[Development] QWebChannel setTimeout

Hausmann Simon Simon.Hausmann at digia.com
Sun Jul 6 08:07:22 CEST 2014

I think setTimeout would be a good function to add to qml's Javascript environment, as imperative js api next to the timer element.

That said, the risk is high that people use it to drive animations or work around other things.

Anyway, patches welcome :)


  Opprinnelig melding
Fra: Milian Wolff
Sendt: 02:33 lørdag 5. juli 2014
Til: development at qt-project.org
Emne: Re: [Development] QWebChannel setTimeout

On Friday 04 July 2014 13:41:28 Bernd Lamecker wrote:
> Hi,
> is there a special reason why you call
> setTimeout(function() { channel.exec({type: QWebChannelMessageTypes.idle});
> }, 0); (Line 179 & 197 in qwebchannel.js)
> via setTimeout? For me this also works without doing a singleshot.
> But the setTimeout function is not defined when including qwebchannel.js
> into a qml application.

You can have a look at what I did in Client.qml in https://codereview.qt-project.org/#/c/81523/13//ALL - there I implement setTimeout in terms of a QML
Timer object. Ugly and doesn't support multiple simultaneous calls to
setTimeout, but at least it works. I really think we should have a setTimeout
in QML, if alone to better support embedding external JavaScript libraries.

Now, on the topic of _why_ I use setTimeout there: This comes from the
original implementation of the QWebChannel we did for a customer of ours on an
embedded platform which showed serious performance issues and scheduling
problems, i.e. sending WebSocket messages repeatedly to a QtWebKit client
completely hogged up the CPU in the WebSocket code and all other applications
froze. This might need to be revisited, esp. considering that we'll get much
faster QtWebKit IPC instead of WebSockets for the common case. So maybe, all
of that is not required anymore. But before I blindly remove this stuff, I
guess we should first work on the core functionality and then eventually
cleanup such legacy code (and write more benchmarks etc. to verify we are not
breaking anything).


Milian Wolff | milian.wolff at kdab.com | Software Engineer
KDAB (Deutschland) GmbH&Co KG, a KDAB Group company
Tel. Germany +49-30-521325470, Sweden (HQ) +46-563-540090
KDAB - Qt Experts - Platform-independent software solutions

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