Using Pointer Events
This document demonstrates how to use pointer events
and <canvas>
to build a multi-touch enabled drawing application. This example is identical to the application described in the Touch events Overview
except this example uses the pointer events
input event model (instead of touch events
. Another difference is that because pointer events are pointer device agnostic, the application accepts both touch input, pen and mouse input, the latter two for free.
This application will only work on a browser that supports pointer events
.
A live version of this application is available on jsfiddle. The source code is available on Github and pull requests and bug reports are welcome.
Definitions
- 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 or mouse.
Example
Create a canvas
The touch-action
property is set to none
to prevent the browser from applying its default touch behavior to the application.
<canvas id="canvas" width="600" height="600" style="border:solid black 1px; touch-action:none"> 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("pointerdown", handleStart, false); el.addEventListener("pointerup", handleEnd, false); el.addEventListener("pointercancel", handleCancel, false); el.addEventListener("pointermove", 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 = new Array();
When a pointerdown
event occurs, indicating that a new touch on the surface has occurred, the handleStart()
function below is called.
function handleStart(evt) { log("pointerdown."); var el = document.getElementsByTagName("canvas")[0]; var ctx = el.getContext("2d"); log("pointerdown: id = " + evt.pointerId); ongoingTouches.push(copyTouch(evt)); var color = colorForTouch(evt); ctx.beginPath(); ctx.arc(touches[i].pageX, touches[i].pageY, 4, 0, 2 * Math.PI, false); // a circle at the start ctx.arc(evt.clientX, evt.clientY, 4, 0, 2 * Math.PI, false); // a circle at the start ctx.fillStyle = color; ctx.fill(); }
After storing some of the event's processing in the ongoingTouches
for later processing, the start point is drawn 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 pointers move
Each time one or more pointers moves, a pointermove
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) { var el = document.getElementsByTagName("canvas")[0]; var ctx = el.getContext("2d"); var color = colorForTouch(evt); var idx = ongoingTouchIndexById(evt.pointerId); log("continuing touch: idx = " + idx); if (idx >= 0) { ctx.beginPath(); log("ctx.moveTo(" + ongoingTouches[idx].pageX + ", " + ongoingTouches[idx].pageY + ");"); ctx.moveTo(ongoingTouches[idx].pageX, ongoingTouches[idx].pageY); log("ctx.lineTo(" + evt.clientX + ", " + evt.clientY + ");"); ctx.lineTo(evt.clientX, evt.clientY); ctx.lineWidth = 4; ctx.strokeStyle = color; ctx.stroke(); ongoingTouches.splice(idx, 1, copyTouch(evt)); // swap in the new touch record log("."); } else { log("can't figure out which touch to continue: idx = " + idx); } }
This function 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 PointerEvent.pointerId
property. This property is a unique integer for each pointer event, 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 pointerup
event is sent. We handle this event by calling the handleEnd()
function below. Its job is to draw the last line segment for the touch that ended and remove the touch point from the ongoing touch list.
function handleEnd(evt) { log("pointerup"); var el = document.getElementsByTagName("canvas")[0]; var ctx = el.getContext("2d"); var color = colorForTouch(evt); var idx = ongoingTouchIndexById(evt.pointerId); if (idx >= 0) { ctx.lineWidth = 4; ctx.fillStyle = color; ctx.beginPath(); ctx.moveTo(ongoingTouches[idx].pageX, ongoingTouches[idx].pageY); ctx.lineTo(evt.clientX, evt.clientY); ctx.fillRect(evt.clientX - 4, evt.clientY - 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 pointercancel
event is sent, and we call the handleCancel()
function below.
function handleCancel(evt) { log("pointercancel: id = " + evt.pointerId); var idx = ongoingTouchIndexById(evt.pointerId); 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.pointerId % 16; var g = Math.floor(touch.pointerId / 3) % 16; var b = Math.floor(touch.pointerId / 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.pointerId + " = " + 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 PointerEvent.pointerId
value of 10, the resulting string is "#aaa".
Copying a touch object
Some browsers may 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.pointerId, pageX: touch.clientX, pageY: touch.clientY }; }
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; }
Specifications
Specification | Status | Comment |
---|---|---|
Pointer Events – Level 2 The definition of 'PointerEvent' in that specification. |
Editor's Draft | Non-stable version. |
Pointer Events The definition of 'PointerEvent' in that specification. |
Recommendation | Initial definition. |
Browser compatibility
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari (WebKit) |
---|---|---|---|---|---|
Basic support | Not supported[2] | (Yes) [1] | 10ms 11 |
Not supported | Not supported |
Feature | Android | Android Webview | Chrome for Android | Firefox Mobile (Gecko) | Firefox OS | IE Mobile | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|---|---|---|
Basic support | Not supported | Not supported | Not supported | Not supported | Not supported | 10 | Not supported | Not supported |
[1] This feature is currently hidden behind a flag — to enable it and experiment, go to about:config and enable dom.w3c_pointer_events.enabled.
[2] In development, see: crbug.com/196799.
See also
License
© 2016 Mozilla Contributors
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-us/docs/web/api/pointer_events/using_pointer_events