覆盖绘图应用程序的drawRect

Question

我正在构建一个演示绘图应用程序。我正在使用 touchesMoved:withEvent 来收集我的积分并将它们添加到 CGMutablePathRef。为了描边路径，我重写了 DrawRect，将路径添加到上下文中并描边路径：

   override func drawRect(rect: CGRect) {
    self.backgroundColor?.set()
    UIRectFill(rect)

    let context : CGContextRef = UIGraphicsGetCurrentContext()
    for line in pathArray {
        CGContextAddPath(context, line.structPath)
        CGContextSetLineWidth(context, line.structLineWidth)
        CGContextSetStrokeColorWithColor(context, line.structLineColor.CGColor)
        CGContextSetAlpha(context, lineOpacity)

    }
    CGContextSetLineCap(context, kCGLineCapRound)
    CGContextStrokePath(context)

    self.empty = false
}

override func touchesBegan(touches: Set<UITouch>, withEvent event: UIEvent?) {
    if let touch = touches.first as UITouch! {
        previousPoint = touch.previousLocationInView(self)
        previousPreviousPoint = touch.previousLocationInView(self)
        currentPoint = touch.locationInView(self)
    }
    self.touchesMoved(touches, withEvent: event)
}

override func touchesMoved(touches: Set<UITouch>, withEvent event: UIEvent?) {


     if let touch = touches.first as UITouch! {

        previousPreviousPoint = previousPoint
        previousPoint = touch.previousLocationInView(self)
        currentPoint = touch.locationInView(self)

        let mid1 : CGPoint = getMidPoint(previousPoint, p2: previousPreviousPoint)
        let mid2 : CGPoint = getMidPoint(currentPoint, p2: previousPoint)

        let subpath : CGMutablePathRef = CGPathCreateMutable()
        CGPathMoveToPoint(subpath, nil, mid1.x, mid1.y)
        CGPathAddQuadCurveToPoint(subpath, nil, previousPoint.x, previousPoint.y, mid2.x, mid2.y)

        let bounds : CGRect = CGPathGetBoundingBox(subpath)
        let drawBox : CGRect = CGRectInset(bounds, -2.0 * lineWidth, -2.0 * lineWidth)

        let newLine = line(newPath: subpath)
        pathArray.append(newLine)
        self.setNeedsDisplayInRect(drawBox)
    }
}

上面的代码按预期工作，除了我看到一个意外的结果。获取 bandbox 并设置 CGRectInset 的“draw box”会更改已绘制的其他路径的 lineColor：

我了解（某种程度上）为什么会发生这种情况，但无法找到解决此问题的方法。任何建议将不胜感激！

Answer 1

您想分别描边每条路径，如下所示：

override func drawRect(rect: CGRect) {
    let context : CGContextRef = UIGraphicsGetCurrentContext()

    // Set parameters in the context that are the same for all lines
    CGContextSetLineCap(context, kCGLineCapRound)
    CGContextSetAlpha(context, lineOpacity)

    for line in pathArray {
        // Set parameters in the context that are specific to this line
        CGContextSetLineWidth(context, line.structLineWidth)
        CGContextSetStrokeColorWithColor(context, line.structLineColor.CGColor)

        // Add the line's path to the context's current path:
        CGContextAddPath(context, line.structPath)
        // And stroke it.
        CGContextStrokePath(context)
        // (This has the side effect of clearing the context's current path.)
    }
}

CGContext 跟踪当前路径。将其视为您无法直接访问的CGMutablePath。您可以通过调用 CGContextAddPath 或许多其他函数来影响它。

CGContext 的当前路径实际上是不可见的，直到您通过调用 CGContextStrokePath 之类的函数来告诉上下文绘制它。那时，上下文使用当前路径和上下文中的其他值（描边颜色、alpha、线宽等）来确定要绘制的内容，并绘制出来。

您的代码将每一行的路径添加到当前路径中，因此您最终得到了包含多个断开连接的子路径的当前路径。然后你在最后调用了一次CGContextStrokePath，所有这些子路径都是使用你最近设置的参数值绘制的。所以你只看到了最后一行的宽度和颜色。