase == UITouchPhaseEnded || touch.phase == UITouchPhaseCancelled)
            touchesFinished++;
    }
    NSLog(@"%d / %d", touchesStarted, touchesFinished);
}

Every single touch event will go through both phases of started and finished/cancelled, and so your counts should match up as soon as your fingers are off the screen.