多点触摸问题

我想知道,当用户摇动iPhone时,触摸屏幕的哪个部分。

我是这样做的:

-(void) accelerometer: (UIAccelerometer*)accelerometer didAccelerate: (UIAcceleration*)acceleration
{
    float shakeStrength = sqrt( acceleration.x * acceleration.x + acceleration.y * acceleration.y + acceleration.z * acceleration.z );

    if (shakeStrength >= 1.5f)
    {
        if (isLeftHandTouches && isRightHandTouches)
        {
            DebugLog(@"both hands shake");
        } else if (isLeftHandTouches)
        {
            DebugLog(@"left hand shake");
        } else if (isRightHandTouches)
        {
            DebugLog(@"right hand shake");
        }
    }
}

-(void) touchesBegan: (NSSet *)touches withEvent: (UIEvent *)event
{
    NSSet *allTouches = [event allTouches];

    for (int i = 0; i < [allTouches count]; i++)
    {
        if ([ [ [allTouches allObjects] objectAtIndex: i] locationInView: [self view] ].x <= 240.0f)
        {
            isLeftHandTouches = YES;
        } else
        {
            isRightHandTouches = YES;
        }
    }
}

-(void) touchesEnded: (NSSet *)touches withEvent: (UIEvent *)event
{
    NSSet *allTouches = [event allTouches];

    for (int i = 0; i < [allTouches count]; i++)
    {
        if ([ [ [allTouches allObjects] objectAtIndex: i] locationInView: [self view] ].x <= 240.0f)
        {
            isLeftHandTouches = NO;
        } else
        {
            isRightHandTouches = NO;
        }
    }
}

如果用户在进行另一次握手之前移除双手,一切都会正常工作,但是如果我的双手放在屏幕上并移除其中一只,一切都会变得混乱。

也就是说,我在屏幕上用双手握手,然后我只想用一只手握手。在这种情况下,震动不算数——就好像屏幕上没有触摸一样。我假设当我从屏幕上取下一只手时,两个“触摸”都会被移除。

问题是什么?我如何解决?

谢谢。