Qt Quick 2 Legend Example

The Qt Quick 2 legend example shows how to make an interactive legend for a graph.

The interesting thing about this example is displaying the legend. We'll concentrate on that and skip explaining the basic functionality - for more detailed QML example documentation, see Qt Quick 2 Scatter Example.

Running the Example

To run the example from Qt Creator, open the Welcome mode and select the example from Examples. For more information, visit Building and Running an Example.

Legend

The legend is simply a column of custom LegendItem items inside a transparent rectangle. Each item is supplied with a series and the graph theme:

 ColumnLayout {
     anchors.fill: parent
     anchors.margins: parent.border.width
     spacing: 0
     clip: true
     LegendItem {
         Layout.fillWidth: true
         Layout.fillHeight: true
         series: station1
         theme: barGraph.theme
     }
     LegendItem {
         Layout.fillWidth: true
         Layout.fillHeight: true
         series: station2
         theme: barGraph.theme
     }
     LegendItem {
         Layout.fillWidth: true
         Layout.fillHeight: true
         series: station3
         theme: barGraph.theme
     }
 }

The legend items consist of a marker rectangle, which indicates the color of the series, and a text field, which shows the name of the series. The colors we get from the series and the theme supplied at legend item initialization:

 property Theme3D theme
 property Bar3DSeries series
     ...
 RowLayout {
     anchors.fill: parent
     spacing: 0
     clip: true
     Item {
         id: markerSpace
         Layout.minimumWidth: 20
         Layout.minimumHeight: 20
         Layout.fillWidth: true
         Layout.fillHeight: true
         Layout.alignment: Qt.AlignVCenter
         Rectangle {
             x: parent.x + parent.width / 4
             y: parent.y + parent.height / 4
             width: parent.width / 2
             height: width
             border.color: "black"
             color: series.baseColor
         }
     }
     Item {
         height: markerSpace.height
         Layout.fillWidth: true
         Layout.fillHeight: true
         Layout.alignment: Qt.AlignVCenter
         Layout.minimumWidth: 100
         Text {
             anchors.fill: parent
             text: series.name
             verticalAlignment: Text.AlignVCenter
             clip: true
             color: theme.labelTextColor
             font: theme.font
         }
     }
 }

We want the legend to be interactive, so we add additional logic to enable selection of a series by clicking on a legend item, as well as highlighting the legend item corresponding to the selected series.

The highlight depends on the selection state of the series, so we define two states, which follow the Bar3DSeries::selectedBar property and adjust the legendItem color appropriately:

 states: [
     State  {
         name: "selected"
         when: series.selectedBar != series.invalidSelectionPosition
         PropertyChanges {
             target: legendItem
             color: series.singleHighlightColor
         }
     },
     State  {
         name: "unselected"
         when: series.selectedBar == series.invalidSelectionPosition
         PropertyChanges {
             target: legendItem
             color: theme.labelBackgroundColor
         }
     }
 ]

To make the legend item interactive, we define a MouseArea to detect clicks on it and adjust the series selection accordingly:

 MouseArea {
     id: mouseArea
     anchors.fill: legendItem
     onClicked: {
         if (legendItem.state === "selected") {
             series.selectedBar = series.invalidSelectionPosition
         } else {
             series.selectedBar = previousSelection
         }
     }
 }

The previousSelection used above is another custom property of LegendItem, which we update whenever selection changes on the series. This way we remember the last selected bar of each series:

 Connections {
     target: series
     onSelectedBarChanged: {
         if (position != series.invalidSelectionPosition) {
             previousSelection = position
         }
     }
 }

Example project @ code.qt.io