qt - How to declare enumerations in QML, without any JavaScript?

Qt - How to declare enumerations in QML, without any JavaScript?

In QML, enumerations can be declared using the enum keyword within a QML file. This allows you to define symbolic names for integer values, which can then be used throughout your QML code. Here's how you can declare and use enumerations in QML:

Example of Declaring Enumerations in QML

// MyEnums.qml import QtQuick 2.0 // Define an enumeration enum Color { Red, Green, Blue } // Another enumeration enum Shape { Circle, Rectangle, Triangle }

Using Enumerations in QML Components

After defining your enumerations in a separate QML file (e.g., MyEnums.qml), you can import and use them in other QML components or files:

import QtQuick 2.0 import "MyEnums.qml" as Enums Rectangle { width: 200 height: 200 Text { text: Enums.Color.Red // Using enum value anchors.centerIn: parent } Text { text: Enums.Shape.Circle // Using another enum value anchors { horizontalCenter: parent.horizontalCenter top: parent.bottom topMargin: 10 } } }

Explanation

  1. Define Enumerations: In MyEnums.qml, use the enum keyword to define your enumerations (Color and Shape in this example). Each enumeration can contain a list of symbolic names that correspond to integer values starting from 0.

  2. Import and Use: In other QML files where you need to use these enumerations, import the file containing the enumerations (MyEnums.qml). You can then refer to the enumeration values using dot notation (Enums.Color.Red, Enums.Shape.Circle, etc.).

Notes

  • Scoped Names: By importing the enumeration file with an alias (as Enums in the example), you can access the enum values using Enums. prefix.

  • Integer Values: By default, the first value in the enumeration starts with 0, and subsequent values increment by 1 (Red = 0, Green = 1, etc.). You can explicitly assign integer values if needed (Red = 1, Green = 2, ...).

  • JavaScript and QML: While QML is inherently declarative, it allows JavaScript expressions when necessary. However, for simple enum declarations and usage, you can avoid JavaScript and stick to pure QML declarations.

Using enumerations in QML helps maintain clarity and improves code readability by replacing magic numbers with meaningful symbolic names, enhancing the maintainability of your QML codebase.

Examples

  1. How to define enumerations in QML using enums?

    • Description: Demonstrates how to declare and use enumerations directly in QML using the enum keyword.
    • Code:
      // MyEnums.qml QtObject { enum Color { Red, Green, Blue } } // Usage in another QML file import "." MyEnums { property MyEnums.Color currentColor: MyEnums.Color.Red }

  2. How to use enums from a separate QML file in Qt?

    • Description: Shows how to define enums in a separate QML file and import and use them in other QML components.
    • Code:
      // Enums.qml QtObject { enum Fruit { Apple, Banana, Orange } } // Usage in another QML file import "Enums.qml" as Enums Item { property Enums.Fruit chosenFruit: Enums.Fruit.Apple }

  3. How to access enum values in QML from C++?

    • Description: Guides on how to expose and access QML enums from C++ code when integrating Qt with C++.
    • Code: (C++ side)
      // main.cpp #include <QGuiApplication> #include <QQmlApplicationEngine> #include <QQmlContext> enum Color { Red, Green, Blue }; int main(int argc, char *argv[]) { QGuiApplication app(argc, argv); QQmlApplicationEngine engine; engine.rootContext()->setContextProperty("Color", Color); engine.load(QUrl(QStringLiteral("qrc:/main.qml"))); if (engine.rootObjects().isEmpty()) return -1; return app.exec(); }
      Code: (QML side)
      // main.qml import QtQuick 2.15 import QtQuick.Controls 2.15 ApplicationWindow { visible: true width: 640 height: 480 title: "Enum Example" Text { text: Color.Red } }

  4. How to use enum properties in QML ListView?

    • Description: Shows how to bind enum values to QML components such as ListView items.
    • Code:
      // Enums.qml QtObject { enum Fruit { Apple, Banana, Orange } } // Usage in ListView import "Enums.qml" as Enums ListView { model: [Enums.Fruit.Apple, Enums.Fruit.Banana, Enums.Fruit.Orange] delegate: Text { text: modelData === Enums.Fruit.Apple ? "Apple" : modelData === Enums.Fruit.Banana ? "Banana" : modelData === Enums.Fruit.Orange ? "Orange" : "" } }

  5. How to switch on enum values in QML?

    • Description: Demonstrates how to use switch statements with QML enums to conditionally handle different enum values.
    • Code:
      // Enums.qml QtObject { enum Day { Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday } } // Usage in QML switch statement import "Enums.qml" as Enums Rectangle { property Enums.Day today: Enums.Day.Monday Text { text: { switch (today) { case Enums.Day.Monday: return "Start of the week"; case Enums.Day.Friday: return "TGIF"; default: return "Another day"; } } } }

  6. How to bind enum values to ComboBox in QML?

    • Description: Guides on populating and binding QML ComboBox with enum values.
    • Code:
      // Enums.qml QtObject { enum Month { January, February, March, April, May, June, July, August, September, October, November, December } } // Usage in QML ComboBox import "Enums.qml" as Enums ComboBox { model: Object.keys(Enums.Month) textRole: "name" onCurrentIndexChanged: { console.log("Selected month:", Enums.Month[model.get(index).name]) } }

  7. How to use enums in QML signal handlers?

    • Description: Shows how to define and use enums in signal handlers to handle different states or actions in QML.
    • Code:
      // Enums.qml QtObject { enum State { Idle, Running, Stopped } } // Usage in QML signal handler import "Enums.qml" as Enums Rectangle { property Enums.State currentState: Enums.State.Idle MouseArea { onClicked: { currentState = Enums.State.Running } } }

  8. How to create constants using enums in QML?

    • Description: Illustrates how to use QML enums to define and utilize constants throughout QML components.
    • Code:
      // Enums.qml QtObject { enum Constants { Pi = 3.14, SpeedOfLight = 299792458 } } // Usage in QML import "Enums.qml" as Enums Text { text: "The value of Pi is " + Enums.Constants.Pi }

  9. How to dynamically set enum values in QML properties?

    • Description: Guides on dynamically setting enum values in QML properties based on runtime conditions or user interactions.
    • Code:
      // Enums.qml QtObject { enum Direction { Up, Down, Left, Right } } // Usage in QML property binding import "Enums.qml" as Enums Rectangle { property Enums.Direction currentDirection: Enums.Direction.Up MouseArea { onClicked: { if (condition) { currentDirection = Enums.Direction.Left } else { currentDirection = Enums.Direction.Right } } } }

  10. How to define and use enums with properties in QML?

    • Description: Shows how to define QML properties using enums for maintaining and accessing state or configuration.
    • Code:
      // Enums.qml QtObject { enum Status { Active, Inactive, Pending } } // Usage in QML properties import "Enums.qml" as Enums Rectangle { property Enums.Status currentStatus: Enums.Status.Active Text { text: { return currentStatus === Enums.Status.Active ? "Active" : currentStatus === Enums.Status.Inactive ? "Inactive" : currentStatus === Enums.Status.Pending ? "Pending" : "" } } }

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