changing QML variables from c++

shokarta

hello,

as I have in QML File:

property bool dbOnline
onDbOnlineChanged: { console.log("variable changed"); }

and then somehow from C++ i wish to change this variable so the change will be noticed by QML and will debug the "variable changed"

can you show an example how to do that please?

mrjj

Hi
Did you try
https://doc.qt.io/qt-5/qqmlproperty.html

https://doc.qt.io/qt-5/qtqml-cppintegration-interactqmlfromcpp.html

shokarta

@mrjj hello,
yes I have checked those, however they are object properties,
i dont know how to implement this fo common QML variable :(

mrjj

@shokarta
Im not sure you can as all sample i have seen is been via what shown in those links/docs.
But Im not super QML expert so give it some days and see if you get
a solution.

shokarta

BUMP,
noone know if this is realy possible?
Why should not be realy...

KroMignon

@shokarta said in changing QML variables from c++:

noone know if this is realy possible?

May be possible, but I don't see use case for this.
The "Qt way" would be to have a C++ QObject which hold the "shared" variable and which will be exposed to QML.
Doing this from QML side is not a good practice in my eyes.

Please read this https://doc.qt.io/qt-5/qtqml-cppintegration-overview.html.
This could help you to do things in the best way.
And never forget this fundamental rule: if you fighting against the Qt/QML framework, then you are probably doing things in the wrong way.

juste my 2 cts

Jarek B

@shokarta
Basically what you are trying to accomplish is having variable on the QML side that will be a state of something else (that is probably kept on the C++ side). You are introducing another variable that needs to be synchronized, and its only purpose is to cache something that should be available on the C++ side.

Imagine you have QObject of class DBConnector that is exposed to a QML and this object have bool property called online. Now on the QML side you can simply do

    Connections {
        target: App.Context.dbConnector
        onOnlineChanged: {
            console.log("variable changed")
        }
    }

App.Context is in this case just a simple class with bunch of properties/objects that are accessible in the QML by:

import Application 1.0 as App

It can be registered using qmlRegisterSingletonType on the C++ side
https://doc.qt.io/qt-5/qqmlengine.html#qmlRegisterSingletonType-1
For Qt < 5.14 you have to use static method or free function instead of lambda.

You can also use QQmlContext::setContextPropery but it is encouraged to use much safer alternatives.

EDIT:
I have no knowledge about QQmlContext::setContextPropery been marked as deprecated (I oversimplified some news I have heard). Only thing I know for sure is that there are much safer ways of achieving the same results that will be encouraged in the current and next releases.

ODБOï

hi
@Jarek-B said in changing QML variables from c++:

Connections {
target: App.Context.dbConnector
onOnlineChanged: {
console.log("variable changed")
}
}

if you use Qt 5.15 Connections syntax has changed

 Connections {
         target: App.Context.dbConnector
         function onOnlineChanged() {
             console.log("variable changed")
         }
     }