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---
title: CanvasRenderingContext2D.lineDashOffset
slug: Web/API/CanvasRenderingContext2D/lineDashOffset
tags:
- Canvas
- CanvasRenderingContext2D
translation_of: Web/API/CanvasRenderingContext2D/lineDashOffset
---
<div>{{APIRef}}</div>
<p><code><strong>CanvasRenderingContext2D</strong></code><strong><code>.lineDashOffset</code></strong> 是 Canvas 2D API 设置虚线偏移量的属性，例如可以实现 “<a href="http://en.wikipedia.org/wiki/Marching_ants">蚂蚁线</a>“ 的效果。</p>
<h2 id="语法">语法</h2>
<pre class="syntaxbox"><var><em>ctx</em>.lineDashOffset = value;</var>
</pre>
<dl>
<dt><code>value</code></dt>
<dd>偏移量是float精度的数字。 初始值为 <code>0.0。</code></dd>
</dl>
<h2 id="示例">示例</h2>
<h3 id="使用_lineDashOffset_属性">使用 <code>lineDashOffset</code> 属性</h3>
<p>这是一段简单的代码片段，使用 <code>lineDashOffset</code> 属性绘制虚线。</p>
<h4 id="HTML">HTML</h4>
<pre class="brush: html"><canvas id="canvas"></canvas>
</pre>
<h4 id="JavaScript">JavaScript</h4>
<pre class="brush: js; highlight:[5]">var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
ctx.setLineDash([4, 16]);
ctx.lineDashOffset = 2;
ctx.beginPath();
ctx.moveTo(0,100);
ctx.lineTo(400, 100);
ctx.stroke();
</pre>
<p>修改下面的代码并在线查看 canvas 变化：</p>
<div class="hidden">
<h6 id="Playable_code">Playable code</h6>
<pre class="brush: html"><canvas id="canvas" width="400" height="200" class="playable-canvas"></canvas>
<div class="playable-buttons">
<input id="edit" type="button" value="Edit" />
<input id="reset" type="button" value="Reset" />
</div>
<textarea id="code" class="playable-code" style="height: 120px">
ctx.setLineDash([4, 16]);
ctx.lineDashOffset = 2;
ctx.beginPath();
ctx.moveTo(0,100);
ctx.lineTo(400, 100);
ctx.stroke();</textarea>
</pre>
<pre class="brush: js">var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var textarea = document.getElementById("code");
var reset = document.getElementById("reset");
var edit = document.getElementById("edit");
var code = textarea.value;
function drawCanvas() {
ctx.clearRect(0, 0, canvas.width, canvas.height);
eval(textarea.value);
}
reset.addEventListener("click", function() {
textarea.value = code;
drawCanvas();
});
edit.addEventListener("click", function() {
textarea.focus();
})
textarea.addEventListener("input", drawCanvas);
window.addEventListener("load", drawCanvas);
</pre>
</div>
<p>{{ EmbedLiveSample('Playable_code', 700, 380) }}</p>
<h3 id="Marching_ants" name="Marching_ants">“蚂蚁线”</h3>
<p>”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。</p>
<div class="hidden">
<h6 id="HTML_2">HTML</h6>
<pre class="brush: html"><canvas id="canvas" class="playable-canvas"></canvas></pre>
</div>
<pre class="brush: js">var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var 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 > 16) {
offset = 0;
}
draw();
setTimeout(march, 20);
}
march();</pre>
<p>{{ EmbedLiveSample('Marching_ants', 700, 200) }}</p>
<h2 id="规范描述">规范描述</h2>
<table class="standard-table">
<tbody>
<tr>
<th scope="col">Specification</th>
<th scope="col">Status</th>
<th scope="col">Comment</th>
</tr>
<tr>
<td>{{SpecName('HTML WHATWG', "scripting.html#dom-context-2d-linedashoffset", "CanvasRenderingContext2D.lineDashOffset")}}</td>
<td>{{Spec2('HTML WHATWG')}}</td>
<td></td>
</tr>
</tbody>
</table>
<h2 id="浏览器兼容性">浏览器兼容性</h2>
<p>{{Compat("api.CanvasRenderingContext2D.lineDashOffset")}}</p>
<h2 id="参见">参见</h2>
<ul>
<li>接口定义， {{domxref("CanvasRenderingContext2D")}}</li>
<li>{{domxref("CanvasRenderingContext2D.getLineDash()")}}</li>
<li>{{domxref("CanvasRenderingContext2D.setLineDash()")}}</li>
<li><a href="https://wiki.developer.mozilla.org/en-US/docs/Web/API/Canvas_API/Tutorial/Applying_styles_and_colors">Applying styles and color</a></li>
</ul>
|
