[webkit-gtk] Is JS callback run from GUI thread?

Gaute Hope eg at gaute.vetsj.com
Thu Oct 19 11:19:29 PDT 2017

Michael Catanzaro writes on oktober 19, 2017 17:54:
> On Thu, Oct 19, 2017 at 2:29 AM, Gaute Hope <eg at gaute.vetsj.com> wrote:
>> I am trying to use javascript in synchronized fashion, by waiting in 
>> the calling thread using condition_variables and mutex (C++):
>> ```
>>  void run_js () {
>>    /* this runs in GTK gui thread */
>>    webkit_web_view_run_javascript (...., callback, ...)
>>    /* wait for notification from callback */
>>  }
>>  void callback (GObject *, GAsyncResult *r) {
>>       /* non-GUI processing of result */
>>    /* notify run_js thread */
>>  }
>> ```
>> However, if I wait for notifications in run_js the callback is never 
>> called. Is the JS callback somehow called from the main GTK thread?
> Yes, probably.
> Like GTK+ itself, WebKitGTK+ is not threadsafe and it must only be used 
> from the main GTK+ thread. If you need to schedule some work from a 
> secondary thread, then you need to post it to the main thread by using 
> e.g. g_idle_add().

Hi, thanks for your answer. I don't want execute JS from multiple 
threads, I want to run javascript *synchronously* from the main GTK+ 
thread. Since the javascript is run async, I assume it is executed in a 
different thread/process than GTK+ main. 

However, it seems that I cannot wait for a javascript callback to happen 
in the GTK+ main thread without iterating the GTK main loop. I am using 
webkit with JS as an API (to avoid setting up webextensions and the IPC 
that follows with it), and I want to make sure that the JS is run and 
its effect on the UI are visible (or ready for drawing at least) before 
I allow any more user events to happen.

Regards, Gaute

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