[Interest] QML Settings fails persistance on Android

[Paolo Angelelli]

I also have had several issues (mostly persistence issues) with Qt.labs.settings.
In the end using QSettings directly seemed to work alright, and so i kept using
that.

A pity since Qt.labs.settings looks much easier to use, but nevertheless..

On Fri, 31 Aug 2018 13:26:05 +0200
René Hansen <renehh at gmail.com> wrote:

> Hi,
> 
> 
> I've run into a number of weird cases where properties on a *Settings*
> element doesn't properly persist between launches of an app.
> 
> Initially I used aliasing quite heavily for it's syntactic ease, as per the
> main example from the docs. However, I found that sometimes only part of
> the the properties would properly persist for next launch. I could never
> quite reproduce the issues consistently and it had a sort of
> race-condition'y feel about it. So I gave up an aliases.
> 
> Instead I've opted for a direct one-way load binding on launch and then a
> write-back to the Settings when the app closes down. This seems to work
> correctly regarding value persistence. That is, when it actually works...
> 
> Here's the pickle; Android has two means of closing an application. One is,
> that you can directly exit it via the back button, which triggers a Close
> event on the main window, the other is by suspending the app and then
> "clearing" it out from the list of background processes.
> 
> The former produces a valid result, where properties are properly persisted
> to the *Settings*, the other does not.
> 
> I'm guessing this is because the actual write-back happens on the
> destruction of the Settings element, which is, afaict, never triggered if
> the app is cleared during suspension.
> 
> I've made a futile attempt at manually triggering the write-back, at an
> earlier stage of the shutdown, by connecting the the *application.state*,
> e.g.:
> 
> *  Connections {*
> *    target: Qt.application*
> *    onStateChanged: {*
> *      switch(Qt.application.state) {*
> *      case Qt.ApplicationActive:*
> *        console.log("Qt.ApplicationActive")*
> *        break;*
> *      case Qt.ApplicationInactive:*
> *        console.log("Qt.ApplicationInactive")*
> *        break;*
> *      case Qt.ApplicationSuspended:*
> *        console.log("Qt.ApplicationSuspended")*
> 
> *        // WRITE STUFF TO SETTINGS HERE*
> 
> *        break;*
> *      case Qt.ApplicationHidden:*
> *        console.log("Qt.ApplicationHidden")*
> *        break;*
> *      }*
> *    }*
> *  }*
> 
> However, even by doing it this way, "seemingly" before the app is
> suspended, the values are still not written back at all. Presumably it's
> because of missing destruction trigger mentioned above.
> 
> So, what are my options here if I'd like to use *Settings* here?
> 
> I notice QSettings have a sync
> <http://doc.qt.io/qt-5/qsettings.html#sync> method,
> but I don't see anything similar for the QML variant.
> 
> What is the canonical way to solve this problem for Android? Has anyone
> else done this in a practical manner?
> 
> 
> Best regards,
> 
> René Hansen