Touch events

터치를 기반으로 한 양질의 서비스를 제공하기 위해, Touch Events(터치이벤트)는 터치로 인한 움직임을 감지할 능력을 제공합니다.

터치 이벤트 인터페이스는 상대적으로 low-level API이며 multi-touch interaction등의 다양한 동작을 특정해 트리거 할 수 있습니다. multi-touch interaction은 한 손가락이 터치패드에 처음 닫는 순간 시작합니다. 이후 다른 손가락들이 터치패드에 닿고 선택적으로 터치패드를 가로지를 수도 있습니다. interaction은 손가락들이 터치패드에서 떨어지면 끝납니다. interaction동안에 어플리케이션은 touch start, move, end 이벤트들을 받습니다.

터치 이벤트는 동시에 여러 손가락으로 동시에 혹은 여러 지점에 터치 할 수 있다는 것만 제외하면 마우스 이벤트와 유사합니다. TouchEvent (en-US) 인터페이스는 현재 액션과 터치 지점을 캡슐화 합니다. single touch로 대표되는 Touch (en-US) 인터페이스는 브라우저의 뷰포트와 관련된 접촉 지점의 위치와 같은 것들의 정보등을 포함합니다.

Definitions

Surface
터치 스크린, 터치 패드등을 포함합니다.
Touch point
손가락이나 터치 펜을 이용해 터치한 지점을 말합니다.

Interfaces

TouchEvent (en-US)
터치나 surface 위에서의 움직임들 입니다.
Touch (en-US)
touch surface위의 한 지점에 접촉할 때 발생합니다.
TouchList (en-US)
다중의 터치가 동시에 일어 났을 때 발생합니다.

Example

This example tracks multiple touch points 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.

Note: The text below uses the term "finger" when describing the contact with the surface, but it could, of course, also be a stylus or other contact method.

Create a canvas

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

Setting up the event handlers

When the page loads, the startup() function shown below should be called by our <body> element's onload attribute (but in the example we use a button to trigger it, due to limitations of the MDN live example system).

function startup() {
  var el = document.getElementsByTagName("canvas")[0];
  el.addEventListener("touchstart", handleStart, false);
  el.addEventListener("touchend", handleEnd, false);
  el.addEventListener("touchcancel", handleCancel, false);
  el.addEventListener("touchmove", handleMove, false);
  log("initialized.");
}

This simply sets up all the event listeners for our <canvas> element so we can handle the touch events as they occur.

Tracking new touches

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

var ongoingTouches = [];

When a touchstart (en-US) 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.");
  var el = document.getElementsByTagName("canvas")[0];
  var ctx = el.getContext("2d");
  var touches = evt.changedTouches;

  for (var i = 0; i < touches.length; i++) {
    log("touchstart:" + i + "...");
    ongoingTouches.push(copyTouch(touches[i]));
    var color = colorForTouch(touches[i]);
    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();
    log("touchstart:" + i + ".");
  }
}

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 (en-US) property.

After that, we iterate over all the Touch (en-US) objects in the list, pushing them onto an array of active touch points 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 moves, a touchmove (en-US) 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();
  var el = document.getElementsByTagName("canvas")[0];
  var ctx = el.getContext("2d");
  var touches = evt.changedTouches;

  for (var i = 0; i < touches.length; i++) {
    var color = colorForTouch(touches[i]);
    var 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
      log(".");
    } 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 in order 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 (en-US) 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 touch point 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 (en-US) event is sent. We handle both of these the same way: by calling the handleEnd() function below. Its job is to draw the last line segment for each touch that ended and remove the touch point from the ongoing touch list.

function handleEnd(evt) {
  evt.preventDefault();
  log("touchend");
  var el = document.getElementsByTagName("canvas")[0];
  var ctx = el.getContext("2d");
  var touches = evt.changedTouches;

  for (var i = 0; i < touches.length; i++) {
    var color = colorForTouch(touches[i]);
    var 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 simply remove the old entry from the ongoing touch list, without adding in the updated information. The result is that we stop tracking that touch point.

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.");
  var touches = evt.changedTouches;

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

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

Convenience functions

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

In order 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) {
  var r = touch.identifier % 16;
  var g = Math.floor(touch.identifier / 3) % 16;
  var 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
  var color = "#" + r + g + b;
  log("color for touch with identifier " + touch.identifier + " = " + color);
  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 (en-US) value of 10, the resulting string is "#aaa".

Copying a touch object

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

function copyTouch(touch) {
  return { identifier: touch.identifier, pageX: touch.pageX, pageY: touch.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 (var i = 0; i < ongoingTouches.length; i++) {
    var id = ongoingTouches[i].identifier;

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

Showing what's going on

function log(msg) {
  var p = document.getElementById('log');
  p.innerHTML = msg + "\n" + p.innerHTML;
}

If your browser supports it, you can see it live.

jsFiddle example

Additional tips

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

Handling clicks

Since calling preventDefault() on a touchstart (en-US) or the first touchmove (en-US) event of a series prevents the corresponding mouse events from firing, it's common to call preventDefault() on touchmove (en-US) rather than touchstart (en-US). That way, mouse events can still fire and things like links will continue to work. Alternatively, some frameworks have taken to refiring 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;

  var newEvt = document.createEvent("MouseEvents");
  var type = null;
  var 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);
}

Calling preventDefault() only on a second touch

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.

Specifications

Specification Status Comment
Touch Events – Level 2
The definition of 'Touch' in that specification.
Draft Added radiusX, radiusY, rotationAngle, force properties
Touch Events
The definition of 'Touch' in that specification.
Recommendation Initial definition.

Browser compatibility

BCD tables only load in the browser

Touch events are typically available on devices with a touch screen, but many browsers make the touch events API unavailable on all desktop devices, even those with touch screens.

The reason for this is that some websites use the availability of parts of the touch events API as an indicator that the browser is running on a mobile device. If the touch events API is available, these websites will assume a mobile device and serve mobile-optimized content. This may then provide a poor experience for users of desktop devices that have touch screens.

To support both touch and mouse across all types of devices, use pointer events instead.