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Re: [UndoManager] Re-introduce DOMTransaction interface?

From: Adam Barth <w3c@adambarth.com>
Date: Wed, 4 Jul 2012 22:00:44 -0700
Message-ID: <CAJE5ia8iC0r48kWeAT9N=GtbSoE7gvPNOaqmXUwrVwdBerYS5g@mail.gmail.com>
To: olli@pettay.fi
Cc: Ryosuke Niwa <rniwa@webkit.org>, public-webapps <public-webapps@w3.org>, Ojan Vafai <ojan@chromium.org>, Jonas Sicking <jonas@sicking.cc>, Ehsan Akhgari <ehsan@mozilla.com>, Erik Arvidsson <arv@chromium.org>, Sukolsak Sakshuwong <sukolsak@gmail.com>, Aryeh Gregor <ayg@aryeh.name>
On Wed, Jul 4, 2012 at 5:25 PM, Olli Pettay <Olli.Pettay@helsinki.fi> wrote:
> On 07/05/2012 03:11 AM, Ryosuke Niwa wrote:
>>
>> On Wed, Jul 4, 2012 at 5:00 PM, Olli Pettay <Olli.Pettay@helsinki.fi
>> <mailto:Olli.Pettay@helsinki.fi>> wrote:
>>
>>     On 07/05/2012 01:38 AM, Ryosuke Niwa wrote:
>>
>>         Hi all,
>>
>>         Sukolsak has been implementing the Undo Manager API in WebKit but
>> the fact undoManager.transact() takes a pure JS object with callback
>>         functions is
>>         making it very challenging.  The problem is that this object needs
>> to be kept alive by either JS reference or DOM but doesn't have a backing
>> C++
>>         object.  Also, as far as we've looked, there are no other
>> specification that uses the same mechanism.
>>
>>
>>     I don't understand what is difficult.
>>     How is that any different to
>>     target.addEventListener("foo", { handleEvent: function() {}})
>>
>>
>> It will be very similar to that except this object is going to have 3
>> callbacks instead of one.
>>
>> The problem is that the event listener is a very special object in WebKit
>> for which we have a lot of custom binding code. We don't want to implement a
>> similar behavior for the DOM transaction because it's very error prone.
>
>
> So, it is very much implementation detail.
> (And I still don't understand how a callback can be so hard in this case.
> There are plenty of different kinds of callback objects.
>  new MutationObserver(some_callback_function_object) )

I haven't tested, by my reading of the MutationObserver implementation
in WebKit is that it leaks.  Specifically:

MutationObserver --retains--> MutationCallback --retains-->
some_callback_function_object --retains--> MutationObserver

I don't see any code that breaks this cycle.

DOM events have a bunch of delicate code to avoid break these
reference cycles and avoid leaks.  We can re-invent that wheel here,
but it's going to be buggy and leaky.

I appreciatie that these jQuery-style APIs are fashionable at the
moment, but API fashions come and go.  If we use this approach, we'll
need to maintain this buggy, leaky code forever.  Instead, we can save
ourselves a lot of pain by just using events, like the rest of the web
platform.

Adam


>>         Since I want to make the API consistent with the rest of the
>> platform and the implementation maintainable in WebKit, I propose the
>> following
>>         changes:
>>
>>            * Re-introduce DOMTransaction interface so that scripts can
>> instantiate new DOMTransaction().
>>            * Introduce AutomaticDOMTransaction that inherits from
>> DOMTransaction and has a constructor that takes two arguments: a function
>> and an
>>         optional label
>>
>>
>>         After this change, authors can write:
>>         scope.undoManager.transact(new AutomaticDOMTransaction{__function
>> () {
>>
>>               scope.appendChild("foo");
>>         }, 'append "foo"'));
>>
>>
>>     Looks somewhat odd. DOMTransaction would be just a container for a
>> callback?
>>
>>
>> Right. If we wanted, we can make DOMTransaction an event target and
>> implement execute, undo, & redo as event listeners to further simplify the
>> matter.
>
>
> That could make the code more consistent with rest of the platform, but the
> API would become harder to use.
>
>>
>> - Ryosuke
>>
>
Received on Thursday, 5 July 2012 05:01:44 GMT

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