Regarding Qml bindings - qt

Question

Why the below code is not working when clicked on cancel button ie., parameterValueText.text is not setting to value 0.
I am assuming both val and parameterValueText.text are binded eachother. If I am wrong, please correct me
Please find below steps
Step 1: Edit a value in Text Input, for ex: 1
Step 2: on Keyboard accepted, parameterValueText.onAccepted is called
Step 3: Click on Save, saveBtn.onClicked is called
Step 4: Click on Cancel, cancelBtn.onClicked is called but the parameterValueText.text is not changed to value 0.
main.qml
import QtQuick 2.12
import QtQuick.Window 2.12
import QtQuick.Controls 2.12
import QtQuick.Layouts 1.12
import QtQuick.VirtualKeyboard 2.1
ApplicationWindow {
id: window
width: 480
height: 272
visible: true
title: qsTr("Keyboard")
property int val: 0
Column {
Item {
id: itemId
height: 20
width: window.width
Rectangle{
width: 100
height: itemId.height
border.color:"black"
TextInput {
id: parameterValueText
text: val //Assuming text and val are binded ??
inputMethodHints: Qt.ImhDigitsOnly
anchors.fill: parent
horizontalAlignment: Text.AlignHCenter
verticalAlignment: Text.AlignVCenter
onAccepted: console.log("Value = ", parameterValueText.text)
}
}
}
Row {
spacing: 10
Button {
id: saveBtn
text: "Save"
onClicked: console.log("Save = ", parameterValueText.text)
}
Button {
id: cancelBtn
text: "Cancel"
onClicked: val = 0 //Why the value is not changing in parameterValueText.text ?
}
}
}
InputPanel
{
id:inputPanel
y:parent.height
width: window.width
//Background for Virtual Keyboard
Component{
id:keyboardBackground
Rectangle{
color:"#f4f6f3"//ScreenConfiguration.backGroundCanvas
}
}
states: State {
name: "visible"
when: inputPanel.active
PropertyChanges {
target: inputPanel
y: parent.height - inputPanel.height
}
PropertyChanges {
target: inputPanel
}
}
transitions: Transition {
from: ""
to: "visible"
reversible: true
ParallelAnimation {
NumberAnimation {
properties: "y"
duration: 200
easing.type: Easing.InOutQuad
}
}
}
Component.onCompleted: {
keyboard.style.keyboardBackground = keyboardBackground; // the keyboard background
}
}
}
main.cpp
int main(int argc, char *argv[])
{
qputenv("QT_IM_MODULE", QByteArray("qtvirtualkeyboard"));
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
QGuiApplication app(argc, argv);
QQmlApplicationEngine engine;
const QUrl url(QStringLiteral("qrc:/main.qml"));
QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
&app, [url](QObject *obj, const QUrl &objUrl) {
if (!obj && url == objUrl)
QCoreApplication::exit(-1);
}, Qt::QueuedConnection);
engine.load(url);
return app.exec();
}
Add below change in .pro file for enabling virtual keyboard
QT += quick virtualkeyboard
The console log ouputs are below
qml: Value = 1, val = 0
qml: Save = 1, val = 0

Answer 1

The binding isn't maintained after the user changes the text using the TextInput (as commenters suggested).
To achieve similar results to what you were attempting you could set val in the TextInput's onAccepted slot, and similarly update both val and parameterValueText.text in the onClicked slots for your buttons as appropriate. For example:
TextInput {
...
onAccepted: {
val = text
}
}
//...
Button {
id: cancelBtn
text: "Cancel"
onClicked: {
val = 0;
parameterValueText.text = val;
}
}
This won't maintain the initial bindings because the values are assigned in imperative javascript, but should achieve resetting values as expected.

Answer 2

As continuation on the answer of #dabbler, you can also recreate the binding in the onAccepted handler, but yeah it's still weird.
parameterValueText.text = Qt.binding(function() { return val; })
As another solution, you could have the val property bind to the text of the input (which then starts with "0"):
readonly property int val: parseInt(parameterValueText.text)
But that only works if the val property is intended to be used as readonly (otherwise you will loose the binding again). In case you want the val property to be set from outside, you could do the following:
onValChanged: parameterValueText.text = val
Note that this is assignment, not binding!