多点触控跟踪问题

Question

我在写作时使用多点触控，所以基本上我正在做的是，我正在用手支持写作，因为通常，它是如何用户权限的，我点击了这个链接How to ignore certain UITouch Points in multitouch sequence

所以，我正在做的是，我在 touchesBegan 中跟踪一个触摸对象，并仅在 touchesMoved 中使用它。一切正常，但有时在写作时，我得到了这一行

在上图中，你可以看到我的手在屏幕上写字时突然出现的粗线

这里是代码

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    NSLog(@"Touches began");   

    UITouch* topmostTouch = self.trackingTouch;
    for (UITouch *touch in touches)
    {
        bufIdx = 0;
        isFirstTouchPoint = YES;
        ctr = 0;

        pts[0] = [touch locationInView:self];


        if(!topmostTouch || [topmostTouch locationInView:self].y > pts[0].y)
        {
            topmostTouch = touch;
            //touchStartPoint1 = pts[0];
        }
        else
        {
            pts[0] = pts[3];
        }
    }

   if (self.trackingTouch != nil && self.trackingTouch != topmostTouch) 
   {
     [self discardDrawing];        
   }

    self.trackingTouch = topmostTouch;
}

-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{    
        // Find the touch that we track for drawing

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

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

                for ( int i = 0; i < 4; i++)
                {
                    pointsBuffer[bufIdx + i] = pts[i];
                }

                bufIdx += 4;

                dispatch_async(drawingQueue, ^{
                    //UIBezierPath *offsetPath = [UIBezierPath bezierPath]; // ................. (2)

                    self.currentPath = [[DrawingPath alloc] init];                   
                    [self.currentPath setPathColor:self.lineColor];


                    if (bufIdx == 0) return;

                    LineSegment ls[4];
                    for ( int i = 0; i < bufIdx; i += 4)
                    {
                        if (isFirstTouchPoint) // ................. (3)
                        {                           
                            ls[0] = (LineSegment){pointsBuffer[0], pointsBuffer[0]};
                            [self.currentPath.path moveToPoint:ls[0].firstPoint];                      
                            isFirstTouchPoint = NO;                   
                        }
                        else
                        {
                            ls[0] = lastSegmentOfPrev;

                        }


                        float frac1 = self.lineWidth/clamp(len_sq(pointsBuffer[i], pointsBuffer[i+1]), LOWER, UPPER); // ................. (4)
                        float frac2 = self.lineWidth/clamp(len_sq(pointsBuffer[i+1], pointsBuffer[i+2]), LOWER, UPPER);
                        float frac3 = self.lineWidth/clamp(len_sq(pointsBuffer[i+2], pointsBuffer[i+3]), LOWER, UPPER);



                        ls[1] = [self lineSegmentPerpendicularTo:(LineSegment){pointsBuffer[i], pointsBuffer[i+1]} ofRelativeLength:frac1]; // ................. (5)
                        ls[2] = [self lineSegmentPerpendicularTo:(LineSegment){pointsBuffer[i+1], pointsBuffer[i+2]} ofRelativeLength:frac2];
                        ls[3] = [self lineSegmentPerpendicularTo:(LineSegment){pointsBuffer[i+2], pointsBuffer[i+3]} ofRelativeLength:frac3];



                        [self.currentPath.path  moveToPoint:ls[0].firstPoint]; // ................. (6)
                        [self.currentPath.path  addCurveToPoint:ls[3].firstPoint controlPoint1:ls[1].firstPoint controlPoint2:ls[2].firstPoint];
                        [self.currentPath.path  addLineToPoint:ls[3].secondPoint];
                        [self.currentPath.path  addCurveToPoint:ls[0].secondPoint controlPoint1:ls[2].secondPoint controlPoint2:ls[1].secondPoint];
                        [self.currentPath.path  closePath];

                        lastSegmentOfPrev = ls[3]; // ................. (7)
                    }


                     dispatch_async(dispatch_get_main_queue(), ^{
                            bufIdx = 0;

                            [m_pathArray addObject:self.currentPath];
                            [self setNeedsDisplay];

                        });
                    });


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

所以朋友们，请帮帮我。

Answer 1

您需要调试才能确认，但它似乎与您在touchesBegan:withEvent: 中的逻辑有关。在这种方法中，您检查每次有新的触摸以确定它是否是“最高的”，但是当最高的触摸确实发生变化时，您似乎不会中止/重置任何现有的绘图。

您最好的方法可能是使用日志记录来确定“最高”触摸是否/何时发生变化以及这对进行中的生产线有什么影响。

另外，在touchesMoved: 中，您不需要for (UITouch *touch in touches) 循环，因为您已经引用了self.trackingTouch，因此您可以直接使用它。

Answer 2

如果我正确理解了您的问题，那么每当用户在您的画布上使用多个手指时，您就会出现不希望的笔触。既然你已经有了self.trackingTouch，那么忽略所有不是self.trackingTouch 的触摸是否合理？

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    if (self.trackingTouch)
    {
        // another touch is active so we ignore this one
        return;
    }
    // ...the rest of your code
}

-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch *touch = self.trackingTouch;
    if (![touches containsObject:touch])
    {
        // updates to touches other than self.trackingTouch
        return;
    }

    // ...process only self.trackingTouch in the rest of your code
}

-(void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch *touch = self.trackingTouch;
    if (![touches containsObject:touch])
    {
        // updates to touches other than self.trackingTouch
        return;
    }

    // ...process only self.trackingTouch in the rest of your code

   self.trackingTouch = nil;
}

-(void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event
{
    // ...same as touchesEnded:withEvent:
}