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canvas_editContext.html
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canvas_editContext.html
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<head>
<style>
canvas:hover {
cursor: default;
}
</style>
</head>
(Note: EditContext can be enabled through <b>#enable-experimental-web-platform-features</b> in about://flags)
<br><br>
Click somewhere in the Canvas below, start typing Chinese/Japanese/Korean,<br>
(and change candidate characters) to see how EditContext works with Canvas to handle IME input: <br><br>
To try the "phrase" mode in Japanese IME (on Windows),<br>
1) type "konosushihaoishiidesu" and press "space".<br>
2) use left/right key to move between phrases and press "space" to open the candidate window<br>
3) use up/down key to choose candidate, press "Esc" to close the candidate window or press "Enter" to commit<br><br>
<canvas id="myCanvas" width="400" height="200" style="border:1px solid #000000;">
</canvas>
<script>
let canvas = document.getElementById("myCanvas");
let canvasContext2D = canvas.getContext("2d");
canvasContext2D.font = "30px Arial";
let charWidth = 30; // The width/height of the (Japanese) character is 30px.
let charHeight = 30;
let caretLength = 30;
let editContext = new EditContext();
canvas.editContext = editContext;
let anchorX = 0; // the anchor point of the composition
let anchorY = 0;
let selection = 0; // selection i.e the index of the caret position
function renderCaret(x, y) {
canvasContext2D.beginPath();
canvasContext2D.moveTo(x + 2, y + 9);
canvasContext2D.lineTo(x + 2, y - caretLength);
canvasContext2D.strokeStyle = 'green';
canvasContext2D.stroke();
}
function renderUnderlineDecoration(rangeStart, rangeEnd, style, thickness) {
let lineStartX = anchorX + rangeStart * charWidth;
let offsetX = 2; // space between underlines of different phrases
let lineEndX = anchorX + rangeEnd * charWidth - offsetX;
let offsetY = 7; // space between chars and the underline
let lineY = anchorY + offsetY;
let thickWidth = 3;
let thinWidth = 2;
// Japanese IME returns 'Squiggle' while Chinese returns 'Dotted' for active composition.
// For simplicity we draw dotted line for both.
if (style == 'Squiggle' || style == 'Dotted') {
canvasContext2D.setLineDash([1, 1]); // dotted line pattern
canvasContext2D.beginPath();
canvasContext2D.moveTo(lineStartX,lineY);
canvasContext2D.lineTo(lineEndX, lineY);
canvasContext2D.stroke();
canvasContext2D.setLineDash([]); // reset line pattern
} else if (style == 'Solid') { // Draw solid lines for "phrases" in Japenese IME
canvasContext2D.lineWidth = (thickness == 'Thick')? thickWidth : thinWidth;
canvasContext2D.beginPath();
canvasContext2D.moveTo(lineStartX, lineY);
canvasContext2D.lineTo(lineEndX, lineY);
canvasContext2D.stroke();
canvasContext2D.lineWidth = 1; // reset line width.
}
}
function render() {
canvasContext2D.clearRect(0,0, canvas.width, canvas.height);
// rener texts in EditContext's buffer
canvasContext2D.strokeText(editContext.text, anchorX, anchorY);
let caretX = anchorX + selection * charWidth;
let caretY = anchorY;
renderCaret(caretX, caretY);
// update layout info through EditContext so IME can show candidate window in the correct position.
let controlBound = canvas.getBoundingClientRect();
selectoinClientX = caretX + canvas.offsetLeft;
selectoinClientY = caretY + canvas.offsetTop;
let selectionBound = DOMRect.fromRect({x:selectoinClientX, y:selectoinClientY, width:1, height:10});
controlBound = scaleDOMRect(controlBound, window.devicePixelRatio);
selectionBound = scaleDOMRect(selectionBound, window.devicePixelRatio);
editContext.updateBounds(controlBound, selectionBound);
}
// when EditContext is active (i.e. the associated element is focused)
// it'll receive textupdate events when there is user input.
editContext.addEventListener("textupdate", e => {
console.log(`editcontext.text:[${editContext.text}]`);
console.log(`textupdate:[${e.updateText}], composition range (${e.updateRangeStart}, ${e.updateRangeEnd}), selection (${e.newSelectionStart}, ${e.newSelectionEnd})`);
selection = e.newSelectionStart;
render();
});
function computeCharacterBound(offset) {
let margin = 10;
let charX = canvas.offsetLeft + anchorX + offset * charWidth;
let charY = canvas.offsetTop + anchorY;
return DOMRect.fromRect({x:charX, y:charY-charHeight, width:charWidth, height:charHeight+margin});
}
editContext.addEventListener("characterboundsupdate", e => {
console.log(`editcontext.text:[${editContext.text}]`);
console.log(`characterboundsupdate: composition range (${e.rangeStart}, ${e.rangeEnd})`);
let rangeStart = e.rangeStart;
let rangeEnd = e.rangeEnd;
// Calculate bounds for each character in the range.
if (rangeEnd > rangeStart) {
let charBounds = [];
for (offset = rangeStart; offset < rangeEnd; offset++) {
let bound = computeCharacterBound(offset);
bound = scaleDOMRect(bound, window.devicePixelRatio);
charBounds.push(bound);
}
editContext.updateCharacterBounds(rangeStart, charBounds);
console.log("charcterBoundsRangeStart:");
console.log(editContext.characterBoundsRangeStart);
console.log("charcterBounds:");
console.log(editContext.characterBounds());
}
});
function handleTextFormat(textFormat) {
console.log(`textformatupdate: range(${textFormat.rangeStart}, ${textFormat.rangeEnd}),${textFormat.underlineColor}, ${textFormat.backgroundColor}, ${textFormat.textColor},${textFormat.underlineThickness}, ${textFormat.underlineStyle}`);
renderUnderlineDecoration(textFormat.rangeStart, textFormat.rangeEnd, textFormat.underlineStyle, textFormat.underlineThickness);
}
editContext.addEventListener("textformatupdate", e => {
let textFormats = e.getTextFormats();
console.log(`textformatupdate: size(${textFormats.length})`);
textFormats.forEach((textFormat) => {
handleTextFormat(textFormat);
});
});
canvas.addEventListener('click', function(event) {
let elemLeft = canvas.offsetLeft + canvas.clientLeft;
let elemTop = canvas.offsetTop + canvas.clientTop;
anchorX = event.pageX - elemLeft;
anchorY = event.pageY - elemTop;
render();
});
// To handle zoom levels
function scaleDOMRect(rect, scale) {
rect.x = rect.x * scale;
rect.y = rect.y * scale;
rect.width = rect.width * scale;
rect.height = rect.height * scale;
return rect;
}
</script>