EditContext: characterboundsupdate event

Limited availability

This feature is not Baseline because it does not work in some of the most widely-used browsers.

Experimental: This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The characterboundsupdate event fires when the operating system needs to know the bounds of certain characters within editable text region of the EditContext object.

This happens when the operating system needs to display a platform-specific editing-related UI surface such as an Input Method Editor (IME) window.

When the characterboundsupdate event fires, you should calculate the character bounds for the text, and then call the EditContext.updateCharacterBounds() method to give the operating system the information it needs.

See the documentation of the updateCharacterBounds method for more information about when and how to use the characterboundsupdate event.

Syntax

Use the event name in methods like addEventListener(), or set an event handler property.

js
addEventListener("characterboundsupdate", (event) => {});

oncharacterboundsupdate = (event) => {};

Event type

A CharacterBoundsUpdateEvent. Inherits from Event.

Event properties

In addition to the properties listed below, properties from the parent interface, Event, are available.

CharacterBoundsUpdateEvent.rangeStart Read only

The offset of the first character within the editable region text for which the operating system needs the bounds.

CharacterBoundsUpdateEvent.rangeEnd Read only

The offset of the last character within the editable region text for which the operating system needs the bounds.

Examples

Updating the character bounds when needed

This example shows how to use the updateCharacterBounds method to update the character bounds in the EditContext of a canvas element when the operating system indicates that it requires the information. Note that the event listener callback is only called when using an IME window, or other platform-specific editing UI surfaces, to compose text.

html
<canvas id="editor-canvas"></canvas>
js
const FONT_SIZE = 40;
const FONT = `${FONT_SIZE}px Arial`;

const canvas = document.getElementById("editor-canvas");
const ctx = canvas.getContext("2d");
ctx.font = FONT;

const editContext = new EditContext();
canvas.editContext = editContext;

function computeCharacterBound(offset) {
  // Measure the width from the start of the text to the character.
  const widthBeforeChar = ctx.measureText(
    editContext.text.substring(0, offset),
  ).width;

  // Measure the character width.
  const charWidth = ctx.measureText(editContext.text[offset]).width;

  const charX = canvas.offsetLeft + widthBeforeChar;
  const charY = canvas.offsetTop;

  // Return a DOMRect representing the character bounds.
  return DOMRect.fromRect({
    x: charX,
    y: charY - FONT_SIZE,
    width: charWidth,
    height: FONT_SIZE,
  });
}

editContext.addEventListener("characterboundsupdate", (e) => {
  const charBounds = [];
  for (let offset = e.rangeStart; offset < e.rangeEnd; offset++) {
    charBounds.push(computeCharacterBound(offset));
  }

  console.log("The required character bounds are", charBounds);
  editContext.updateCharacterBounds(e.rangeStart, charBounds);
});

Specifications

Specification
EditContext API
# dom-editcontext-oncharacterboundsupdate

Browser compatibility

BCD tables only load in the browser