Pointer events

Pointer events are a modern way to handle input from a variety of pointing devices, such as a mouse, a pen/stylus, a touchscreen, and so on.

The brief history

Let’s make a small overview, so that you understand the general picture and the place of Pointer Events among other event types.

• Long ago, in the past, there were only mouse events.

  Then touch devices became widespread, phones and tablets in particular. For the existing scripts to work, they generated (and still generate) mouse events. For instance, tapping a touchscreen generates mousedown. So touch devices worked well with web pages.

  But touch devices have more capabilities than a mouse. For example, it’s possible to touch multiple points at once (“multi-touch”). Although, mouse events don’t have necessary properties to handle such multi-touches.

• So touch events were introduced, such as touchstart, touchend, touchmove, that have touch-specific properties (we don’t cover them in detail here, because pointer events are even better).

  Still, it wasn’t enough, as there are many other devices, such as pens, that have their own features. Also, writing code that listens for both touch and mouse events was cumbersome.

• To solve these issues, the new standard Pointer Events was introduced. It provides a single set of events for all kinds of pointing devices.

As of now, Pointer Events Level 2 specification is supported in all major browsers, while the newer Pointer Events Level 3 is in the works and is mostly compatible with Pointer Events level 2.

Unless you develop for old browsers, such as Internet Explorer 10, or for Safari 12 or below, there’s no point in using mouse or touch events any more – we can switch to pointer events.

Then your code will work well with both touch and mouse devices.

That said, there are some important peculiarities that one should know in order to use Pointer Events correctly and avoid surprises. We’ll make note of them in this article.

Pointer event types

Pointer events are named similarly to mouse events:

As we can see, for every mouse<event>, there’s a pointer<event> that plays a similar role. Also there are 3 additional pointer events that don’t have a corresponding mouse... counterpart, we’ll explain them soon.

Pointer event properties

Pointer events have the same properties as mouse events, such as clientX/Y, target, etc., plus some others:

• pointerId – the unique identifier of the pointer causing the event.

  Browser-generated. Allows us to handle multiple pointers, such as a touchscreen with stylus and multi-touch (examples will follow).

• pointerType – the pointing device type. Must be a string, one of: “mouse”, “pen” or “touch”.

  We can use this property to react differently on various pointer types.

• isPrimary – is true for the primary pointer (the first finger in multi-touch).

Some pointer devices measure contact area and pressure, e.g. for a finger on the touchscreen, there are additional properties for that:

• width – the width of the area where the pointer (e.g. a finger) touches the device. Where unsupported, e.g. for a mouse, it’s always 1.
• height – the height of the area where the pointer touches the device. Where unsupported, it’s always 1.
• pressure – the pressure of the pointer tip, in range from 0 to 1. For devices that don’t support pressure must be either 0.5 (pressed) or 0.
• tangentialPressure – the normalized tangential pressure.
• tiltX, tiltY, twist – pen-specific properties that describe how the pen is positioned relative to the surface.

These properties aren’t supported by most devices, so they are rarely used. You can find the details about them in the specification if needed.

Multi-touch

One of the things that mouse events totally don’t support is multi-touch: a user can touch in several places at once on their phone or tablet, or perform special gestures.

Pointer Events allow handling multi-touch with the help of the pointerId and isPrimary properties.

Here’s what happens when a user touches a touchscreen in one place, then puts another finger somewhere else on it:

1. At the first finger touch:
   • pointerdown with isPrimary=true and some pointerId.
2. For the second finger and more fingers (assuming the first one is still touching):
   • pointerdown with isPrimary=false and a different pointerId for every finger.

Please note: the pointerId is assigned not to the whole device, but for each touching finger. If we use 5 fingers to simultaneously touch the screen, we have 5 pointerdown events, each with their respective coordinates and a different pointerId.

The events associated with the first finger always have isPrimary=true.

We can track multiple touching fingers using their pointerId. When the user moves and then removes a finger, we get pointermove and pointerup events with the same pointerId as we had in pointerdown.

Here’s the demo that logs pointerdown and pointerup events:

Please note: you must be using a touchscreen device, such as a phone or a tablet, to actually see the difference in pointerId/isPrimary. For single-touch devices, such as a mouse, there’ll be always same pointerId with isPrimary=true, for all pointer events.

Event: pointercancel

The pointercancel event fires when there’s an ongoing pointer interaction, and then something happens that causes it to be aborted, so that no more pointer events are generated.

Such causes are:

• The pointer device hardware was physically disabled.
• The device orientation changed (tablet rotated).
• The browser decided to handle the interaction on its own, considering it a mouse gesture or zoom-and-pan action or something else.

We’ll demonstrate pointercancel on a practical example to see how it affects us.

Let’s say we’re implementing drag’n’drop for a ball, just as in the beginning of the article Drag'n'Drop with mouse events.

Here is the flow of user actions and the corresponding events:

1. The user presses on an image, to start dragging
   • pointerdown event fires
2. Then they start moving the pointer (thus dragging the image)
   • pointermove fires, maybe several times
3. And then the surprise happens! The browser has native drag’n’drop support for images, that kicks in and takes over the drag’n’drop process, thus generating pointercancel event.
   • The browser now handles drag’n’drop of the image on its own. The user may even drag the ball image out of the browser, into their Mail program or a File Manager.
   • No more pointermove events for us.

So the issue is that the browser “hijacks” the interaction: pointercancel fires in the beginning of the “drag-and-drop” process, and no more pointermove events are generated.

Here’s the drag’n’drop demo with logging of pointer events (only up/down, move and cancel) in the textarea:

We’d like to implement the drag’n’drop on our own, so let’s tell the browser not to take it over.

Prevent the default browser action to avoid pointercancel.

We need to do two things:

1. Prevent native drag’n’drop from happening:
   • We can do this by setting ball.ondragstart = () => false, just as described in the article Drag'n'Drop with mouse events.
   • That works well for mouse events.
2. For touch devices, there are other touch-related browser actions (besides drag’n’drop). To avoid problems with them too:
   • Prevent them by setting #ball { touch-action: none } in CSS.
   • Then our code will start working on touch devices.

After we do that, the events will work as intended, the browser won’t hijack the process and doesn’t emit pointercancel.

This demo adds these lines:

As you can see, there’s no pointercancel any more.

Now we can add the code to actually move the ball, and our drag’n’drop will work for mouse devices and touch devices.

Pointer capturing

Pointer capturing is a special feature of pointer events.

The idea is very simple, but may seem quite odd at first, as nothing like that exists for any other event type.

The main method is:

• elem.setPointerCapture(pointerId) – binds events with the given pointerId to elem. After the call all pointer events with the same pointerId will have elem as the target (as if happened on elem), no matter where in document they really happened.

In other words, elem.setPointerCapture(pointerId) retargets all subsequent events with the given pointerId to elem.

The binding is removed:

• automatically when pointerup or pointercancel events occur,
• automatically when elem is removed from the document,
• when elem.releasePointerCapture(pointerId) is called.

Now what is it good for? It’s time to see a real-life example.

Pointer capturing can be used to simplify drag’n’drop kind of interactions.

Let’s recall how one can implement a custom slider, described in the Drag'n'Drop with mouse events.

We can make a slider element to represent the strip and the “runner” (thumb) inside it:

<div class="slider">
  <div class="thumb"></div>
</div>

With styles, it looks like this:

And here’s the working logic, as it was described, after replacing mouse events with similar pointer events:

1. The user presses on the slider thumb – pointerdown triggers.
2. Then they move the pointer – pointermove triggers, and our code moves the thumb element along.
   • …As the pointer moves, it may leave the slider thumb element, go above or below it. The thumb should move strictly horizontally, remaining aligned with the pointer.

In the mouse event based solution, to track all pointer movements, including when it goes above/below the thumb, we had to assign mousemove event handler on the whole document.

That’s not a cleanest solution, though. One of the problems is that when a user moves the pointer around the document, it may trigger event handlers (such as mouseover) on some other elements, invoke totally unrelated UI functionality, and we don’t want that.

This is the place where setPointerCapture comes into play.

• We can call thumb.setPointerCapture(event.pointerId) in pointerdown handler,
• Then future pointer events until pointerup/cancel will be retargeted to thumb.
• When pointerup happens (dragging complete), the binding is removed automatically, we don’t need to care about it.

So, even if the user moves the pointer around the whole document, events handlers will be called on thumb. Nevertheless, coordinate properties of the event objects, such as clientX/clientY will still be correct – the capturing only affects target/currentTarget.

Here’s the essential code:

thumb.onpointerdown = function(event) {
  // retarget all pointer events (until pointerup) to thumb
  thumb.setPointerCapture(event.pointerId);

  // start tracking pointer moves
  thumb.onpointermove = function(event) {
    // moving the slider: listen on the thumb, as all pointer events are retargeted to it
    let newLeft = event.clientX - slider.getBoundingClientRect().left;
    thumb.style.left = newLeft + 'px';
  };

  // on pointer up finish tracking pointer moves
  thumb.onpointerup = function(event) {
    thumb.onpointermove = null;
    thumb.onpointerup = null;
    // ...also process the "drag end" if needed
  };
};

// note: no need to call thumb.releasePointerCapture,
// it happens on pointerup automatically

The full demo:

In the demo, there’s also an additional element with onmouseover handler showing the current date.

Please note: while you’re dragging the thumb, you may hover over this element, and its handler does not trigger.

So the dragging is now free of side effects, thanks to setPointerCapture.

At the end, pointer capturing gives us two benefits:

1. The code becomes cleaner as we don’t need to add/remove handlers on the whole document any more. The binding is released automatically.
2. If there are other pointer event handlers in the document, they won’t be accidentally triggered by the pointer while the user is dragging the slider.

Pointer capturing events

There’s one more thing to mention here, for the sake of completeness.

There are two events associated with pointer capturing:

• gotpointercapture fires when an element uses setPointerCapture to enable capturing.
• lostpointercapture fires when the capture is released: either explicitly with releasePointerCapture call, or automatically on pointerup/pointercancel.

Summary

Pointer events allow handling mouse, touch and pen events simultaneously, with a single piece of code.

Pointer events extend mouse events. We can replace mouse with pointer in event names and expect our code to continue working for mouse, with better support for other device types.

For drag’n’drops and complex touch interactions that the browser may decide to hijack and handle on its own – remember to cancel the default action on events and set touch-action: none in CSS for elements that we engage.

Additional abilities of pointer events are:

• Multi-touch support using pointerId and isPrimary.
• Device-specific properties, such as pressure, width/height, and others.
• Pointer capturing: we can retarget all pointer events to a specific element until pointerup/pointercancel.

As of now, pointer events are supported in all major browsers, so we can safely switch to them, especially if IE10- and Safari 12- are not needed. And even with those browsers, there are polyfills that enable the support of pointer events.