[Interest] Some questions on QSettings, qmlRegisterType() and setContextProperty

Jayesh Bhoot mister.bhoot at gmail.com
Fri Aug 9 17:12:53 CEST 2013

I will try explaining my confusion through the application I am currently  

My application (based on Qt5.1 + Qt Quick Controls) interacts with  
Facebook API to manage a Facebook Page. I am trying to keep the QML code  
(for UI) as separate as possible from the C++ core.

Now, an OAuth2 implementation is required to be able to interact with  
Facebook API. For that, I have a C++ OAuth2 class, the constructor of  
which has the following signature:

OAuth2::OAuth2(QString appId, QString redirectUrl, QStringList  

Now, as the OAuth process requires a browser, I have also implemented an  
OAuthBrowser.qml, which uses OAuth2 to complete an authorization.

I have the following options to expose OAuth2 class to OAuth2Browser:

1. Instantiate OAuth2 and use setContextProperty() to expose the instance  
to OAuth2Browser. However, this means my C++ code has to deal with the UI  
code. The more baffling issue is that OAuth2Browser is a secondary window.  
When a user clicks on a "Authorize" window on the MainWindow, then an  
AppController C++ object (connected to MainWindow) will launch the  
OAuth2Browser window. Thus, the instantiation code of OAuth2Browser would  
go deep down inside a AppController method. It would have been good if  
only main.cpp had to deal with the window creation.

2. Use qmlRegisterType(). In this case, I can't pass parameters to the  
constructor. So, I will have to implement an init() method that would  
initialize an OAuth2 object. Then, I would call this init() method in  
OAuth2Browser's Component.onCompleted() method.
However, in this approach, I will have to expose QSettings to the UI code  
- QML window, so that the required parameters to init() method can be  
retrieved. I have huge skepticism on whether directly exposing application  
settings to QML UI is a good idea.

3. Implicitly use QSettings within the OAuth2 constructor. This way, I  
won't have to pass any parameters, and I would be able to use  
qmlRegisterType(). However, this means I am doing some magic stuff "behind  
the curtains". Instead of explicitly passing QSettings instance, I am  
using it wherever the hell I want to, thus hiding the initialization  
detail from public API.

An alternative based on the 3rd option was advised on IRC - use an  
initFromSettings() type of method to initialize an instance if no  
parameter is passed to the constructor. That way, the initialization is  
not hidden, and initFromSettings() can confidently use QSettings within  
itself. Now, I can happily use qmlRegisterType() to instantiate OAuth2 in  

So, what is the better approach?
1. Is exposing QSettings directly to QML UI a good idea?
2. I personally prefer qmlRegisterType() to setContextProperty() - that  
way, the lifetime of a registered class's instance is maintained solely by  
QML. However, the former is less likely to be used due to the lack of  
support of parameterized constructors, unless some form of init() is used  
explicitly for initialization. Is that a good design?

I apologise in advance for an excruciatingly long post. But I thought it  
best to ask here.


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