Simplest way for two-way binding a text field

xtofl

Hi, I'm having trouble to have updates sent to a bound property without explicitly defining an event handler.

class Model(QtCore.QObject):
    """simplest model with a text property"""

    def __init__(self):
        super().__init__()
        self._member_a = 'YES'

    def a(self):
        return self._member_a

    def set_a(self, v):
        print(f'changing to {v}')
        self._member_a = v

    @QtCore.Signal
    def a_changed(self):
        pass

    a = QtCore.Property(str, a, set_a, notify=a_changed)


model = Model()

app = QtGui.QGuiApplication(sys.argv)
view = QtQuick.QQuickView()
view.rootContext().setContextProperty('model', model)

view.setResizeMode(QtQuick.QQuickView.SizeRootObjectToView)
with Path('Main.qml').open('w') as f:
    f.write('''
        import QtQuick 2.0
        import QtQuick.Controls 2.0
        TextField {
          text: model.a
          // I don't want this: 
          // onEditingFinished: model.a = text
        }''')

view.setSource('Main.qml')
view.resize(100, 100)
view.show()
result = app.exec_()

My setup is with PySide6 6..0.1:

$ poetry show
astroid           2.5    An abstract syntax tree for Python with inference support.
isort             5.7.0  A Python utility / library to sort Python imports.
lazy-object-proxy 1.5.2  A fast and thorough lazy object proxy.
mccabe            0.6.1  McCabe checker, plugin for flake8
pylint            2.7.0  python code static checker
pyside6           6.0.1  Python bindings for the Qt cross-platform application and UI framework
shiboken6         6.0.1  Python / C++ bindings helper module
toml              0.10.2 Python Library for Toms Obvious, Minimal Language
wrapt             1.12.1 Module for decorators, wrappers and monkey patching.

I tried to find proper guidance in the documentation, but failed.

Is there a better way than to define the onEditingFinished handler?

Thanks!

eyllanesc

@xtofl In QML the binding is unidirectional: from right to left:

Foo {
     id: foo
     a: bar.b
}

This binding indicates that every time the property "b" (which must be a signal) of the object "bar" then the property "a" of the object "foo" will be updated with that value. In python it would be something like:

bar.b_Signal.connect(lambda: setattr(foo, "a", foo.b))

If you want to implement a 2-way binding then create a class that listens when one of those properties change ("a" or "b") and then updates the other.

SGaist

Hi,

@xtofl said in Simplest way for two-way binding a text field:

def set_a(self, v):
print(f'changing to {v}')
self._member_a = v

You are not emitting the change signal.

The classic implementation is:

def set_a(self, value):
    if self._member_a == value:
        return
    self._member_a = value
    self.a_changed.emit(value)

xtofl

Thank you. Only... this makes things even less simple. Are you saying that this is the simplest way?

SGaist

In what way is it less simple ?

The only simpler would be:

def set_a(self, value):
    if self._member_a != value:
        self._member_a = value
        self.a_changed.emit(value)

xtofl

Thanks - the emit is a necessary addition, I agree. I meant that adding a line by definition makes code less simple.

But what I really don't understand is why I would need the onEditingFinished handler in the qml itself. I was hoping to get the 2-way binding working without explicit event handling, by just binding Textfield.text property to model.a property.

eyllanesc

@xtofl In QML the binding is unidirectional: from right to left:

Foo {
     id: foo
     a: bar.b
}

This binding indicates that every time the property "b" (which must be a signal) of the object "bar" then the property "a" of the object "foo" will be updated with that value. In python it would be something like:

bar.b_Signal.connect(lambda: setattr(foo, "a", foo.b))

If you want to implement a 2-way binding then create a class that listens when one of those properties change ("a" or "b") and then updates the other.