Drawing text - Web APIs 编辑

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After having seen how to apply styles and colors in the previous chapter, we will now have a look at how to draw text onto the canvas.

Drawing text

The canvas rendering context provides two methods to render text:

fillText(text, x, y [, maxWidth])

Fills a given text at the given (x,y) position. Optionally with a maximum width to draw.

strokeText(text, x, y [, maxWidth])

Strokes a given text at the given (x,y) position. Optionally with a maximum width to draw.

A fillText example

The text is filled using the current fillStyle.

function draw() {
  var ctx = document.getElementById('canvas').getContext('2d');
  ctx.font = '48px serif';
  ctx.fillText('Hello world', 10, 50);
}

<canvas id="canvas" width="300" height="100"></canvas>

draw();

A strokeText example

The text is filled using the current strokeStyle.

function draw() {
  var ctx = document.getElementById('canvas').getContext('2d');
  ctx.font = '48px serif';
  ctx.strokeText('Hello world', 10, 50);
}

<canvas id="canvas" width="300" height="100"></canvas>

draw();

Styling text

In the examples above we are already making use of the font property to make the text a bit larger than the default size. There are some more properties which let you adjust the way the text gets displayed on the canvas:

font = value

The current text style being used when drawing text. This string uses the same syntax as the CSS font property. The default font is 10px sans-serif.

textAlign = value

Text alignment setting. Possible values: start, end, left, right or center. The default value is start.

textBaseline = value

Baseline alignment setting. Possible values: top, hanging, middle, alphabetic, ideographic, bottom. The default value is alphabetic.

direction = value

Directionality. Possible values: ltr, rtl, inherit. The default value is inherit.

These properties might be familiar to you, if you have worked with CSS before.

The following diagram from the WHATWG demonstrates the various baselines supported by the textBaseline property.

A textBaseline example

Edit the code below and see your changes update live in the canvas:

ctx.font = '48px serif';
ctx.textBaseline = 'hanging';
ctx.strokeText('Hello world', 0, 100);

Playable code

<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">
ctx.font = "48px serif";
ctx.textBaseline = "hanging";
ctx.strokeText("Hello world", 0, 100);</textarea>

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);

Advanced text measurements

In the case you need to obtain more details about the text, the following method allows you to measure it.

measureText()

Returns a TextMetrics object containing the width, in pixels, that the specified text will be when drawn in the current text style.

The following code snippet shows how you can measure a text and get its width.

function draw() {
  var ctx = document.getElementById('canvas').getContext('2d');
  var text = ctx.measureText('foo'); // TextMetrics object
  text.width; // 16;
}

Gecko-specific notes

In Gecko (the rendering engine of Firefox, Firefox OS and other Mozilla based applications), some prefixed APIs were implemented in earlier versions to draw text on a canvas. These are now deprecated and removed, and are no longer guaranteed to work.

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