[PyQt] QWebEnginePage API for callbacks

[Kevin Mcintyre]

I too am curious, my software uses QWebElement extensively, and was a
little surprised how page visibility there is available.

On Mon, Jan 5, 2015 at 8:27 PM, Florian Bruhin <me at the-compiler.org> wrote:

> * David Cortesi <davecortesi at gmail.com> [2015-01-05 19:59:13 -0800]:
> > On Mon, Jan 5, 2015 at 3:26 PM, Andreas Pakulat <apaku at gmx.de> wrote:
> >
> > >
> > > I'm a bit at a loss what to do following a call to such a method. What
> is
> > >> the best way to "idle" until the callback comes? n.b. I don't see any
> > >> WebEngine specific guidance at the PyQt5 doc [2].
> > >>
> > >
> > > I'd say just as all other callbacks in Qt (i.e. signal/slots), return
> > > control to the qt event loop.
> > >
> >
> > Could you clarify a bit? The doc [1] for QWebEnginePage.toPlainText()
> will
> > at some unknown future time call my callback passing the text of the
> page.
> > But I would make that call in some code that needs that text -- to write
> it
> > to a file, say. So in a custom class based on QWebEngineView, I capture
> the
> > ^S key, perhaps, for Save. I put a findSaveDialog and get a path string
> and
> > open a text stream for output. Then...
> >
> >     self.page().toPlainText( lambda s : self.save_plain_text = s )
> >     # Here --  twiddle my thumbs until I can...
> >     output_stream << self.save_plain_text
> >
> > Is this the kind of thing you have in mind?
> >
> >     self.save_plain_text = None
> >     self.page().toPlainText( lambda s : self.save_plain_text = s )
> >     while self.save_plain_text is None : QCoreApplication.processEvents()
> >     output_stream << self.save_plain_text
>
> Probably something like this:
>
>     def handle_ctrl_s(self):
>         self.save_filename = get_filename_from_user()
>         self.page().toPlainText(self.do_save)
>
>     def do_save(self, data):
>         with open(self.save_filename, 'w') as f:
>             f.write(data)
>
> Or this using functools.partial, i.e. it won't break if the user
> presses Ctrl-S again before the text is ready:
>
>     def handle_ctrl_s(self):
>         filename = get_filename_from_user()
>         self.page().toPlainText(partial(self.do_save, filename))
>
>     def do_save(self, filename, data):
>         with open(filename, 'w') as f:
>             f.write(data)
>
> I think this is a general concept which is very important to grasp
> (and also took me a while) when programming with something like PyQt:
>
> Try to make everything happen async - the majority of the time should
> be spent in the Qt event loop (i.e. the app.exec_() call). Any code
> you write gets called from that event loop usually, and should finish
> "immediately". So here, you don't "wait" until the data is ready, you
> tell Qt "call me back when the data is ready", return to the event
> loop, and *then* save it when it *is* ready.
>
> Florian
>
> --
> http://www.the-compiler.org | me at the-compiler.org (Mail/XMPP)
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