What is necessary to update QML properties from the host language?

Question

I am working on a GUI project with PySide (Qt) and planned to keep all backend/model code in python. This looked easy enough with normal Qt Widgets since it is easy to save a reference to the widget and then update properties as necessary. However I then saw QtQuick and have started to go that direction. The problem that I am having is that I can not seem to figure out how to access QML elements from Python. For example lets say I have a QML file with just a Text element and I want to update that on some frequency. How would I gain access to the Text element to be able to set the text property? What is the standard way to accomplish this?

UPDATE - The marked answer is essentially what I am using with some additions. Not long before the answer was posted I found the reference page that explained all options (http://qt-project.org/doc/qt-4.8/qtbinding.html). The approach I am taking is to setup a class with a property that I want to be updated in the QML element, pass the object to setContextProperty(), reference the object.property in QML then be sure that a notify signal is setup so any change to the object will be reflected in QML. I'll post my code later so others can see it for future reference.

Jairo Jairo · Accepted Answer · 2014-07-08T07:17:41

You can only access to most-top element properties. The "right" way should be using alias properties. If you want, for example, define a Label element (just a preconfigured Text element) you should do something like this:

// Label.qml
Item{
  id: root
  property alias text: txLabel.text
  Text{
    id: txLabel
    anchors{ fill: parent; margins: 4 }
    verticalAlignment: Text.AlignVCenter
    horizontalAlignment: Text.AlignHCenter
    // ... other stuff
  }

}

Every time you set to or get from Label.text, you are working with txLabel.text indeed.

I don't know how to expose variables from PySide to QML engine, but it should be like C++: setContextProperty() function or something else. So, in order to use our Label component. We can do:

// OurPage.qml
Item{
    id: root
    width: 800; height: 600
    Text{
        id: txCaption
        anchors{ centerIn: parent }
        text: qsTr("My name is:")
    }
    Label{
        anchors{ top: textCaption }
        text: myName
    }
}

Where myName is a backend variable exported with setContextProperty("myName", pyVarMyName).

I hope this help you.

What is necessary to update QML properties from the host language?

1 Answers