Qt Quick How-tos | Qt Quick | Qt 6.10.3

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Finishing Touches

# Qt Quick How-tos

This page aims to provide an easily discoverable, useful reference that shows the simplest and best way of performing specific tasks in Qt Quick. Each solution provides QML and/or C++ code snippets where applicable, and every snippet is automatically tested by Qt to ensure they remain functional.

How do I:

- Call a C++ function from QML when a Button is clicked
- See which item has active focus
- Create a time picker like Android's TimePickerDialog
- Use a C++ enum in JavaScript
- Create a Gauge

## Call a C++ function from QML when a Button is clicked

Assuming that the C++ type should be globally available to the QML files in the application, the simplest way is to make it a QML singleton with QML_SINGLETON. For example, in the header file, `backend.h`:

     #include <QObject>
     #include <QQmlEngine>

     class Backend : public QObject
     {
         Q_OBJECT
         QML_ELEMENT
         QML_SINGLETON

     public:
         Q_INVOKABLE void doStuff();
     };

`backend.cpp`:

     #include "backend.h"

     #include <QDebug>

     void Backend::doStuff()
     {
         qDebug() << "Did stuff!";
     }

You can then call that function from any QML file:

     import QtQuick.Controls

     import MyModule

     ApplicationWindow {
         width: 400
         height: 400
         title: qsTr("C++ Button example")

         Button {
             text: qsTr("Click me")
             onClicked: Backend.doStuff()
         }
     }

If the C++ type only needs to be available to a small set of QML files, consider using QML_ELEMENT. For more ways of exposing C++ types to QML, see Choosing the Correct Integration Method Between C++ and QML.

This example assumes that the `Backend` type is available in a QML module. With CMake, this is done via qt_add_qml_module. For an example that demonstrates this in detail, see Building a QML application.

## See which item has active focus

Write a property change signal handler for the window's activeFocusItem property:

     import QtQuick
     import QtQuick.Controls

     ApplicationWindow {
         width: 400
         height: 400
         visible: true
         title: qsTr("Active focus debugging example")

         onActiveFocusItemChanged: print("activeFocusItem: " + activeFocusItem)

         Row {
             TextField {
                 objectName: "textField1"
             }
             TextField {
                 objectName: "textField2"
             }
         }
     }

This will print the item which currently has active focus to the console. To ensure that the output is useful, give each item a descriptive objectName.

## Create a time picker like Android's TimePickerDialog

We've prepared an example that consists of a few QML files which demonstrate how to do this. They can be used in your application in the following manner:

     import QtQuick
     import QtQuick.Layouts
     import QtQuick.Controls.Material

     ApplicationWindow {
         id: window
         width: 600
         height: 600
         visible: true
         title: qsTr("Time Picker Example")

         Material.theme: darkThemeSwitch.checked ? Material.Dark : Material.Light

         // Shows the selected time and opens the dialog.
         TimeComponentLabel {
             id: openDialogLabel
             width: parent.width - 80
             anchors.centerIn: parent
             font.pixelSize: Qt.application.font.pixelSize * 8
             renderTypeQuality: Text.VeryHighRenderTypeQuality
             interactive: !timePickerDialog.opened

             text: Qt.formatTime(new Date(1970, 1, 1, timePickerDialog.hours, timePickerDialog.minutes), "hh:mm")

             onTapped: timePickerDialog.openWithMode(TimePicker.Mode.Hours)
         }

         ColumnLayout {
             // We always want the openDialogLabel to be centered in the window, not us.
             // For that reason, we use anchors rather than putting the root items into a ColumnLayout.
             anchors.horizontalCenter: parent.horizontalCenter
             anchors.top: openDialogLabel.bottom
             anchors.topMargin: 24
             spacing: 12

             Switch {
                 id: is24HourSwitch
                 text: qsTr("24 Hour")
                 checked: timePickerDialog.is24Hour
             }
             Switch {
                 id: darkThemeSwitch
                 text: qsTr("Dark")
             }
         }

         TimePickerDialog {
             id: timePickerDialog
             anchors.centerIn: parent
             is24Hour: is24HourSwitch.checked

             onTimeAccepted: print("A time was chosen - do something here!")
         }
     }

TimePickerDialog in its light theme. | TimePickerDialog in its dark theme.

## Use a C++ enum in JavaScript

To expose a C++ enum to JavaScript (that is, QJSEngine, not QQmlEngine or QQmlApplicationEngine), use newQMetaObject() and registerModule():

         QJSEngine engine;
         engine.installExtensions(QJSEngine::AllExtensions);

         QJSValue metaObjects = engine.newObject();
         const QJSValue backendJsMetaObject = engine.newQMetaObject(&Backend::staticMetaObject);
         metaObjects.setProperty("Backend", backendJsMetaObject);
         // Repeat the two lines above for other types as needed.
         engine.registerModule("MyApp", metaObjects);

         Backend backend(&engine);
         const bool loaded = backend.load();

The enum can then be used from JavaScript:

     export function backendStatusUpdate(backendStatus) {
         if (backendStatus === Backend.Error) {
             console.warn("Error!")
             return
         }

         console.log("Backend loaded successfully")
     }

When using QQmlEngine or QQmlApplicationEngine, there are easier options; see Choosing the Correct Integration Method Between C++ and QML for more information.

`backend.h`:

     #include <QObject>
     #include <QJSEngine>

     class Backend : public QObject
     {
         Q_OBJECT

     public:
         Backend(QJSEngine *engine);

         enum Status {
             Unknown,
             Error,
             Loading,
             Loaded
         };

         Q_ENUM(Status)

         bool load();

     private:
         QJSEngine *mEngine = nullptr;
     };

`backend.cpp`:

     #include "backend.h"

     Backend::Backend(QJSEngine *engine) :
         mEngine(engine)
     {
     }

     bool Backend::load()
     {
         // Do some loading here...

         const QJSValue module = mEngine->importModule(":/script.mjs");
         if (module.isError()) {
             qWarning() << "Error loading script.mjs:" << module.toString();
             return false;
         }

         const QJSValue function = module.property("backendStatusUpdate");
         if (!function.isCallable()) {
             qWarning() << "backendStatusUpdate script function is not callable!";
             return false;
         }

         const QJSValue functionResult = function.call(QJSValueList() << Loaded);
         if (functionResult.isError()) {
             qWarning() << "backendStatusUpdate script function had errors:" << functionResult.toString();
             return false;
         }

         return true;
     }

For more information, see QObject Integration.

## Create a Gauge

We've prepared an example that consists of a few C++ and QML files which demonstrate how to do this. They can be used in your application in the following manner:

     import QtQuick.Controls

     import GaugeHowTo

     ApplicationWindow {
         width: 400
         height: 400
         title: qsTr("Gauge example")

         Gauge {
             minimumValue: 0
             value: 75
             maximumValue: 100
         }
     }

Finishing Touches