使用CGContextSetLineDash绘制一条虚线 [英] Drawing a dashed line with CGContextSetLineDash

问题描述

我正在尝试用 CGContextSetLineDash 画一条虚线。

I'm trying to draw a dashed line with CGContextSetLineDash.

这是我的代码：

float dashPhase = 0.0;
float dashLengths[] = {30, 30};
CGContextSetLineDash(context, dashPhase, dashLengths, 20.0);
self.previousPoint2 = self.previousPoint1;
self.previousPoint1 = previous;
self.currentPoint = current;

self.mid1 = [self pointBetween:self.previousPoint1 andPoint:self.previousPoint2];
self.mid2 = [self pointBetween:self.currentPoint andPoint:self.previousPoint1];

UIBezierPath* newPath = [UIBezierPath bezierPath];

[newPath moveToPoint:self.mid1];
[newPath addLineToPoint:self.mid2];
[newPath setLineWidth:self.brushSize];

但是，如果我画得很慢，它们的虚线就不会出现（见下图顶部）但是如果我快速绘制，它们会出现（见下图底部）。

However, if I draw slowly, they dashed lines do not appear (see top of image below) but if I draw quickly, they do appear (see bottom of image below).

为什么会发生这种情况？

Why is this happening?

推荐答案

您已设置 dashPhase = 0。，因此每次开始换行时，模式都会以30个单位绘制的段开头，然后是30单位未上漆的部分。如果线段很短，则整个线都会被绘制。

You have set dashPhase = 0., therefore each time you start a new line, the pattern starts with a 30 unit painted segment, followed by a 30 unit unpainted segment. If the line segments are short, the entire line will be painted.

因此要么使用单个路径，要么只追加线段，要么计算每个新线子路径 dashPhase 在哪里开始模式。

So either you use a single path, where you only append line segments, or you compute for each new subpath the dashPhase where to start the pattern.

（不应该是 CGContextSetLineDash 是 dashLengths [] 的长度，即 2 ？）

(Shouldn't the last parameter of CGContextSetLineDash be the length of the dashLengths[], i.e. 2 ?)

更新：正如我们在讨论中所发现的那样，问题的解决方案确实是将线段添加到最后一个贝塞尔曲线路径，只要user绘制相同的曲线：

UPDATE: As we figured out in the discussion, the solution to the problem was indeed to add line segments to the last bezier path as long as the user draws the same curve:

-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
    // ...
    // Compute nextPoint to draw ...
    UIBezierPath *lastPath = [self.paths lastObject]; 
    [lastPath addLineToPoint:self.nextPoint];
    // ...
}

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