Touch events

Surface

The touch-sensitive surface. This may be a screen or trackpad.

Touch point

A point of contact with the surface. This may be a finger (or elbow, ear, nose, whatever, but typically a finger) or stylus.

TouchEvent

Represents an event that occurs when the state of touches on the surface changes.

Touch

Represents a single point of contact between the user and the touch surface.

TouchList

Represents a group of touches; this is used when the user has, for example, multiple fingers on the surface at the same time.

This example tracks multiple touchpoints at a time, allowing the user to draw in a <canvas> with more than one finger at a time. It will only work on a browser that supports touch events.

When the page loads, the startup() function shown below will be called. This sets up all the event listeners for our <canvas> element so we can handle the touch events as they occur.

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);

Tracking new touches

We'll keep track of the touches in-progress.

const ongoingTouches = [];

When a touchstart event occurs, indicating that a new touch on the surface has occurred, the handleStart() function below is called.

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();
  }
}

This calls event.preventDefault() to keep the browser from continuing to process the touch event (this also prevents a mouse event from also being delivered). Then we get the context and pull the list of changed touch points out of the event's TouchEvent.changedTouches property.

After that, we iterate over all the Touch objects in the list, pushing them onto an array of active touchpoints and drawing the start point for the draw as a small circle; we're using a 4-pixel wide line, so a 4-pixel radius circle will show up neatly.

Drawing as the touches move

Each time one or more fingers move, a touchmove event is delivered, resulting in our handleMove() function being called. Its responsibility in this example is to update the cached touch information and to draw a line from the previous position to the current position of each touch.

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");
    }
  }
}

This iterates over the changed touches as well, but it looks in our cached touch information array for the previous information about each touch to determine the starting point for each touch's new line segment to be drawn. This is done by looking at each touch's Touch.identifier property. This property is a unique integer for each touch and remains consistent for each event during the duration of each finger's contact with the surface.

This lets us get the coordinates of the previous position of each touch and use the appropriate context methods to draw a line segment joining the two positions together.

After drawing the line, we call Array.splice() to replace the previous information about the touchpoint with the current information in the ongoingTouches array.

Handling the end of a touch

When the user lifts a finger off the surface, a touchend event is sent. We handle this by calling the handleEnd() function below. Its job is to draw the last line segment for each touch that ended and remove the touchpoint from the ongoing touch list.

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");
    }
  }
}

This is very similar to the previous function; the only real differences are that we draw a small square to mark the end and that when we call Array.splice(), we remove the old entry from the ongoing touch list, without adding in the updated information. The result is that we stop tracking that touchpoint.

Handling canceled touches

If the user's finger wanders into browser UI, or the touch otherwise needs to be canceled, the touchcancel event is sent, and we call the handleCancel() function below.

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
  }
}

Since the idea is to immediately abort the touch, we remove it from the ongoing touch list without drawing a final line segment.

This example uses two convenience functions that should be looked at briefly to help make the rest of the code more clear.

Selecting a color for each touch

To make each touch's drawing look different, the colorForTouch() function is used to pick a color based on the touch's unique identifier. This identifier is an opaque number, but we can at least rely on it differing between the currently-active touches.

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;
}

The result from this function is a string that can be used when calling <canvas> functions to set drawing colors. For example, for a Touch.identifier value of 10, the resulting string is "#a31".

Copying a touch object

Some browsers (mobile Safari, for one) re-use touch objects between events, so it's best to copy the properties you care about, rather than referencing the entire object.

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

Finding an ongoing touch

The ongoingTouchIndexById() function below scans through the ongoingTouches array to find the touch matching the given identifier then returns that touch's index into the array.

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
}

Showing what's going on

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

You can test this example on mobile devices by touching the box below.

This section provides additional tips on how to handle touch events in your web application.

Since calling preventDefault() on a touchstart or the first touchmove event of a series prevents the corresponding mouse events from firing, it's common to call preventDefault() on touchmove rather than touchstart. That way, mouse events can still fire and things like links will continue to work. Alternatively, some frameworks have taken to re-firing touch events as mouse events for this same purpose. (This example is oversimplified and may result in strange behavior. It is only intended as a guide.)

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);
}

One technique for preventing things like pinchZoom on a page is to call preventDefault() on the second touch in a series. This behavior is not well defined in the touch events spec and results in different behavior for different browsers (i.e., iOS will prevent zooming but still allow panning with both fingers; Android will allow zooming but not panning; Opera and Firefox currently prevent all panning and zooming.) Currently, it's not recommended to depend on any particular behavior in this case, but rather to depend on meta viewport to prevent zooming.