User Agents that run on terminals which provide touch input to use web applications typically use interpreted mouse events to allow users to access interactive web applications. However, these interpreted events, being normalized data based on the physical touch input, tend to have limitations on delivering the intended user experience. Additionally, it is not possible to handle concurrent input regardless of device capability, due to constraints of mouse events: both system level limitations and legacy compatibility.
Meanwhile, native applications are capable of handling both cases with the provided system APIs.
The Touch Events specification provides a solution to this problem by specifying interfaces to allow web applications to directly handle touch events, and multiple touch points for capable devices.
This specification defines conformance criteria that apply to a single product: the user agent that implements the interfaces that it contains.
Implementations that use ECMAScript to implement the APIs defined in this specification must implement them in a manner consistent with the ECMAScript Bindings defined in the Web IDL specification [[!WEBIDL]] as this specification uses that specification and terminology.
A conforming implementation is required to implement all fields defined in this specification.
WindowProxy is defined in [[!HTML5]].
This interface describes an individual touch point for a touch event. Touch objects are immutable; after one is created, its attributes must not change.
This interface defines a list of individual points of contact for a touch event. TouchList objects are immutable; after one is created, its contents must not change.
This interface defines the touchstart, touchend, touchmove, and touchcancel event types. TouchEvent objects are immutable; after one is created and initialized, its attributes must not change.
a list of Touches for every point of contact which contributed to the event.
For the touchstart event this must be a list of the touch points that just became active with the current event. For the touchmove event this must be a list of the touch points that have moved since the last event. For the touchend and touchcancel events this must be a list of the touch points that have just been removed from the surface.
true if the alt (Alternate) key modifier is activated;
otherwise false
true if the meta (Meta) key modifier is activated;
otherwise false. On some platforms this attribute may
map to a differently-named key modifier.
true if the ctrl (Control) key modifier is activated;
otherwise false
true if the shift (Shift) key modifier is activated;
otherwise false
The examples below demonstrate the relations between the different TouchList members defined in a TouchEvent.
This example demonstrates the utility and relations between the touches and targetTouches members defined in the TouchEvent interface. The following code will generate different output based on the number of touch points on the touchable element and the document:
<div id='touchable'>
This element is touchable.
</div>
document.getElementById('touchable').addEventListener('touchstart', function(ev) {
if (ev.touches.item(0) == ev.targetTouches.item(0))
{
/**
* If the first touch on the surface is also targeting the
* "touchable" element, the code below should execute.
* Since targetTouches is a subset of touches which covers the
* entire surface, TouchEvent.touches >= TouchEvents.targetTouches
* is always true.
*/
document.write('Hello Touch Events!');
}
if (ev.touches.length == ev.targetTouches.length)
{
/**
* If all of the active touch points are on the "touchable"
* element, the length properties should be the same.
*/
document.write('All points are on target element')
}
if (ev.touches.length > 1)
{
/**
* On a single touch input device, there can only be one point
* of contact on the surface, so the following code can only
* execute when the terminal supports multiple touches.
*/
document.write('Hello Multiple Touch!');
}
}, false);
This example demonstrates the utility of changedTouches and it's relation with the other TouchList members of the TouchEvent interface. The code is a example which triggers whenever a touch point is removed from the defined touchable element:
<div id='touchable'>
This element is touchable.
</div>
document.getElementById('touchable').addEventListener('touchend', function(ev) {
/**
* Example output when three touch points are on the surface,
* two of them being on the "touchable" element and one point
* in the "touchable" element is lifted from the surface:
*
* Touch points removed: 1
* Touch points left on element: 1
* Touch points left on document: 2
*/
document.write('Removed: ' + ev.changedTouches.length);
document.write('Remaining on element: ' + ev.targetTouches.length);
document.write('Remaining on document: ' + ev.touches.length);
}, false);
The following table provides a summary of the types of possible TouchEvent types defined in this specification. All events should accomplish the bubbling phase. Some events are not cancelable (see preventDefault).
| Event Type | Sync / Async | Bubbling phase | Trusted proximal event target types | DOM interface | Cancelable | Default Action |
|---|---|---|---|---|---|---|
| touchstart | Sync | Yes | Document, Element |
TouchEvent | Yes | undefined |
| touchend | Sync | Yes | Document, Element |
TouchEvent | Yes | Varies: mousemove (If point has been moved), mousedown, mouseup, click |
| touchmove | Sync | Yes | Document, Element |
TouchEvent | Yes | undefined |
| touchcancel | Sync | Yes | Document, Element |
TouchEvent | No | none |
A user agent must dispatch this event type to indicate when the user places a touch point on the touch surface.
The target of this event must be an Element. If the touch point is within a frame, the event should be dispatched to an element in the child browsing context of that frame.
If the preventDefault method is called on this event, it should prevent any default actions caused by any touch events associated with the same active touch point, including mouse events or scrolling.
A user agent must dispatch this event type to indicate when the user removes a touch point from the touch surface, also including cases where the touch point physically leaves the touch surface, such as being dragged off of the screen.
The target of this event must be the same Element on which the touch point started when it was first placed on the surface, even if the touch point has since moved outside the interactive area of the target element.
The touch point or points that were removed must be included in the changedTouches attribute of the TouchEvent, and must not be included in the touches and targetTouches attributes.
A user agent must dispatch this event type to indicate when the user moves a touch point along the touch surface.
The target of this event must be the same Element on which the touch point started when it was first placed on the surface, even if the touch point has since moved outside the interactive area of the target element.
Note that the rate at which the user agent sends touchmove events is implementation-defined, and may depend on hardware capabilities and other implementation details.
If the preventDefault method is called on the first touchmove event of an active touch point, it should prevent any default action caused by any touchmove event associated with the same active touch point, such as scrolling.
A user agent must dispatch this event type to indicate when a touch point has been disrupted in an implementation-specific manner, such as a synchronous event or action originating from the UA canceling the touch, or the touch point leaving the document window into a non-document area which is capable of handling user interactions. (e.g. The UA's native user interface, plug-ins) A user agent may also dispatch this event type when the user places more touch points on the touch surface than the device or implementation is configured to store, in which case the earliest Touch object in the TouchList should be removed.
The target of this event must be the same Element on which the touch point started when it was first placed on the surface, even if the touch point has since moved outside the interactive area of the target element.
The touch point or points that were removed must be included in the changedTouches attribute of the TouchEvent, and must not be included in the touches and targetTouches attributes.
The Document interface [[!DOM-LEVEL-3-CORE]] contains methods by which the user can create Touch and TouchList objects.
The user agent may dispatch both touch events and mouse events [[!DOM-LEVEL-2-EVENTS]] in response to the same user input. If the user agent dispatches both touch events and mouse events in response to a single user action, then the touchstart event type must be dispatched before any mouse event types for that action. If the preventDefault method of touchstart or touchmove is called, the user agent should not dispatch any mouse event that would be a consequential result of the the prevented touch event.
If a Web application can process touch events, it can intercept them, and no corresponding mouse events would need to be dispatched by the user agent. If the Web application is not specifically written for touch input devices, it can react to the subsequent mouse events instead.
If the user agent intreprets a sequence of touch events as a click, then it should dispatch mousemove, mousedown, mouseup, and click events (in that order) at the location of the touchend event for the corresponding touch input. If the contents of the document have changed during processing of the touch events, then the user agent may dispatch the mouse events to a different target than the touch events.
The default actions and ordering of any further touch and mouse events are implementation-defined, except as specified elsewhere.
The working group maintains a list of open issues in this specification. These issues may be addressed in future revisions of the specification.
Many thanks to the WebKit engineers for developing the model used as a basis for this spec, Neil Roberts (SitePen) for his summary of WebKit touch events, Peter-Paul Koch (PPK) for his write-ups and suggestions, Robin Berjon for developing the ReSpec.js spec authoring tool, and the WebEvents WG for their many contributions.
Many others have made additional comments as the spec developed, which have led to steady improvements. Among them are Matthew Schinckel, Andrew Grieve, and Cathy Chan. If I inadvertently omitted your name, please let me know.