[Interest] QML Settings fails persistance on Android

[maitai]

Hi, 

Here I have a slot in my backend (c++) that is called each time the app
goes to background (from override onPause() in java). I do sync() there.

Philippe. 

Le 31-08-2018 13:26, René Hansen a écrit :

> 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 [1] 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 
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Links:
------
[1] http://doc.qt.io/qt-5/qsettings.html#sync
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