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];
}

Jansen

Updated my response to include a new theory. – Matthaeus

I'm sure you'd receive more answers if you fix your accept rating. It's currently 0%. – Hagiology

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.

Matthaeus

I do have the following: - (void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event { [self touchesEnded:touches withEvent:event]; } (added to original code sample now) – Jansen

Sometimes neither ended nor cancelled is called - it's a bug in iOS – Squeamish

Answer 2

One thing to remember... You may receive some touches that have multiple taps. Don't forget to take tapCount into account.

However, if you still have the problem, you can consider all touches from the event, though it presents some other management issues...

HACK ALERT

I have coded the following HACK to work around this. Sometimes the touchesEnded does not get called, BUT, the touches show up as part of all the touches in the event.

Note, that you can now process the same "canceled" or "ended" touch multiple times. If that is a problem, you have to keep your own state of "pending" touches, and remove them when done.

Yeah, it all is pretty bad, but I don't know how to overcome this issue without a similar hack. The basic solution is to look at all the touches in each event, and process them based on their phase, calling the appropriate ended/canceled when they are seen.

- (void) touchesEndedOrCancelled:(NSSet *)touches
{
    __block NSMutableSet *ended = nil;
    __block NSMutableSet *canceled = nil;
    [touches enumerateObjectsUsingBlock:^(UITouch *touch, BOOL *stop) {
        if (touch.phase == UITouchPhaseEnded) {
            if (!ended) ended = [NSSet setWithObject:touch];
            else [ended addObject:touch];
        } else if (touch.phase == UITouchPhaseCancelled) {
            if (!canceled) canceled = [NSSet setWithObject:touch];
            else [canceled addObject:touch];
        }
    }];
    if (ended) [self touchesEnded:ended withEvent:nil];
    if (canceled) [self touchesCancelled:canceled withEvent:nil];
}

Then, call it at the end of touchesBegan and touchesMoved...

[self touchesEndedOrCancelled:event.allTouches];

For this to work, touchesEnded/Canceled needs to not choke on a nil event. Also, the "other" needs to be handled. In touchesEnded...

[self touchesCancelled:[event.allTouches objectsPassingTest:^BOOL(UITouch *touch, BOOL *stop) {
    return touch.phase == UITouchPhaseCancelled;
}] withEvent:nil];

and in touchesCanceled...

[self touchesEnded:[event.allTouches objectsPassingTest:^BOOL(UITouch *touch, BOOL *stop) {
    return touch.phase == UITouchPhaseEnded;
}] withEvent:nil];

Nutt

Answer 3

You may need to provide more detail but....

If you run your finger off the edge of the screen this event will show touch started and touch moved but no end as it didn't actually end (lift finger). This could be your problem, that the event didn't happen. Plus there is a limit to the amount of touches in multi touch if you press say 10 times its then up to the system to determine what events are really and what are false, it seems like you are using the hardware incorrectly.

Homograft