1 files changed, 90 insertions, 0 deletions
diff --git a/src/selectionmode/backgroundcolorhelper.cpp b/src/selectionmode/backgroundcolorhelper.cpp
new file mode 100644
index 000000000..8a7d69758
--- /dev/null
+++ b/src/selectionmode/backgroundcolorhelper.cpp
@@ -0,0 +1,90 @@
+/*
+    This file is part of the KDE project
+    SPDX-FileCopyrightText: 2022 Felix Ernst <[email protected]>
+
+    SPDX-License-Identifier: LGPL-2.1-only OR LGPL-3.0-only OR LicenseRef-KDE-Accepted-LGPL
+*/
+
+#include "backgroundcolorhelper.h"
+
+#include <KColorScheme>
+
+#include <QGuiApplication>
+#include <QPalette>
+#include <QtGlobal>
+#include <QWidget>
+
+BackgroundColorHelper *BackgroundColorHelper::instance()
+{
+    if (!s_instance) {
+        s_instance = new BackgroundColorHelper;
+    }
+    return s_instance;
+}
+
+
+void setBackgroundColorForWidget(QWidget *widget, QColor color)
+{
+    QPalette palette;
+    palette.setBrush(QPalette::Active,   QPalette::Window, color);
+    palette.setBrush(QPalette::Inactive, QPalette::Window, color);
+    palette.setBrush(QPalette::Disabled, QPalette::Window, color);
+    widget->setAutoFillBackground(true);
+    widget->setPalette(palette);
+}
+
+void BackgroundColorHelper::controlBackgroundColor(QWidget *widget)
+{
+    setBackgroundColorForWidget(widget, m_backgroundColor);
+
+    Q_ASSERT_X(std::find(m_colorControlledWidgets.begin(), m_colorControlledWidgets.end(), widget) == m_colorControlledWidgets.end(), "controlBackgroundColor",
+               "Duplicate insertion is not necessary because the background color should already automatically update itself on paletteChanged");
+    m_colorControlledWidgets.emplace_back(widget);
+}
+
+BackgroundColorHelper::BackgroundColorHelper()
+{
+    updateBackgroundColor();
+    QObject::connect(qApp, &QGuiApplication::paletteChanged, [=](){ slotPaletteChanged(); });
+}
+
+void BackgroundColorHelper::slotPaletteChanged()
+{
+    updateBackgroundColor();
+    for (auto i = m_colorControlledWidgets.begin(); i != m_colorControlledWidgets.end(); ++i) {
+        if (!*i) {
+            i = m_colorControlledWidgets.erase(i);
+            continue;
+        }
+        setBackgroundColorForWidget(*i, m_backgroundColor);
+    }
+}
+
+void BackgroundColorHelper::updateBackgroundColor()
+{
+    // We use colors from the color scheme for mixing so it fits the theme.
+    const auto colorScheme = KColorScheme(QPalette::Normal, KColorScheme::Window);
+    const auto activeBackgroundColor = colorScheme.background(KColorScheme::BackgroundRole::ActiveBackground).color();
+    // We use the positive color for mixing so the end product doesn't look like a warning or error.
+    const auto positiveBackgroundColor = colorScheme.background(KColorScheme::BackgroundRole::PositiveBackground).color();
+
+    // Make sure the new background color has a meaningfully different hue than the activeBackgroundColor.
+    const int hueDifference = positiveBackgroundColor.hue() - activeBackgroundColor.hue();
+    int newHue;
+    if (std::abs(hueDifference) > 80) {
+        newHue = (activeBackgroundColor.hue() + positiveBackgroundColor.hue()) / 2;
+    } else {
+        newHue = hueDifference > 0 ?
+            activeBackgroundColor.hue() + 40 :
+            activeBackgroundColor.hue() - 40;
+        newHue %= 360; // hue needs to be between 0 and 359 per Qt documentation.
+    }
+
+    m_backgroundColor = QColor::fromHsv(newHue,
+                                        // Saturation should be closer to the active color because otherwise the selection mode color might overpower it.
+                                        .7 * activeBackgroundColor.saturation() + .3 * positiveBackgroundColor.saturation(),
+                                        (activeBackgroundColor.value() + positiveBackgroundColor.value()) / 2,
+                                        (activeBackgroundColor.alpha() + positiveBackgroundColor.alpha()) / 2);
+}
+
+BackgroundColorHelper *BackgroundColorHelper::s_instance = nullptr;