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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">&lt;canvas id="canvas"&gt;&lt;/canvas&gt;
+</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 style="display: none;">
+<h6 id="Playable_code">Playable code</h6>
+
+<pre class="brush: html">&lt;canvas id="canvas" width="400" height="200" class="playable-canvas"&gt;&lt;/canvas&gt;
+&lt;div class="playable-buttons"&gt;
+  &lt;input id="edit" type="button" value="Edit" /&gt;
+  &lt;input id="reset" type="button" value="Reset" /&gt;
+&lt;/div&gt;
+&lt;textarea id="code" class="playable-code" style="height: 120px"&gt;
+ctx.setLineDash([4, 16]);
+ctx.lineDashOffset = 2;
+
+ctx.beginPath();
+ctx.moveTo(0,100);
+ctx.lineTo(400, 100);
+ctx.stroke();&lt;/textarea&gt;
+</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">&lt;canvas id="canvas" class="playable-canvas"&gt;&lt;/canvas&gt;</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 &gt; 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>
+
+<div class="hidden">
+<p>此页面上的兼容性表是根据结构化数据生成的。 如果您想提供数据，请查看<a href="https://github.com/mdn/browser-compat-data">https://github.com/mdn/browser-compat-data</a> 并向我们发送请求请求。</p>
+</div>
+
+<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>