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Proposed change in removeEventHandler semantics

From: Raph Levien <raph@levien.com>
Date: Sun, 19 Mar 2000 06:45:57 -0800
Message-Id: <200003191445.GAA12524@levien.com>
To: www-dom@w3.org
Cc: Philippe Le Hegaret <plh@w3.org>, Tom Pixley <joki@netscape.com>, Tim Janik <timj@gtk.org>
Hi w3c dom people,

   As a result of a conversation between Philippe Le Hegaret, Tim
Janik, and myself yesterday, I am sending this mail to propose a
change in the semantics of removeEventHandler. Briefly, the current
semantics call for the handler to be invoked in some cases after it is
removed. I propose that it should not be.

   Let me start by outlining a concrete example. Suppose the document
contains an "X" button that is intended to hide one of the elements of
the document. Further suppose that there is a Model/View/Controller
architecture with the DOM as the Model, and separate View and
Controller modules that are connected via the DOM event mechanism.

   Thus, both the Controller and the View have listeners on the
button. The Controller listener implements the semantics of removing
the object, while the View listener keeps the display up to date,
including (in this example) visualizing mouse events.

   Then, the following sequence is at least plausible: The user clicks
on the "X". The Controller event listener is invoked first, which
performs the operations needed to hide the object. In particular, it
detaches the View from the element, changing the View state so that
the element is no longer shown, and invoking removeEventListener on
the View's handlers.

   Finally, the Controller's event handler completes, and the DOM
invokes the event listener of the View. However, the View's state is
no longer consistent with the element being shown. Thus, unless the
View takes special care to detect the fact that the event should no
longer be handled, unexpected behavior is likely.

   Note that this unexpected behavior is quite dependent on the order
of invocation of the handlers on an element. If the View's handler is
invoked first, nothing bad will happen.

   The Gtk+ signal mechanism is quite analogous to DOM2 Events, and is
extensively used for plumbing UI events, as might be expected from
advanced usage of the DOM. Tim Janik, a long time co-maintainer of
Gtk+, has collected quite a bit of experience in this area,
particularly in dealing with re-entrant invocations and similar
interactions. His experience is that invoking a handler after removal
nearly always contains the possibility of unexpected behavior, and is
rarely (if ever) actually useful. Gtk+'s signal mechanism currently
enforces the invariant that a handler is never invoked after removal,
and these semantics work quite well in practice.

   For more information on Gtk+, please refer to the Gtk+ webpage at

   After discussing the issues with Tim and Philippe, I believe that
the current DOM semantics may well cause real problems in Gill, as I
expect that quite a lot of functionality might be invoked from event
listeners, including modifications to the relationship between the
Model and the View. Gill is the Gnome implementation of SVG, a spec in
preparation by the W3C for advanced graphics. Gill is based on a
strict Model/View/Controller architecture based on the DOM, and the
consistency between Model and View is maintained entirely from DOM2
Events. The Gill webpage is at http://www.levien.com/svg/

   Thus, I propose the following modification to the specification.
The current language in the description of removeEventListener reads:

      "If an EventListener is removed from an EventTarget which is
      currently processing an event the removed listener will still be
      triggered by the current event."

   This should be changed to read "will not be triggered by the
current event". Further, the statement, "an event listener will never
be invoked after it is removed" should be added as a global invariant.

   I believe that the pain of making this change now is insignificant
to that of changing the semantics in a later DOM level. I advocate
making the change now, before the status advances past Candidate

   Thanks in advance for considering this issue.

Received on Friday, 24 March 2000 12:05:38 UTC

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