---
title: CanvasRenderingContext2D.arcTo()
slug: Web/API/CanvasRenderingContext2D/arcTo
translation_of: Web/API/CanvasRenderingContext2D/arcTo
---
<div>{{APIRef}}</div>

<p><code><strong>CanvasRenderingContext2D</strong></code><strong><code>.arcTo()</code></strong> 是 Canvas 2D API 根据控制点和半径绘制圆弧路径，使用当前的描点(前一个moveTo或lineTo等函数的止点)。根据当前描点与给定的控制点1连接的直线，和控制点1与控制点2连接的直线，作为使用指定半径的圆的<strong>切线</strong>，画出两条切线之间的弧线路径。</p>

<h2 id="语法">语法</h2>

<pre class="syntaxbox">void <var><em>ctx</em>.arcTo(x1, y1, x2, y2, radius);</var>
</pre>

<h3 id="Parameters">Parameters</h3>

<dl>
 <dt><code>x1</code></dt>
 <dd>第一个控制点的 x 轴坐标。</dd>
 <dt><code>y1</code></dt>
 <dd>第一个控制点的 y 轴坐标。</dd>
 <dt><code>x2</code></dt>
 <dd>第二个控制点的 x 轴坐标。</dd>
 <dt><code>y2</code></dt>
 <dd>第二个控制点的 y 轴坐标。</dd>
 <dt><code>radius</code></dt>
 <dd>圆弧的半径。</dd>
</dl>

<h2 id="示例">示例</h2>

<h3 id="Using_the_arc_method" name="Using_the_arc_method">使用 <code>arcTo</code> 方法</h3>

<p>这是一段绘制圆弧的简单的代码片段。 基础点是蓝色的，两个控制点是红色的。</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:[6]">var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");

ctx.setLineDash([])
ctx.beginPath();
ctx.moveTo(150, 20);
ctx.arcTo(150,100,50,20,30);
ctx.stroke();

ctx.fillStyle = 'blue';
// base point
ctx.fillRect(150, 20, 10, 10);

ctx.fillStyle = 'red';
// control point one
ctx.fillRect(150, 100, 10, 10);
// control point two
ctx.fillRect(50, 20, 10, 10);
//
ctx.setLineDash([5,5])
ctx.moveTo(150, 20);
ctx.lineTo(150,100);
ctx.lineTo(50, 20);
ctx.stroke();
ctx.beginPath();
ctx.arc(120,38,30,0,2*Math.PI);
ctx.stroke();
</pre>

<p>{{ EmbedLiveSample('Using_the_arc_method', 315, 165) }}</p>

<h3 id="Trying_the_arcTo_parameters" name="Trying_the_arcTo_parameters">尝试 <code>arcTo</code> 参数</h3>

<p>修改下面的代码并在线查看 canvas 的变化：</p>

<div class="hidden">
<pre class="brush: html">&lt;canvas id="canvas" class="playable-canvas" height="200" width="400"&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"&gt;
ctx.setLineDash([])
ctx.beginPath();
ctx.moveTo(150, 20);
ctx.arcTo(150,100,50,20,30);
ctx.stroke();

ctx.fillStyle = 'blue';
// base point
ctx.fillRect(150, 20, 10, 10);

ctx.fillStyle = 'red';
// control point one
ctx.fillRect(150, 100, 10, 10);
// control point two
ctx.fillRect(50, 20, 10, 10);
//
ctx.setLineDash([5,5])
ctx.moveTo(150, 20);
ctx.lineTo(150,100);
ctx.lineTo(50, 20);
ctx.stroke();
ctx.beginPath();
ctx.arc(120,38,30,0,2*Math.PI);
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('Trying_the_arcTo_parameters', 700, 360) }}</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-arcto", "CanvasRenderingContext2D.arcTo")}}</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>

{{Compat("api.CanvasRenderingContext2D.arcTo")}}

<h2 id="参见">参见</h2>

<ul>
 <li>接口定义， {{domxref("CanvasRenderingContext2D")}}</li>
</ul>