perf(ui): cache stylesheets to reduce memory usage

Issue #5078

src/widget/style.cpp

@@ -92,15 +92,23 @@ QStringList Style::getThemeColorNames()
 QList<QColor> Style::themeColorColors = {QColor(), QColor("#004aa4"), QColor("#97ba00"),
                                          QColor("#c23716"), QColor("#4617b5")};
 
-QString Style::getStylesheet(const QString& filename, const QFont& baseFont)
+// stylesheet filename, font -> stylesheet
+// QString implicit sharing deduplicates stylesheets rather than constructing a new one each time
+std::map<std::pair<const QString, const QFont>, const QString> Style::stylesheetsCache;
+
+const QString Style::getStylesheet(const QString& filename, const QFont& baseFont)
 {
-    QFile file(filename);
-    if (!file.open(QFile::ReadOnly | QFile::Text)) {
-        qWarning() << "Stylesheet " << filename << " not found";
-        return QString();
+    const std::pair<const QString, const QFont> cacheKey(filename, baseFont);
+    auto it = stylesheetsCache.find(cacheKey);
+    if (it != stylesheetsCache.end())
+    {
+        // cache hit
+        return it->second;
     }
-
-    return resolve(file.readAll(), baseFont);
+    // cache miss, new styleSheet, read it from file and add to cache
+    const QString newStylesheet = resolve(filename, baseFont);
+    stylesheetsCache.insert(std::make_pair(cacheKey, newStylesheet));
+    return newStylesheet;
 }
 
 QColor Style::getColor(Style::ColorPalette entry)
@@ -128,8 +136,15 @@ QFont Style::getFont(Style::Font font)
     return fonts[font];
 }
 
-QString Style::resolve(QString qss, const QFont& baseFont)
+const QString Style::resolve(const QString& filename, const QFont& baseFont)
 {
+    QFile file{filename};
+    if (!file.open(QFile::ReadOnly | QFile::Text)) {
+        qWarning() << "Stylesheet " << filename << " not found";
+        return QString("");
+    }
+    QString qss = file.readAll();
+
     if (dict.isEmpty()) {
         dict = {// colors
                 {"@green", Style::getColor(Style::Green).name()},
@@ -162,7 +177,6 @@ QString Style::resolve(QString qss, const QFont& baseFont)
     for (const QString& key : dict.keys()) {
         qss.replace(QRegularExpression(QString("%1\\b").arg(key)), dict[key]);
     }
-
     return qss;
 }
 

src/widget/style.h

@@ -59,17 +59,18 @@ class Style
     };
 
     static QStringList getThemeColorNames();
-    static QString getStylesheet(const QString& filename, const QFont& baseFont = QFont());
+    static const QString getStylesheet(const QString& filename, const QFont& baseFont = QFont());
     static QColor getColor(ColorPalette entry);
     static QFont getFont(Font font);
-    static QString resolve(QString qss, const QFont& baseFont = QFont());
+    static const QString resolve(const QString& filename, const QFont& baseFont = QFont());
     static void repolish(QWidget* w);
     static void setThemeColor(int color);
     static void setThemeColor(const QColor& color);
     static void applyTheme();
     static QPixmap scaleSvgImage(const QString& path, uint32_t width, uint32_t height);
 
     static QList<QColor> themeColorColors;
+    static std::map<std::pair<const QString, const QFont>, const QString> stylesheetsCache;
 
 signals:
     void themeChanged();

src/widget/widget.cpp

@@ -143,8 +143,7 @@ void Widget::init()
     connect(actionQuit, &QAction::triggered, qApp, &QApplication::quit);
 
     layout()->setContentsMargins(0, 0, 0, 0);
-    ui->friendList->setStyleSheet(
-        Style::resolve(Style::getStylesheet(":/ui/friendList/friendList.css")));
+    ui->friendList->setStyleSheet(Style::getStylesheet(":/ui/friendList/friendList.css"));
 
     profilePicture = new MaskablePixmapWidget(this, QSize(40, 40), ":/img/avatar_mask.svg");
     profilePicture->setPixmap(QPixmap(":/img/contact_dark.svg"));