Restore property binding when changed

kkettinger

Hello,

for a settings page, i have a TextField which is connected to a c++ property:

Q_PROPERTY(QString serialNumber READ getSerialNumber WRITE setSerialNumber NOTIFY serialNumberChanged)

QML:

TextField {
    id: textFieldSerialNumber
    value: backend.serialNumber
}

When the page is loaded, the value is displayed and changed-signals are received.

Now the user should have an option to discard any changes made to the TextField. There is now an extra button:

Button {
    id: buttonDiscard
    onClicked: textFieldSerialNumber= backend.serialNumber
}

This restores textFieldSerialNumber to the original value of the backend, but now it seems i've lost the binding to the c++ property and i can't receive any changes from the backend anymore.

I see this when using another button for resetting to default values:

Button {
    id: buttonReset
    onClicked: backend.resetSerialNumber()
}

This changes the c++ property to the default value and emits a changed() signal to qml. But because the user clicked "discard", the property is not updated anymore.

Is there any solution only with properties?

I appreciate any help.

KroMignon

@kkettinger said in Restore property binding when changed:

Is there any solution only with properties?

My way to doing this would be:

property var aux: backend.serialNumber
TextField {
    id: textFieldSerialNumber
    value: backend.serialNumber
}

Button {
    id: buttonDiscard
    onClicked: textFieldSerialNumber.value = backend.serialNumber // Set current value (no more updates)
}

Button {
    id: buttonReset
    onClicked:  {
        textFieldSerialNumber.value = backend.serialNumber // set current value
        textFieldSerialNumber.value = Qt.binding(function() { return backend.serialNumber} ) //"restore" connection (updated on changes)
    }
}

sierdzio

This is one of the classic problems of QML :-) Check Andre's slides: https://www.kdab.com/kdab-qt-world-summit-2019/#component The video went down for some reason :-(

In general, there is no ready-made solution for this in Qt, but there are several ways to address this problem and it can be fixed quite easily.

johngod

Please check if something like this works:

property var aux: backend.serialNumber
TextField {
    id: textFieldSerialNumber
    value: aux
}

Button {
    id: buttonDiscard
    onClicked: aux = "some original value"
}

Button {
    id: buttonReset
    onClicked: aux = backend.serialNumber //"restore" connection 
}

KroMignon

@kkettinger said in Restore property binding when changed:

Is there any solution only with properties?

My way to doing this would be:

property var aux: backend.serialNumber
TextField {
    id: textFieldSerialNumber
    value: backend.serialNumber
}

Button {
    id: buttonDiscard
    onClicked: textFieldSerialNumber.value = backend.serialNumber // Set current value (no more updates)
}

Button {
    id: buttonReset
    onClicked:  {
        textFieldSerialNumber.value = backend.serialNumber // set current value
        textFieldSerialNumber.value = Qt.binding(function() { return backend.serialNumber} ) //"restore" connection (updated on changes)
    }
}

Kai Nickel

You may want to have a look at the Qt.binding() function to re-activate a broken binding. See https://doc.qt.io/qt-5/qtqml-syntax-propertybinding.html#creating-property-bindings-from-javascript for details.

kkettinger

Using Qt.binding() does get the job done, thank you :)