desktop: automatically pick a good theme title color

The preference setting seemed far too strange to do this. And not very user
friendly. So instead we figure out if this is a dark theme or not by looking at
text and background colors in the palette, and make sure we get notified if
that changes.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

desktop-widgets/tab-widgets/TabBase.cpp

@@ -5,6 +5,7 @@ TabBase::TabBase(QWidget *parent) : QWidget(parent)
 {
 }
 
-void TabBase::updateUi()
+void TabBase::updateUi(QString titleColor)
 {
+	Q_UNUSED(titleColor)
 }

desktop-widgets/tab-widgets/TabBase.h

@@ -13,7 +13,7 @@ public:
 	TabBase(QWidget *parent = 0);
 	virtual void updateData() = 0;
 	virtual void clear() = 0;
-	virtual void updateUi();
+	virtual void updateUi(QString titleColor);
 };
 
 #endif

desktop-widgets/tab-widgets/TabDiveInformation.cpp

@@ -243,14 +243,10 @@ void TabDiveInformation::updateData()
 		showCurrentWidget(false, 0);  // Show current star widget at lefthand side
 }
 
-void TabDiveInformation::updateUi()
+void TabDiveInformation::updateUi(QString titleColor)
 {
-	// Put together appropriate CSS stylesheets: NB: colors below in same order as the enum in prefs.h
-	QStringList colors = { "mediumblue", "lightblue", "black" };	// If using dark theme, set color appropriately
-	QString colorText = colors[prefs.headerstyle_color];
-
 	QString CSSSetSmallLabel = "QLabel:enabled { color: ";
-	CSSSetSmallLabel.append(colorText + "; font-size: ");
+	CSSSetSmallLabel.append(titleColor + "; font-size: ");
 	CSSSetSmallLabel.append(QString::number((int)(0.5 + ui->diveHeadingLabel->geometry().height() * 0.66)) + "px;}");
 	ui->groupBox_visibility->setStyleSheet(ui->groupBox_visibility->styleSheet() + CSSSetSmallLabel);
 	ui->groupBox_current->setStyleSheet(ui->groupBox_current->styleSheet() + CSSSetSmallLabel);

desktop-widgets/tab-widgets/TabDiveInformation.h

@@ -16,7 +16,7 @@ public:
 	~TabDiveInformation();
 	void updateData() override;
 	void clear() override;
-	void updateUi() override;
+	void updateUi(QString titleColor) override;
 private slots:
 	void divesChanged(const QVector<dive *> &dives, DiveField field);
 	void cylinderChanged(dive *d);

desktop-widgets/tab-widgets/maintab.cpp

@@ -47,6 +47,12 @@ struct Completers {
 	QCompleter *tags;
 };
 
+static bool paletteIsDark(const QPalette &p)
+{
+	// we consider a palette dark if the text color is lighter than the windows background
+	return p.window().color().valueF() < p.windowText().color().valueF();
+}
+
 MainTab::MainTab(QWidget *parent) : QTabWidget(parent),
 	editMode(false),
 	ignoreInput(false),
@@ -72,6 +78,9 @@ MainTab::MainTab(QWidget *parent) : QTabWidget(parent),
 	extraWidgets << new TabDiveComputer(this);
 	ui.tabWidget->addTab(extraWidgets.last(), tr("Device names"));
 
+	// make sure we know if this is a light or dark mode
+	isDark = paletteIsDark(palette());
+
 	// call colorsChanged() for the initial setup now that the extraWidgets are loaded
 	colorsChanged();
 
@@ -704,13 +713,24 @@ void MainTab::clearTabs()
 		widget->clear();
 }
 
+void MainTab::changeEvent(QEvent *ev)
+{
+	if (ev->type() == QEvent::PaletteChange) {
+		// check if this is a light or dark mode
+		bool dark = paletteIsDark(palette());
+		if (dark != isDark) {
+			// things have changed, so setup the colors correctly
+			isDark = dark;
+			colorsChanged();
+		}
+	}
+	QTabWidget::changeEvent(ev);
+}
+
 // setup the colors of 'header' elements in the tab widget
 void MainTab::colorsChanged()
 {
-	// Put together appropriate CSS stylesheets: NB: colors below in same order as the enum in prefs.h
-	QStringList colors = { "mediumblue", "lightblue", "black" };	// If using dark theme, set color appropriately
-	QString colorText = colors[prefs.headerstyle_color];
-
+	QString colorText = isDark ? QStringLiteral("lightblue") : QStringLiteral("mediumblue");
 	QString lastpart = colorText + " ;}";
 
 	// only set the color if the widget is enabled
@@ -731,5 +751,5 @@ void MainTab::colorsChanged()
 
 	// finally call the individual updateUi() functions so they can overwrite these style sheets
 	for (TabBase *widget: extraWidgets)
-		widget->updateUi();
+		widget->updateUi(colorText);
 }

desktop-widgets/tab-widgets/maintab.h

@@ -78,6 +78,8 @@ private:
 	dive_trip *currentTrip;
 	QList<TabBase*> extraWidgets;
 	void divesEdited(int num); // Opens a warning window if more than one dive was edited
+	void changeEvent(QEvent *ev) override;
+	bool isDark;
 };
 
 #endif // MAINTAB_H