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From Braden Shepherdson <bra...@chromium.org>
Subject Re: Wiping plugins on navigation
Date Thu, 06 Sep 2012 15:41:24 GMT
I disagree with the latency fears for regenerating plugins. I've just
sampled twenty plugins for Android and a handful for iOS (I don't know
Objective-C, so I can't reliably say much about these), and the large
majority of Android plugins don't define constructors at all. Those that do
are usually empty. I only saw one that had actual code, and it was simply
storing the result of a .getInstance() call on some other class into a
member variable. Similarly, I didn't see any member variables with
expensive initializers.

Therefore I don't think we need to worry about the cost of destroying and
recreating a couple dozen classes with (nearly) empty constructors.

As to Jimmy Jarvis' suggestion on the bug, I don't know if we can do IDs
that persist across refreshes. We could make callback IDs include the name
of the plugin, but each plugin over its lifetime may make many requests and
have multiple outstanding requests at a time. That requires a count, and we
can't persist that count across page reloads since it and the map of
callbacks reside in Javascript. We could use a hash function so that
callback IDs are only very unlikely to collide, but that seems less than
ideal.

One advantage of the destroy-and-regenerate approach to plugins is that
they cannot get themselves into a bad state on navigation, because they
will always be starting fresh. As noted elsewhere, on purely
Javascript-powered platforms, is it even possible for plugins to persist
across navigation without playing games like using persistent storage?


On Thu, Sep 6, 2012 at 11:27 AM, Braden Shepherdson <braden@chromium.org>wrote:

> Repeating here a comment on the bug:
>
> Jimmy Jarvis<https://issues.apache.org/jira/secure/ViewProfile.jspa?name=jiminyjarvis>
added
> a comment - 05/Sep/12 23:02 - edited
>
> JSON-P uses a unique callback identifier, similar to the proposed fix
> above, and is far more reliable and easier to debug than reset with
> repeating IDs. It would be unfortunate overhead to have to reinitialize our
> native logic every time a page reloads. The app would become far less
> responsive, at least for us. If a plugin is a bunch of stubs, it's not an
> big deal as you say – however, if it is a longer running setup process ----
> I'd prefer ignoring unmatched callbacks. I agree sending the plugin a Reset
> or Terminate message would enable the plugin authors an opportunity to do
> proper cleanup on pending requests, but even if they do not, unique ID's
> (like JSON-P) are a better solution than repeating IDs.
>
>
> On Wed, Sep 5, 2012 at 9:33 PM, Andrew Grieve <agrieve@chromium.org>wrote:
>
>> Plugins could probably use static fields if they wanted to maintain some
>> state between page changes. I think it should be extremely rare to do so
>> though. e.g. For the platforms that implement their plugins in JS, is it
>> even possible to maintain state between page changes?
>>
>> One nice outcome of this is that it will make our mobile-spec pages less
>> likely to affect each other.
>>
>>
>> On Wed, Sep 5, 2012 at 6:59 PM, Brian LeRoux <b@brian.io> wrote:
>>
>> > Yes I do not refuting Cordova needs to work for both single page and
>> > multipage apps. Just saying there is a solution to this problem. ;P
>> >
>> >
>> > On Wed, Sep 5, 2012 at 3:49 PM, Jesse <purplecabbage@gmail.com> wrote:
>> > > Whether it is an edge case, or a common case, multi-page apps are a
>> > > reality, so we definitely need to notify plugins when the page is
>> > > changing.
>> > > I don't necessarily agree that the plugin should be destroyed and
>> > > recreated though, I can think of several cases where persistence would
>> > > be nice to have.
>> > >
>> > > I also do not see this as a security issue.  Security is already
>> > > governed by the white-list, so non-trusted pages cannot access device
>> > > functions.  If a plugin needs additional security, then it should be
>> > > built into the plugin, and not the responsibility of the framework.
>> > > ... Thinking of a SuperCookie plugin which uses the domain of the
>> > > currently loaded page before deciding what to return, or something
>> > > similar.
>> > >
>> > > My 2 sense,
>> > >   Jesse
>> > >
>> > >
>> > > On Wed, Sep 5, 2012 at 3:30 PM, Bryce Curtis <curtis.bryce@gmail.com>
>> > wrote:
>> > >> Sometimes multi-page apps are needed or you navigate from your app
to
>> > >> another page.  One bug we ran into was that callback ids are reused
>> > >> when loading a new page.  So, a plugin trying to send data back to
>> the
>> > >> original page could be calling a recycled plugin with erroneous data.
>> > >> In addition to the bugs, there is also a security issue with a
>> > >> subsequent page being able to access a plugin that was used in a
>> > >> previous page.
>> > >>
>> > >> The app/page lifecycle events are propagated to the plugins, and the
>> > >> plugins are destroyed when loading a new page.  However, looking at
>> > >> the code, it appears this may be broken now.  (At least for Android).
>> > >>
>> > >> On Wed, Sep 5, 2012 at 3:40 PM, Braden Shepherdson <
>> braden@chromium.org>
>> > wrote:
>> > >>> Sure, and I'm a fan of single-page apps (I do work for Google,
after
>> > >>> all...), but this causes very chaotic, hard-to-track bugs, so it
>> makes
>> > >>> sense to be robust over a refresh/navigation.
>> > >>>
>> > >>>
>> > >>> On Wed, Sep 5, 2012 at 5:25 PM, Brian LeRoux <b@brian.io>
wrote:
>> > >>>
>> > >>>> One thing to note, we tend to advise ppl author single page
web
>> apps
>> > >>>> which makes state visibility change an app logic concern (and
avoid
>> > >>>> this issue from manifesting). Generally, we can say a page
refresh
>> is
>> > >>>> not a great user experience in apps.
>> > >>>>
>> > >>>> On Wed, Sep 5, 2012 at 1:15 PM, Braden Shepherdson <
>> > braden@chromium.org>
>> > >>>> wrote:
>> > >>>> > This is intended as a continuation of the discussion started
in
>> > >>>> > https://issues.apache.org/jira/browse/CB-1318 .
>> > >>>> >
>> > >>>> > The bug in question is one where one page starts a long
native
>> side
>> > >>>> action
>> > >>>> > such as a network call. Then the user navigates the app
to
>> another
>> > page.
>> > >>>> > When the long action completes, the call returns and the
>> appropriate
>> > >>>> > Javascript callback is looked up and called.
>> > >>>> >
>> > >>>> > However when the page is navigated, the counter that provides
>> > supposedly
>> > >>>> > unique names for callbacks is reset, allowing a callback
on the
>> new
>> > page
>> > >>>> to
>> > >>>> > have the same name as the callback from the old page.
It then
>> gets
>> > called
>> > >>>> > incorrectly, potentially introducing weird and transient
bugs.
>> > >>>> >
>> > >>>> > The proposed solution is to do the following on navigation:
>> > >>>> > - Call a destroy() call on all plugins, which by default
does
>> > nothing.
>> > >>>> This
>> > >>>> > allows the plugins a chance to cancel any outstanding
network
>> > requests or
>> > >>>> > do any other cleanup work.
>> > >>>> > - Delete the plugin instance and recreate it.
>> > >>>> >
>> > >>>> > In the bug I also said one step would be to wipe the callback
>> table
>> > in
>> > >>>> the
>> > >>>> > Javascript, but that isn't necessary since it would have
been
>> wiped
>> > by
>> > >>>> the
>> > >>>> > navigation anyway.
>> > >>>> >
>> > >>>> > This issue is cross-platform-ish. It (probably) doesn't
apply to
>> > >>>> web-based
>> > >>>> > platforms like WebOS or Bada, because the plugins are
Javascript
>> > shims
>> > >>>> > rather than native code, and are wiped on navigation like
any
>> other
>> > >>>> > Javascript. However this issue does exist on at least
Android and
>> > iOS,
>> > >>>> and
>> > >>>> > probably a few others as well.
>> > >>>> >
>> > >>>> > I'm proposing to implement the solution outlined above
on Android
>> > and
>> > >>>> iOS.
>> > >>>> > I don't have the devices or environment to do any other
>> platforms,
>> > nor
>> > >>>> am I
>> > >>>> > sure which are necessary. The maintainers of other platforms
will
>> > have to
>> > >>>> > consider this problem for their platform. I would also
update the
>> > core
>> > >>>> > plugins to define a destroy() method if they have relevant
>> cleanups
>> > to
>> > >>>> make.
>> > >>>> >
>> > >>>> > Thoughts on the approach, things I'm missing?
>> > >>>> >
>> > >>>> > Braden
>> > >>>>
>> > >
>> > >
>> > >
>> > > --
>> > > @purplecabbage
>> > > risingj.com
>> >
>>
>
>

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