Widgets/Style: add QCachedPainter

Add a helper class QCachedPainter to simplify the usage of QPixmapCache.

Task-number: QTBUG-129680
Change-Id: Id6835ec05af2fff3fc43c36b5fedaec598cb580b
Reviewed-by: Axel Spoerl <[email protected]>

3 files changed, 88 insertions, 0 deletions

diff --git a/src/widgets/styles/qcommonstyle.cpp b/src/widgets/styles/qcommonstyle.cpp
index 701dc2c45da..24968128da3 100644
--- a/src/widgets/styles/qcommonstyle.cpp
+++ b/src/widgets/styles/qcommonstyle.cpp
@@ -6233,6 +6233,7 @@ int QCommonStyle::layoutSpacing(QSizePolicy::ControlType /* control1 */, QSizePo
 void QCommonStyle::polish(QPalette &pal)
 {
     QStyle::polish(pal);
+    QCachedPainter::cleanupPixmapCache();
 }
 
 /*!
diff --git a/src/widgets/styles/qstyle.cpp b/src/widgets/styles/qstyle.cpp
index 32a994f1114..183b0cbe0c9 100644
--- a/src/widgets/styles/qstyle.cpp
+++ b/src/widgets/styles/qstyle.cpp
@@ -7,8 +7,10 @@
 #include "qwidget.h"
 #include "qbitmap.h"
 #include "qpixmapcache.h"
+#include "qset.h"
 #include "qstyleoption.h"
 #include "private/qstyle_p.h"
+#include "private/qstylehelper_p.h"
 #include "private/qguiapplication_p.h"
 #include <qpa/qplatformtheme.h>
 #ifndef QT_NO_DEBUG
@@ -2422,6 +2424,53 @@ bool QStylePrivate::useFullScreenForPopup()
     return theme && theme->themeHint(QPlatformTheme::UseFullScreenForPopupMenu).toBool();
 }
 
+//
+// QCachedPainter
+QSet<QString> QCachedPainter::s_pixmapCacheKeys;
+QCachedPainter::QCachedPainter(QPainter *painter, const QString &cachePrefix,
+                               const QStyleOption *option, QSize size, QRect paintRect)
+    : m_painter(painter)
+    , m_option(option)
+    , m_paintRect(paintRect)
+{
+    const auto sz = size.isNull() ? option->rect.size() : size;
+    const qreal dpr = QStyleHelper::getDpr(painter);
+    m_pixmapName = QStyleHelper::uniqueName(cachePrefix, option, sz, dpr);
+    m_alreadyCached = QPixmapCache::find(m_pixmapName, &m_pixmap);
+    if (!m_alreadyCached) {
+        m_pixmap = styleCachePixmap(sz, dpr);
+        m_pixmapPainter = std::make_unique<QPainter>(&m_pixmap);
+        m_pixmapPainter->setRenderHint(QPainter::Antialiasing);
+        s_pixmapCacheKeys += m_pixmapName;
+    }
+}
+
+QCachedPainter::~QCachedPainter()
+{
+    finish();
+    if (!m_alreadyCached)
+        QPixmapCache::insert(m_pixmapName, m_pixmap);
+}
+
+void QCachedPainter::finish()
+{
+    m_pixmapPainter.reset();
+    if (!m_pixmapDrawn) {
+        m_pixmapDrawn = true;
+        if (m_paintRect.isNull())
+            m_painter->drawPixmap(m_option->rect.topLeft(), m_pixmap);
+        else
+            m_painter->drawPixmap(m_paintRect, m_pixmap);
+    }
+}
+
+void QCachedPainter::cleanupPixmapCache()
+{
+    for (const auto &key : s_pixmapCacheKeys)
+        QPixmapCache::remove(key);
+    s_pixmapCacheKeys.clear();
+}
+
 QT_END_NAMESPACE
 
 #include "moc_qstyle.cpp"
diff --git a/src/widgets/styles/qstyle_p.h b/src/widgets/styles/qstyle_p.h
index d04dd491520..0412aca209b 100644
--- a/src/widgets/styles/qstyle_p.h
+++ b/src/widgets/styles/qstyle_p.h
@@ -72,6 +72,44 @@ inline QPixmap styleCachePixmap(const QSize &size, qreal pixelRatio)
         } \
     }
 
+// small helper to read out the pixmap to paint from QPixmapCache or re-draw
+// it and put it into the QPixmapCache for later usage
+class Q_WIDGETS_EXPORT QCachedPainter
+{
+public:
+    QCachedPainter(QPainter *painter, const QString &cachePrefix,
+                   const QStyleOption *option, QSize size = {}, QRect paintRect = {});
+    ~QCachedPainter();
+    void finish();
+    bool needsPainting() const
+    {
+        return !m_alreadyCached;
+    }
+    QPainter *operator->()
+    {
+        return painter();
+    }
+    QPainter *painter()
+    {
+        Q_ASSERT_X(m_pixmapPainter, "painter()", "Must only be called when painting on a pixmap to cache");
+        return m_pixmapPainter.get();
+    }
+
+    // clean pixmap cache from all cached pixmaps (e.g. due to palette change)
+    // to make sure the widgets are painted correctly afterwards
+    static void cleanupPixmapCache();
+private:
+    QPainter *m_painter = nullptr;
+    const QStyleOption *m_option = nullptr;
+    std::unique_ptr<QPainter> m_pixmapPainter;
+    QString m_pixmapName;
+    QPixmap m_pixmap;
+    QRect m_paintRect;
+    bool m_alreadyCached;
+    bool m_pixmapDrawn = false;
+    static QSet<QString> s_pixmapCacheKeys;
+};
+
 QT_END_NAMESPACE
 
 #endif //QSTYLE_P_H