How to effectively process UITouches in a multitouch sequence

问题

I am working with multitouch while writing, So basically what I am doing is, I am writing with hand support, because typically, its how user writes, I followed this link How to ignore certain UITouch Points in multitouch sequence

Everything is working fine with single touch, but their is some problem when I write with my hand touching the screen i.e multiple UItouches Below is my code

In touches began, I go through all touches and I find the touch with highest y position, below is my code

Below is my code

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch* topmostTouch = self.trackingTouch;
    for (UITouch *touch in touches)
    {
        ctr = 0;

        touchStartPoint1 = [touch locationInView:self];


        if(!topmostTouch || [topmostTouch locationInView:self].y > touchStartPoint1.y)
        {
            topmostTouch = touch;
            pts[0] = touchStartPoint1;
        }
    }   

    self.trackingTouch = topmostTouch;
}

In touches moved., I will only take self.trackingTouch, which I found in touches Began

My touches Moved code below

-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{ 
    if(self.trackingTouch== nil)
    {
        return;
    }

    CGPoint p = [self.trackingTouch locationInView:self];
    ctr++;
    pts[ctr] = p;

    if (ctr == 4)
    {
        pts[3] = midPoint(pts[2], pts[4]);

        self.currentPath = [[DrawingPath alloc] init];

        [self.currentPath setPathColor:self.lineColor];
        self.currentPath.pathWidth = [NSString stringWithFormat:@"%f",self.lineWidth];


        [self.currentPath.path moveToPoint:pts[0]];
        [self.currentPath.path addCurveToPoint:pts[3] controlPoint1:pts[1] controlPoint2:pts[2]];

        [self setNeedsDisplay];


        pts[0] = pts[3];
        pts[1] = pts[4];
        ctr = 1;
    }
}

For your reference here is the image of writing with single touch and multitouch respectively

You can see that, when I write with single touch, my writing is smooth, the curves are smooth, but when I write with hand resting, I curves get jagged, as you can see in the second image.

So friends, please help me out

回答1:

Assuming the drawing code is all the same the difference is only the extra overhead of processing the extra touches. Looping to process these, looping to draw, everything is at least doubled.. So while you are processing touch and then drawing on finger #2, out here in the real world finger #1 etc is/are still moving... Be mindful that UIKit can only draw at the end of each runLoop. I don't think there is any way around this, also as I told you before, I doubt there are many people who will value the ability to draw with multiple touches, although this may well be my limited, English speaking, Australian perspective on it. Good luck

回答2:

I finally solved my issue,

Here is the code

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch* topmostTouch = self.trackingTouch;
    for (UITouch *touch in touches)
    {


        touchStartPoint1 = [touch locationInView:self];


        if(!topmostTouch || [topmostTouch locationInView:self].y > touchStartPoint1.y)
        {
            ctr = 0;
            topmostTouch = touch;
            pts[0] = touchStartPoint1;
        }
    }   

    self.trackingTouch = topmostTouch;
}

The problem was very simple, just set ctr = 0 , inside the check, like this..

来源：https://stackoverflow.com/questions/21952274/how-to-effectively-process-uitouches-in-a-multitouch-sequence

标签

ios

core-graphics

uitouch

uibezierpath

uiresponder