TextMetrics | Web API

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特性
范例
规范
浏览器兼容性
另请参阅
相关话题

TextMetrics interface represents the dimensions of a piece of text in the canvas; a textMetrics() instance can be retrieved using the CanvasRenderingContext2D.measureText() 方法。

特性

TextMetrics.width 只读: 是 double giving the calculated width of a segment of inline text in CSS pixels. It takes into account the current font of the context.
TextMetrics.actualBoundingBoxLeft 只读: 是 double giving the distance from the alignment point given by the CanvasRenderingContext2D.textAlign property to the left side of the bounding rectangle of the given text, in CSS pixels. The distance is measured parallel to the baseline.
TextMetrics.actualBoundingBoxRight 只读: 是 double giving the distance from the alignment point given by the CanvasRenderingContext2D.textAlign property to the right side of the bounding rectangle of the given text, in CSS pixels. The distance is measured parallel to the baseline.
TextMetrics.fontBoundingBoxAscent 只读: 是 double giving the distance from the horizontal line indicated by the CanvasRenderingContext2D.textBaseline attribute to the top of the highest bounding rectangle of all the fonts used to render the text, in CSS pixels.
TextMetrics.fontBoundingBoxDescent 只读: 是 double giving the distance from the horizontal line indicated by the CanvasRenderingContext2D.textBaseline attribute to the bottom of the bounding rectangle of all the fonts used to render the text, in CSS pixels.
TextMetrics.actualBoundingBoxAscent 只读: 是 double giving the distance from the horizontal line indicated by the CanvasRenderingContext2D.textBaseline attribute to the top of the bounding rectangle used to render the text, in CSS pixels.
TextMetrics.actualBoundingBoxDescent 只读: 是 double giving the distance from the horizontal line indicated by the CanvasRenderingContext2D.textBaseline attribute to the bottom of the bounding rectangle used to render the text, in CSS pixels.
TextMetrics.emHeightAscent 只读: 是 double giving the distance from the horizontal line indicated by the CanvasRenderingContext2D.textBaseline property to the top of the em square in the line box, in CSS pixels.
TextMetrics.emHeightDescent 只读: 是 double giving the distance from the horizontal line indicated by the CanvasRenderingContext2D.textBaseline property to the bottom of the em square in the line box, in CSS pixels.
TextMetrics.hangingBaseline 只读: 是 double giving the distance from the horizontal line indicated by the CanvasRenderingContext2D.textBaseline property to the hanging baseline of the line box, in CSS pixels.
TextMetrics.alphabeticBaseline 只读: 是 double giving the distance from the horizontal line indicated by the CanvasRenderingContext2D.textBaseline property to the alphabetic baseline of the line box, in CSS pixels.
TextMetrics.ideographicBaseline 只读: 是 double giving the distance from the horizontal line indicated by the CanvasRenderingContext2D.textBaseline property to the ideographic baseline of the line box, in CSS pixels.

范例

Baselines illustrated

This example demonstrates the baselines the TextMetrics object holds. The default baseline is the alphabeticBaseline 。另请参阅 CanvasRenderingContext2D.textBaseline 特性。

HTML

<canvas id="canvas" width="550" height="500"></canvas>

JavaScript

const canvas = document.getElementById('canvas');
const ctx = canvas.getContext('2d');
const baselinesAboveAlphabetic = ['fontBoundingBoxAscent', 'actualBoundingBoxAscent',
                   'emHeightAscent', 'hangingBaseline'];
const baselinesBelowAlphabetic = ['ideographicBaseline', 'emHeightDescent',
                   'actualBoundingBoxDescent', 'fontBoundingBoxDescent'];
const baselines = [...baselinesAboveAlphabetic, ...baselinesBelowAlphabetic];
ctx.font = '25px serif';
ctx.strokeStyle = 'red';
baselines.forEach(function (baseline, index) {
  let text = 'Abcdefghijklmnop (' + baseline + ')';
  let textMetrics = ctx.measureText(text);
  let y = 50 + index * 50;
  ctx.beginPath();
  ctx.fillText(text, 0, y);
  let lineY = y - Math.abs(textMetrics[baseline]);
  if (baselinesBelowAlphabetic.includes(baseline)) {
    lineY = y + Math.abs(textMetrics[baseline]);
  }
  ctx.moveTo(0, lineY);
  ctx.lineTo(550, lineY);
  ctx.stroke();
});

结果

Measuring text width

When measuring the x-direction of a piece of text, the sum of actualBoundingBoxLeft and actualBoundingBoxRight can be wider than the width of the inline box ( width ), due to slanted/italic fonts where characters overhang their advance width.

It can therefore be useful to use the sum of actualBoundingBoxLeft and actualBoundingBoxRight as a more accurate way to get the absolute text width:

const canvas = document.getElementById('canvas');
const ctx = canvas.getContext('2d');
const text = 'Abcdefghijklmnop';
ctx.font = 'italic 50px serif';
const textMetrics = ctx.measureText(text);
console.log(textMetrics.width);
// 459.8833312988281
console.log(Math.abs(textMetrics.actualBoundingBoxLeft) +
            Math.abs(textMetrics.actualBoundingBoxRight));
// 462.8833333333333

规范

规范
HTML 实时标准 The definition of 'TextMetrics' in that specification.

浏览器兼容性

The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request. 更新 GitHub 上的兼容性数据

	桌面						移动
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
`TextMetrics`	Chrome 4	Edge 12	Firefox 1.5	IE 9	Opera 9	Safari 3.1	WebView Android Yes	Chrome Android Yes	Firefox Android 31	Opera Android Yes	Safari iOS Yes	Samsung Internet Android Yes
`actualBoundingBoxAscent`	Chrome 77 77 ? — 77 Disabled Disabled Until version 77 (exclusive): this feature is behind the `Experimental Web Platform Features` preference. To change preferences in Chrome, visit chrome://flags.	Edge 79	Firefox 74	IE 不支持 No	Opera ?	Safari Yes	WebView Android 77	Chrome Android 77 77 ? — 77 Disabled Disabled Until version 77 (exclusive): this feature is behind the `Experimental Web Platform Features` preference. To change preferences in Chrome, visit chrome://flags.	Firefox Android 不支持 No	Opera Android ?	Safari iOS Yes	Samsung Internet Android 12.0
`actualBoundingBoxDescent`	Chrome 77 77 ? — 77 Disabled Disabled Until version 77 (exclusive): this feature is behind the `Experimental Web Platform Features` preference. To change preferences in Chrome, visit chrome://flags.	Edge 79	Firefox 74	IE 不支持 No	Opera ?	Safari Yes	WebView Android 77	Chrome Android 77 77 ? — 77 Disabled Disabled Until version 77 (exclusive): this feature is behind the `Experimental Web Platform Features` preference. To change preferences in Chrome, visit chrome://flags.	Firefox Android 不支持 No	Opera Android ?	Safari iOS Yes	Samsung Internet Android 12.0
`actualBoundingBoxLeft`	Chrome 77 77 ? — 77 Disabled Disabled Until version 77 (exclusive): this feature is behind the `Experimental Web Platform Features` preference. To change preferences in Chrome, visit chrome://flags.	Edge 79	Firefox 74	IE 不支持 No	Opera ?	Safari Yes	WebView Android 77	Chrome Android 77 77 ? — 77 Disabled Disabled Until version 77 (exclusive): this feature is behind the `Experimental Web Platform Features` preference. To change preferences in Chrome, visit chrome://flags.	Firefox Android 不支持 No	Opera Android ?	Safari iOS Yes	Samsung Internet Android 12.0
`actualBoundingBoxRight`	Chrome 77 77 ? — 77 Disabled Disabled Until version 77 (exclusive): this feature is behind the `Experimental Web Platform Features` preference. To change preferences in Chrome, visit chrome://flags.	Edge 79	Firefox 74	IE 不支持 No	Opera ?	Safari Yes	WebView Android 77	Chrome Android 77 77 ? — 77 Disabled Disabled Until version 77 (exclusive): this feature is behind the `Experimental Web Platform Features` preference. To change preferences in Chrome, visit chrome://flags.	Firefox Android 不支持 No	Opera Android ?	Safari iOS Yes	Samsung Internet Android 12.0
`alphabeticBaseline`	Chrome Yes Disabled Yes Disabled Disabled This feature is behind the `Experimental Web Platform Features` preference. To change preferences in Chrome, visit chrome://flags.	Edge 79 Disabled 79 Disabled Disabled From version 79: this feature is behind the `Experimental Web Platform Features` preference.	Firefox 74	IE 不支持 No	Opera ?	Safari Yes	WebView Android 不支持 No	Chrome Android Yes Disabled Yes Disabled Disabled This feature is behind the `Experimental Web Platform Features` preference. To change preferences in Chrome, visit chrome://flags.	Firefox Android 不支持 No	Opera Android ?	Safari iOS Yes	Samsung Internet Android 不支持 No
`emHeightAscent`	Chrome Yes Disabled Yes Disabled Disabled This feature is behind the `Experimental Web Platform Features` preference. To change preferences in Chrome, visit chrome://flags.	Edge 79 Disabled 79 Disabled Disabled From version 79: this feature is behind the `Experimental Web Platform Features` preference.	Firefox 74	IE 不支持 No	Opera ?	Safari Yes	WebView Android 不支持 No	Chrome Android Yes Disabled Yes Disabled Disabled This feature is behind the `Experimental Web Platform Features` preference. To change preferences in Chrome, visit chrome://flags.	Firefox Android 不支持 No	Opera Android ?	Safari iOS Yes	Samsung Internet Android 不支持 No
`emHeightDescent`	Chrome Yes Disabled Yes Disabled Disabled This feature is behind the `Experimental Web Platform Features` preference. To change preferences in Chrome, visit chrome://flags.	Edge 79 Disabled 79 Disabled Disabled From version 79: this feature is behind the `Experimental Web Platform Features` preference.	Firefox 74	IE 不支持 No	Opera ?	Safari Yes	WebView Android 不支持 No	Chrome Android Yes Disabled Yes Disabled Disabled This feature is behind the `Experimental Web Platform Features` preference. To change preferences in Chrome, visit chrome://flags.	Firefox Android 不支持 No	Opera Android ?	Safari iOS Yes	Samsung Internet Android 不支持 No
`fontBoundingBoxAscent`	Chrome Yes Disabled Yes Disabled Disabled This feature is behind the `Experimental Web Platform Features` preference. To change preferences in Chrome, visit chrome://flags.	Edge 79 Disabled 79 Disabled Disabled From version 79: this feature is behind the `Experimental Web Platform Features` preference.	Firefox 74	IE 不支持 No	Opera ?	Safari Yes	WebView Android 不支持 No	Chrome Android Yes Disabled Yes Disabled Disabled This feature is behind the `Experimental Web Platform Features` preference. To change preferences in Chrome, visit chrome://flags.	Firefox Android 不支持 No	Opera Android ?	Safari iOS Yes	Samsung Internet Android 不支持 No
`fontBoundingBoxDescent`	Chrome Yes Disabled Yes Disabled Disabled This feature is behind the `Experimental Web Platform Features` preference. To change preferences in Chrome, visit chrome://flags.	Edge 79 Disabled 79 Disabled Disabled From version 79: this feature is behind the `Experimental Web Platform Features` preference.	Firefox 74	IE 不支持 No	Opera ?	Safari Yes	WebView Android 不支持 No	Chrome Android Yes Disabled Yes Disabled Disabled This feature is behind the `Experimental Web Platform Features` preference. To change preferences in Chrome, visit chrome://flags.	Firefox Android 不支持 No	Opera Android ?	Safari iOS 不支持 No	Samsung Internet Android 不支持 No
`hangingBaseline`	Chrome Yes Disabled Yes Disabled Disabled This feature is behind the `Experimental Web Platform Features` preference. To change preferences in Chrome, visit chrome://flags.	Edge 79 Disabled 79 Disabled Disabled From version 79: this feature is behind the `Experimental Web Platform Features` preference.	Firefox 74	IE 不支持 No	Opera ?	Safari Yes	WebView Android 不支持 No	Chrome Android Yes Disabled Yes Disabled Disabled This feature is behind the `Experimental Web Platform Features` preference. To change preferences in Chrome, visit chrome://flags.	Firefox Android 不支持 No	Opera Android ?	Safari iOS Yes	Samsung Internet Android 不支持 No
`ideographicBaseline`	Chrome Yes Disabled Yes Disabled Disabled This feature is behind the `Experimental Web Platform Features` preference. To change preferences in Chrome, visit chrome://flags.	Edge 79 Disabled 79 Disabled Disabled From version 79: this feature is behind the `Experimental Web Platform Features` preference.	Firefox 74	IE 不支持 No	Opera ?	Safari Yes	WebView Android 不支持 No	Chrome Android Yes Disabled Yes Disabled Disabled This feature is behind the `Experimental Web Platform Features` preference. To change preferences in Chrome, visit chrome://flags.	Firefox Android 不支持 No	Opera Android ?	Safari iOS Yes	Samsung Internet Android 不支持 No
`width`	Chrome 4	Edge 12	Firefox 1.5	IE 9	Opera 9	Safari 3.1	WebView Android Yes	Chrome Android Yes	Firefox Android 31	Opera Android Yes	Safari iOS 3.2	Samsung Internet Android Yes

图例

完整支持
不支持
兼容性未知 ?
实验。期望将来行为有所改变。
用户必须明确启用此特征。

另请参阅

Creator method in CanvasRenderingContext2D
<canvas> element and its associated interface, HTMLCanvasElement

元数据

最后修改： Mar 3, 2020

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