--- title: CanvasRenderingContext2D.drawFocusIfNeeded() slug: Web/API/CanvasRenderingContext2D/drawFocusIfNeeded translation_of: Web/API/CanvasRenderingContext2D/drawFocusIfNeeded ---
{{APIRef}}

CanvasRenderingContext2D.drawFocusIfNeeded() 是 Canvas 2D API 用来给当前路径或特定路径绘制焦点的方法，如果给定的元素获取了焦点。

语法

void ctx.drawFocusIfNeeded(element);
void ctx.drawFocusIfNeeded(path, element);

参数

element
是否需要设置焦点的元素。
path
{{domxref("Path2D")}} 路径。

示例

使用 drawFocusIfNeeded 方法

这是一段使用 drawFocusIfNeeded 方法的简单的代码片段。

HTML

<canvas id="canvas">
  <input id="button" type="range" min="1" max="12">
</canvas>

JavaScript

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var button = document.getElementById("button");

button.focus();

ctx.beginPath();
ctx.rect(10, 10, 30, 30);
ctx.drawFocusIfNeeded(button);

修改下面的代码并在线查看 canvas的变化：

Playable code
<canvas id="canvas" width="400" height="200" class="playable-canvas">
<input id="button" type="range" min="1" max="12">
</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">
ctx.beginPath();
ctx.rect(10, 10, 30, 30);
ctx.drawFocusIfNeeded(button);</textarea>

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var textarea = document.getElementById("code");
var button = document.getElementById("button");
var reset = document.getElementById("reset");
var edit = document.getElementById("edit");
var code = textarea.value;
button.focus();

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);

{{EmbedLiveSample('Playable_code', 700, 360)}}

规范描述

Specification Status Comment
{{SpecName('HTML WHATWG', "scripting.html#dom-context-2d-drawfocusifneeded", "CanvasRenderingContext2D.drawFocusIfNeeded")}} {{Spec2('HTML WHATWG')}} Initial definition

浏览器兼容性

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

兼容性注解

参见