CanvasRenderingContext2D:lineDashOffset 属性

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.

Canvas 2D API 的 CanvasRenderingContext2D.lineDashOffset 属性用于设置虚线偏移量或者称为“相位”。

备注: 要绘制线条,需调用 stroke() 方法。

一个浮点数,指定线条虚线的偏移量。默认值为 0.0

示例

偏移虚线

此示例绘制了两条虚线。第一条没有虚线偏移量。第二条虚线的偏移量为 4。

HTML

html
<canvas id="canvas"></canvas>

JavaScript

js
const canvas = document.getElementById("canvas");
const ctx = canvas.getContext("2d");

ctx.setLineDash([4, 16]);

// 无偏移量的虚线
ctx.beginPath();
ctx.moveTo(0, 50);
ctx.lineTo(300, 50);
ctx.stroke();

// 偏移量为 4 的虚线
ctx.beginPath();
ctx.strokeStyle = "red";
ctx.lineDashOffset = 4;
ctx.moveTo(0, 100);
ctx.lineTo(300, 100);
ctx.stroke();

结果

带有偏移的虚线用红色绘制。

蚂蚁线

蚂蚁线”效果是一种动画技巧,经常出现在计算机绘图程序的套索工具中。它能帮助用户根据图片背景动态变化的边界来区分选择的边界。

js
const canvas = document.getElementById("canvas");
const ctx = canvas.getContext("2d");
let offset = 0;

function draw() {
  ctx.clearRect(0, 0, canvas.width, canvas.height);
  ctx.setLineDash([4, 2]);
  ctx.lineDashOffset = offset;
  ctx.strokeRect(10, 10, 100, 100);
}

function march() {
  offset++;
  if (offset > 5) {
    offset = 0;
  }
  draw();
  setTimeout(march, 20);
}

march();

规范

Specification
HTML
# dom-context-2d-linedashoffset-dev

浏览器兼容性

Report problems with this compatibility data on GitHub
desktopmobile
Chrome
Edge
Firefox
Opera
Safari
Chrome Android
Firefox for Android
Opera Android
Safari on iOS
Samsung Internet
WebView Android
WebView on iOS
lineDashOffset

Legend

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Full support
Full support
Uses a non-standard name.
Has more compatibility info.

参见