kcms/nightcolor: Show live temperature preview

Allows live preview of what Night Color will look like when active. Now
users don't have to find a temperature they like by trial-and-error.

Requires https://invent.kde.org/plasma/kwin/-/merge_requests/1931

kcms/nightcolor/package/contents/ui/main.qml

@@ -118,10 +118,20 @@ KCM.SimpleKCM {
                     from: 1000 // TODO get min/max fron kcfg
                     to: 6500
                     stepSize: 100
+                    live: true
 
                     value: kcm.nightColorSettings.nightTemperature
 
-                    onMoved: kcm.nightColorSettings.nightTemperature = value
+                    onMoved: {
+                        kcm.nightColorSettings.nightTemperature = value
+                        previewMessage.state = "visible"
+                        cA.preview(value)
+                    }
+
+                    onPressedChanged: {
+                        previewMessage.state = "invisible"
+                        cA.stopPreview()
+                    }
 
                     Layout.columnSpan: 3
 
@@ -147,6 +157,35 @@ KCM.SimpleKCM {
                 Item {}
             }
 
+            QQC2.Label {
+                id: previewMessage
+                text: i18n("This is what Night Color will look like when active.")
+                opacity: 0
+                state: "invisible"
+                states: [
+                    State {
+                        name: "invisible"
+                        PropertyChanges { target: previewMessage; opacity: 0 }
+                    },
+                    State {
+                        name: "visible"
+                        PropertyChanges { target: previewMessage; opacity: 1 }
+                    }
+                ]
+                transitions: [
+                    Transition {
+                        from: "invisible"
+                        to: "visible"
+                        NumberAnimation { properties: "opacity"; easing.type: Easing.OutCubic; duration: Kirigami.Units.shortDuration }
+                    },
+                    Transition {
+                        from: "visible"
+                        to: "invisible"
+                        NumberAnimation { properties: "opacity"; easing.type: Easing.InCubic; duration: Kirigami.Units.shortDuration }
+                    }
+                ]
+            }
+
             Item {
                 Kirigami.FormData.isSection: true
             }

libcolorcorrect/compositorcoloradaptor.cpp

@@ -108,4 +108,13 @@ void CompositorAdaptor::sendAutoLocationUpdate(double latitude, double longitude
     m_iface->call(QStringLiteral("nightColorAutoLocationUpdate"), latitude, longitude);
 }
 
+void CompositorAdaptor::preview(int temperature)
+{
+    m_iface->call("preview", (uint)temperature);
+}
+
+void CompositorAdaptor::stopPreview()
+{
+    m_iface->call("stopPreview");
+}
 }

libcolorcorrect/compositorcoloradaptor.h

@@ -72,6 +72,22 @@ public:
      **/
     Q_INVOKABLE void sendAutoLocationUpdate(double latitude, double longitude);
 
+    /**
+     * @brief Preview a color temperature for 15s.
+     *
+     * @return void
+     * @since 5.25
+     **/
+    Q_INVOKABLE void preview(int temperature);
+
+    /**
+     * @brief Stop an ongoing preview.
+     *
+     * @return void
+     * @since 5.25
+     **/
+    Q_INVOKABLE void stopPreview();
+
 Q_SIGNALS:
     void errorChanged();
     void errorTextChanged();