タッチイベント

html

<canvas id="canvas" width="600" height="600" style="border:solid black 1px;">
  Your browser does not support canvas element.
</canvas>
<br />
Log:
<pre id="log" style="border: 1px solid #ccc;"></pre>

css

#log {
  height: 200px;
  width: 600px;
  overflow: scroll;
}

js

function startup() {
  const el = document.getElementById("canvas");
  el.addEventListener("touchstart", handleStart);
  el.addEventListener("touchend", handleEnd);
  el.addEventListener("touchcancel", handleCancel);
  el.addEventListener("touchmove", handleMove);
  log("Initialized.");
}

document.addEventListener("DOMContentLoaded", startup);

js

const ongoingTouches = [];

js

function handleStart(evt) {
  evt.preventDefault();
  log("touchstart.");
  const el = document.getElementById("canvas");
  const ctx = el.getContext("2d");
  const touches = evt.changedTouches;

  for (let i = 0; i < touches.length; i++) {
    log(`touchstart: ${i}.`);
    ongoingTouches.push(copyTouch(touches[i]));
    const color = colorForTouch(touches[i]);
    log(`color of touch with id ${touches[i].identifier} = ${color}`);
    ctx.beginPath();
    ctx.arc(touches[i].pageX, touches[i].pageY, 4, 0, 2 * Math.PI, false); // a circle at the start
    ctx.fillStyle = color;
    ctx.fill();
  }
}

js

function handleMove(evt) {
  evt.preventDefault();
  const el = document.getElementById("canvas");
  const ctx = el.getContext("2d");
  const touches = evt.changedTouches;

  for (let i = 0; i < touches.length; i++) {
    const color = colorForTouch(touches[i]);
    const idx = ongoingTouchIndexById(touches[i].identifier);

    if (idx >= 0) {
      log(`continuing touch ${idx}`);
      ctx.beginPath();
      log(
        `ctx.moveTo( ${ongoingTouches[idx].pageX}, ${ongoingTouches[idx].pageY} );`,
      );
      ctx.moveTo(ongoingTouches[idx].pageX, ongoingTouches[idx].pageY);
      log(`ctx.lineTo( ${touches[i].pageX}, ${touches[i].pageY} );`);
      ctx.lineTo(touches[i].pageX, touches[i].pageY);
      ctx.lineWidth = 4;
      ctx.strokeStyle = color;
      ctx.stroke();

      ongoingTouches.splice(idx, 1, copyTouch(touches[i])); // swap in the new touch record
    } else {
      log("can't figure out which touch to continue");
    }
  }
}

js

function handleEnd(evt) {
  evt.preventDefault();
  log("touchend");
  const el = document.getElementById("canvas");
  const ctx = el.getContext("2d");
  const touches = evt.changedTouches;

  for (let i = 0; i < touches.length; i++) {
    const color = colorForTouch(touches[i]);
    let idx = ongoingTouchIndexById(touches[i].identifier);

    if (idx >= 0) {
      ctx.lineWidth = 4;
      ctx.fillStyle = color;
      ctx.beginPath();
      ctx.moveTo(ongoingTouches[idx].pageX, ongoingTouches[idx].pageY);
      ctx.lineTo(touches[i].pageX, touches[i].pageY);
      ctx.fillRect(touches[i].pageX - 4, touches[i].pageY - 4, 8, 8); // and a square at the end
      ongoingTouches.splice(idx, 1); // remove it; we're done
    } else {
      log("can't figure out which touch to end");
    }
  }
}

js

function handleCancel(evt) {
  evt.preventDefault();
  log("touchcancel.");
  const touches = evt.changedTouches;

  for (let i = 0; i < touches.length; i++) {
    let idx = ongoingTouchIndexById(touches[i].identifier);
    ongoingTouches.splice(idx, 1); // remove it; we're done
  }
}

js

function colorForTouch(touch) {
  let r = touch.identifier % 16;
  let g = Math.floor(touch.identifier / 3) % 16;
  let b = Math.floor(touch.identifier / 7) % 16;
  r = r.toString(16); // make it a hex digit
  g = g.toString(16); // make it a hex digit
  b = b.toString(16); // make it a hex digit
  const color = `#${r}${g}${b}`;
  return color;
}

js

function copyTouch({ identifier, pageX, pageY }) {
  return { identifier, pageX, pageY };
}

js

function ongoingTouchIndexById(idToFind) {
  for (let i = 0; i < ongoingTouches.length; i++) {
    const id = ongoingTouches[i].identifier;

    if (id === idToFind) {
      return i;
    }
  }
  return -1; // not found
}

js

function log(msg) {
  const container = document.getElementById("log");
  container.textContent = `${msg} \n${container.textContent}`;
}

js

function onTouch(evt) {
  evt.preventDefault();
  if (
    evt.touches.length > 1 ||
    (evt.type === "touchend" && evt.touches.length > 0)
  )
    return;

  const newEvt = document.createEvent("MouseEvents");
  let type = null;
  let touch = null;

  switch (evt.type) {
    case "touchstart":
      type = "mousedown";
      touch = evt.changedTouches[0];
      break;
    case "touchmove":
      type = "mousemove";
      touch = evt.changedTouches[0];
      break;
    case "touchend":
      type = "mouseup";
      touch = evt.changedTouches[0];
      break;
  }

  newEvt.initMouseEvent(
    type,
    true,
    true,
    evt.originalTarget.ownerDocument.defaultView,
    0,
    touch.screenX,
    touch.screenY,
    touch.clientX,
    touch.clientY,
    evt.ctrlKey,
    evt.altKey,
    evt.shiftKey,
    evt.metaKey,
    0,
    null,
  );
  evt.originalTarget.dispatchEvent(newEvt);
}