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Re: Level 3 Events comment: CustomEvent

From: Philippe Le Hegaret <plh@w3.org>
Date: 18 Sep 2002 11:42:25 -0400
To: David Flanagan <david@oreilly.com>
Cc: WWW DOM <www-dom@w3.org>
Message-Id: <1032363745.2905.171.camel@jfouffa>

On Tue, 2002-07-30 at 17:31, David Flanagan wrote:
> 
> 
> 
> 
> 
> I found the descripton of the CustomEvent interface in the Last Call
> draft of the Level 3 Events spec too be very confusing.  I was only able
> to figure it out after reading the note about it in the description of
> DocumentEvent.createEvent().  I suggest you add more description to
> CustomEvent itself.  In particular, please say when it is necessary to
> implement this interface.

Here is a try for section 1.4.1 (1.6.1 is in the future version). I hope
it's easier to understand the purpose of CustomEvent:

[[
1.6.1. Event creation

In most cases, the events dispatched by the DOM Events implementation are
also created by this implementation. It is however possible to simulate
events such as mouse events by creating the Event objects and dispatch them
using the DOM Events implementation.

The DOM Events model provides two ways for creating Events. An application
can either create Events objects that are known to the implementation, or
create its own objects and have them dispatched by the DOM Events
implementation.

Creating Event objects that are known to the DOM Events implementation is
done using DocumentEvent.createEvent. The application must then initialize
the object by calling the appropriate initialization method before invoking
EventTarget.dispatchEvent. The Events objects created must be known by the
DOM Events implementation otherwise an event exception is thrown.

The DOM application might want to create its own Event objects, in order to
change the default Event implementation provided by the DOM Events
implementation or to generate new event types with specific contextual
information. In any case, the application is responsible for creating and
initializing the Event object. The application can then dispatch the event
using the DOM Events implementation by using EventTarget.dispatchEvent.

However, the DOM Events implementation requires to have access to two
attributes in the Event object in order to accomplish the dispatch
appropriately: Event.currentTarget and Event.eventPhase. Those attributes
are defined as readonly in the Event interface since event listeners must
not change them and it is the responsability of the DOM Events
implementation to update them during the event flow. Therefore, implementing
the Event interface when creating its own events is not enough for an
application since the DOM Events implementation will not be able to update
the current phase and the current node during the dispatch. This problem
could be resolved using two techniques: by extending the object definition
of the Event implementation provided by the DOM Events implementation, or by
the CustomEvent interface. The first method has the disadvantage to bind the
application to a specific DOM Events implementation while the second method
is a generic one and can be used with any DOM Level 3 Events implementation.

Interface DocumentEvent (introduced in DOM Level 2)

     The DocumentEvent interface provides a mechanism by which the user can
     create an Event object of a type supported by the implementation. It is
     expected that the DocumentEvent interface will be implemented on the
     same object which implements the Document interface in an
     implementation which supports the Event model.

     IDL Definition

          // Introduced in DOM Level 2:
          interface DocumentEvent {
            Event              createEvent(in DOMString eventType)
                                                  raises(DOMException);
            // Introduced in DOM Level 3:
            EventListenerGroup createEventListenerGroup();
            // Introduced in DOM Level 3:
            boolean            canDispatch(in DOMString namespaceURI,
                                           in DOMString type);
          };

     Methods
          canDispatch introduced in DOM Level 3
               Test if the implementation will generate events of a
               specified type.
               Parameters
               namespaceURI of type DOMString
                    Specifies the Event.namespaceURI of the event.
               type of type DOMString
                    Specifies the Event.type of the event.
               Return Value
               boolean true if the implementation will generate and
                       dispatch this event type, false otherwise.
               No Exceptions
          createEvent
               Parameters
               eventType of type DOMString
                    The eventType parameter specifies the name of the DOM
                    Events interface to be supported by the created event
                    object, e.g. "Event", "MouseEvent", "MutationEvent" ...
                    If the Event is to be dispatched via the dispatchEvent
                    method the appropriate event init method must be called
                    after creation in order to initialize the Event's
                    values.
                    As an example, a user wishing to synthesize some kind of
                    UIEvent would call createEvent with the parameter
                    "UIEvent". The initUIEventNS method could then be called
                    on the newly created UIEvent object to set the specific
                    type of user interface event to be dispatched,
                    {"http://www.w3.org/2001/xml-events", "DOMActivate"} for
                    example, and set its context information, e.g. detail in
                    this example.
                    The createEvent method is used in creating Events when
                    it is either inconvenient or unnecessary for the user to
                    create an Event themselves. In cases where the
                    implementation provided Event is insufficient, users may
                    supply their own Event implementations for use with the
                    dispatchEvent method. However, the DOM implementation
                    needs access to the attributes currentTarget and
                    eventPhase of the Event interface to propagate
                    appropriately the event in the DOM tree. Therefore users
                    Event implementation might need to support the
                    CustomEvent interface for that effect.

                    Note: For backward compatibility reason, "UIEvents",
                    "MouseEvents", "MutationEvents", and "HTMLEvents"
                    feature names are valid values for the parameter
                    eventType and represent respectively the interfaces
                    "UIEvent", "MouseEvent", "MutationEvent", and "Event".

               Return Value
               Event The newly created event object.
               Exceptions
               DOMException NOT_SUPPORTED_ERR: Raised if the implementation
                            does not support the Event interface requested.
          createEventListenerGroup introduced in DOM Level 3
               This method creates a new EventListenerGroup for use in the
               addEventListenerNS and removeEventListenerNS methods of the
               EventTarget interface (see also Stopping the event flow).
               Return Value
               EventListenerGroup The newly created EventListenerGroup
                                  object.
               No Parameters
               No Exceptions

Interface CustomEvent (introduced in DOM Level 3)

     The CustomEvent interface gives access to the attributes
     Event.currentTarget and Event.eventPhase. It is intended to be used by
     the DOM Events implementation to access the underlying current target
     and event phase while dispatching the event in the tree.

     The methods contained in this interface are not intended to be used by
     a DOM application, especially during the dispatch on the Event object.
     Changing the current target or the current phase may conduct into
     unpredictable results of the event flow. The DOM Events implementation
     should invoke both methods before invoking each event listener on the
     current target to protect DOM applications from malicious event
     listeners.

     IDL Definition

          // Introduced in DOM Level 3:
          interface CustomEvent : Event {
            void               setCurrentTarget(in Node target);
            void               setEventPhase(in unsigned short phase);
          };

     Methods
          setCurrentTarget
               The setCurrentTarget method is used by the DOM Events
               implementation to change the value of Event.currentTarget.
               Parameters
               target of type Node
                    Specifies the new value for the Event.currentTarget
                    attribute.
               No Return Value
               No Exceptions
          setEventPhase
               The setEventPhase method is used by the DOM Events
               implementation to change the value of Event.eventPhase.
               Parameters
               phase of type unsigned short
                    Specifies the new value for the Event.eventPhase
                    attribute.
               No Return Value
               No Exceptions
]]

Philippe
Received on Wednesday, 18 September 2002 11:42:28 GMT

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