移动端触摸事件介绍

Posted 2020-08-27 ddlove

By publishing this Recommendation, W3C expects that the functionality specified in this Touch Interface Recommendation will not be affected by changes to html5 or Web IDL as those specifications proceed to Recommendation. The WG has completed and approved this specification‘s Test Suite and created an Implementation Report that shows that two or more independent implementations pass each test.The Touch Events specification defines a set of low-level events that represent one or more points of contact with a touch-sensitive surface, and changes of those points with respect to the surface and any DOM elements displayed upon it (e.g. for touch screens) or associated with it (e.g. for drawing tablets without displays). It also addresses pen-tablet devices, such as drawing tablets, with consideration toward stylus capabilities.

Introduction

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.

The W3C‘s Protocols and Formats Working Group created a non-normative document that includes a mapping of hardware events (e.g. keyboard events) to touch events. For more information see Touch Events Accessibility Mapping.

This specification defines conformance criteria that apply to a single product: the user agent that implements the interfaces that it contains.

WindowProxy is defined in [[!HTML5]].

WebIDL Conformance

The IDL blocks in this specification are conforming IDL fragments as defined by the WebIDL specification [[!WEBIDL]].

A conforming Web Events user agent must also be a conforming ECMAScript implementation of this IDL fragments in this specification, with the following exception:

• section 4.4.6 of Web IDL requires that IDL attributes are reflected as accessor properties on interface prototype objects. Instead of this, the user agent may reflect IDL attributes as data properties on the platform objects that implement the relevant interface. These data properties must have the same behavior when getting and setting as would be exhibited when invoking the getter and setter of the accessor properties on the platform object.

Note: Both ways of reflecting IDL attributes allow for simply getting and setting the property on the platform object to work. For example, given a Touch object aTouch, evaluating aTouch.target would return the EventTarget for the Touch object. If the user agent implements IDL attributes as accessor properties, then the property access invokes the getter which returns the EventTarget. If the user agent implements IDL attributes as data properties on the platform object with the same behavior as would be found with the accessor properties, then the object would appear to have an own property named "target" whose value is an EventTarget object, and the property access would return this value.

Touch Interface

This interface describes an individual touch point for a touch event. Touch objects are immutable; after one is created, its attributes must not change.

readonly attribute long identifier

An identification number for each touch point. When a touch point becomes active, it must be assigned an identifier that is distinct from any other active touch point. While the touch point remains active, all events that refer to it must assign it the same identifier.

readonly attribute EventTarget target

The EventTarget 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 that element.

readonly attribute double screenX

The horizontal coordinate of point relative to the screen in pixels

readonly attribute double screenY

The vertical coordinate of point relative to the screen in pixels

readonly attribute double clientX

The horizontal coordinate of point relative to the viewport in pixels, excluding any scroll offset

readonly attribute double clientY

The vertical coordinate of point relative to the viewport in pixels, excluding any scroll offset

readonly attribute double pageX

The horizontal coordinate of point relative to the viewport in pixels, including any scroll offset

readonly attribute double pageY

The vertical coordinate of point relative to the viewport in pixels, including any scroll offset

TouchList Interface

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.

A TouchList object‘s supported property indices ([[!WEBIDL]]) are the numbers in the range 0 to one less than the length of the list.

readonly attribute unsigned long length

returns the number of Touches in the list

getter Touch? item (in unsigned long index)

returns the Touch at the specified index in the list or null if the index is not less than the length of the list.

TouchEvent Interface

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.

readonly attribute TouchList touches

a list of Touches for every point of contact currently touching the surface.

readonly attribute TouchList targetTouches

a list of Touches for every point of contact that is touching the surface and started on the element that is the target of the current event.

readonly attribute TouchList changedTouches

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.

readonly attribute boolean altKey

true if the alt (Alternate) key modifier is activated; otherwise false

readonly attribute boolean metaKey

true if the meta (Meta) key modifier is activated; otherwise false. On some platforms this attribute may map to a differently-named key modifier.

readonly attribute boolean ctrlKey

true if the ctrl (Control) key modifier is activated; otherwise false

readonly attribute boolean shiftKey

true if the shift (Shift) key modifier is activated; otherwise false

TouchEvent Implementer‘s Note

User agents should ensure that all Touch objects available from a given TouchEvent are all associated to the same document that the TouchEvent was dispatched to. To implement this, user agents should maintain a notion of the current touch-active document. On first touch, this is set to the target document where the touch was created. When all active touch points are released, the touch-active document is cleared. All TouchEvents are dispatched to the current touch-active document, and each Touch object it contains refers only to DOM elements (and co-ordinates) in that document. If a touch starts entirely outside the currently touch-active document, then it is ignored entirely.

Usage Examples

The examples below demonstrate the relations between the different TouchList members defined in a TouchEvent.

touches and targetTouches of 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);
              

changedTouches of a TouchEvent

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);
              

List of TouchEvent types

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

The touchstart event

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.

The touchend event

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.

If this event is cancelled, any sequence of touch events that includes this event must not be interpreted as a click.

The touchmove event

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.

A user agent should suppress the default action caused by any touchmove event until at least one touchmove event associated with the same active touch point is not cancelled. Whether the default action is suppressed for touchmove events after at least one touchmove event associated with the same active touch point is not cancelled is implementation dependent.

The touchcancel event

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.

Extensions to the Document Interface

The Document interface [[!DOM-LEVEL-3-CORE]] contains methods by which the user can create Touch and TouchList objects.

Touch createTouch()

Creates a Touch object with the specified attributes.

WindowProxy view

EventTarget target

long identifier

double pageX

double pageY

double screenX

double screenY

TouchList createTouchList()

Creates a TouchList object consisting of zero or more Touch objects. Calling this method with no arguments creates a TouchList with no objects in it and length 0 (zero).

Touch... touches

Some user agents implement an initTouchEvent method as part of the TouchEvent interface. When this method is available, scripts can use it to initialize the properties of a TouchEvent object, including its TouchList properties (which can be initialized with values returned from createTouchList). The initTouchEvent method is not yet standardized, but it may appear in some form in a future specification.

Interaction with Mouse Events

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 thepreventDefault method of touchstart, touchmove, or touchend 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.

Glossary

active touch point

A touch point which is currently on the screen and is being tracked by the user agent. The touch point becomes active when the user agent first dispatches a touchstart event indicating its appearance. It ceases to be active after the user agent dispatches a touchend or touchcancel event indicating that the touch point is removed from the surface or no longer tracked.

touch point

The coordinate point at which a pointer (e.g finger or stylus) intersects the target surface of an interface. This may apply to a finger touching a touch-screen, or an digital pen writing on a piece of paper.

preventDefault

If a event is cancelable, the preventDefault method is used to signify that the event is to be canceled, and any default actions defined in the user agent as a result of this event, or consequential events from the canceled event will not occur. Calling this method on non-cancelable events will have no effect.

Issues

The working group maintains a list of open issues in this specification. These issues may be addressed in future revisions of the specification.

Acknowledgements

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, Cathy Chan, and Boris Zbarsky. If we inadvertently omitted your name, please let me know.

The group acknowledges the following contributors to this specification‘s test suite: Matt Brubeck, Olli Pettay, Art Barstow, Cathy Chan and Rick Byers.

Changes Since Last Publication

The following non-substantive changes were made since the 24 January 2013 Last Call Working Draft was published:

• Added a non-normative note for implementers regarding event targets (changeset).
• Added a non-normative note regarding mapping hardware events to touch events (changeset).
• Minor Web IDL bug fixes and clarifications (changesets: 1, 2).
• Added a non-normative note regarding the initTouchEvent method that is not standardized (changesets: 1, 2).