Using File Selectors with Qt Quick Controls

File selectors provide a convenient way of selecting file variants. Qt offers the platform name and the locale as built-in selectors. Qt Quick Controls extends the built-in selectors with the name (lowercase) of the style that an application is running with.

By using file selectors, style-specific tweaks can be applied without creating a hard dependency to a style. From the available file variants, only the selected QML file is loaded by the QML engine. Each file variant can assume the context, that is, a specific style. This typically leads to some code duplication, but on the other hand, cuts the aforementioned hard dependency to the style, and leads to simpler and more efficient QML code. The same technique is used to implement the Qt Quick Controls styles.

The following example demonstrates a custom rounded button that has a styled drop shadow in the Material style, and looks flat in other styles. The files are organized so that the Material version of CustomButton.qml is placed into a +material sub-directory.

 :/main.qml
 :/CustomButton.qml
 :/+material/CustomButton.qml

By default, main.qml will use CustomButton.qml for the CustomButton type. However, when the application is run with the Material style, the material selector will be present and the +material/CustomButton.qml version will be used instead.

 // main.qml
 import QtQuick 2.15
 import QtQuick.Controls 2.15

 ApplicationWindow {
     id: window
     visible: true

     CustomButton {
         text: "Button"
         anchors.centerIn: parent
     }
 }

The base implementation of the custom button is a simple rounded flat button.

 // CustomButton.qml
 import QtQuick 2.15
 import QtQuick.Controls 2.15

 Button {
     id: control

     background: Rectangle {
         radius: width / 2
         implicitWidth: 36
         implicitHeight: 36
         color: control.pressed ? "#ccc" : "#eee"
     }
 }

The Material style's implementation of the custom button imports the Material style, requests a dark theme to get light text, and creates a drop shadow for the background.

 // +material/CustomButton.qml
 import QtQuick 2.15
 import QtGraphicalEffects 1.15
 import QtQuick.Controls 2.15
 import QtQuick.Controls.Material 2.15

 Button {
     id: control

     Material.theme: Material.Dark

     background: Rectangle {
         implicitWidth: 48
         implicitHeight: 48
         color: Material.accentColor
         radius: width / 2

         layer.enabled: control.enabled
         layer.effect: DropShadow {
             verticalOffset: 1
             color: Material.dropShadowColor
             samples: control.pressed ? 20 : 10
             spread: 0.5
         }
     }
 }

Note: It is recommended to use QQmlApplicationEngine, which internally creates a QQmlFileSelector instance. This is all that is needed to take QML file selectors into use.

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Using File Selectors with Qt Quick Controls

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