A customized busy indicator in Qt Quick

For the MuseScore apps, I needed an easily customizable BusyIndicator. The designer came up with something similar to spin.js but of course, we needed it in QML.

Here is spin.js in action:

And here is the QML version.
Since we can't run QML in the browser (yet?), I made a short video. Of course, you can just download the source code and run it yourself.


Qt Quick Controls can be customized with a (ControlName)Style. For a BusyIndicator, it's a BusyIndicatorStyle.

BusyIndicatorStyle {  
    id: style
    property int lines: 11
    property real length: 10 // % of the width of the control
    property real width: 5 // % of the height of the control
    property real radius: 13 // % of the width of the control
    property real corner: 1 // between 0 and 1
    property real speed: 100 // smaller is faster
    property real trail: 0.6 // between 0 and 1
    property bool clockWise: true

    property real opacity: 0.7
    property string color: "#7B756B"
    property string highlightColor: "white"
    property string bgColor: "black"

    indicator: Rectangle {
        color: style.bgColor
        visible: control.running
        Repeater {
            id: repeat
            model: style.lines
            Rectangle {
                property real factor: control.width / 200
                color: style.color
                opacity: style.opacity
                Behavior on color {
                    ColorAnimation {
                        from: style.highlightColor
                        duration: style.speed * style.lines * style.trail
                radius: style.corner * height / 2
                width: style.length * factor
                height: style.width * factor
                x: control.width / 2 + style.radius * factor
                y: control.height / 2 - height / 2
                transform: Rotation {
                    origin.x: -style.radius * factor
                    origin.y: height / 2
                    angle: index * (360 / repeat.count)
                Timer {
                    id: reset
                    interval: style.speed * (style.clockWise ? index : style.lines - index)
                    onTriggered: {
                        parent.opacity = 1
                        parent.color = style.highlightColor
                Timer {
                    id: reset2
                    interval: style.speed
                    onTriggered: {
                        parent.opacity = style.opacity
                        parent.color = style.color
                Timer {
                    id: globalTimer // for a complete cycle
                    interval: style.speed * style.lines
                    onTriggered: {
                    triggeredOnStart: true
                    repeat: true
                Component.onCompleted: {

I exposed the different customization parameters as QML properties.

Then the core of the style is a Repeater. It will draw a given number of Rectangles using the color, corner radius etc... and of course a Rotation.

To animate the spinner, I relied on 3 Timers per rectangle.

  • A global timer fires for every cycle and start a second timer.
  • This second timer will animate the color and opacity of the given rectangle with an interval depending on the index of the rectangle and start a new timer.
  • This last timer will restore the state of the rectangle to default according to the speed parameters.

Source code

The code is on GitHub: CustomBusyIndicator
It could be that there is a better and more QML way of doing the animation. If so, let me know @lasconic

Edit: For a QtWidget approach, see https://github.com/snowwlex/QtWaitingSpinner