Introduce a texture cache for DecorationShadows in SceneOpenGLShadow

The DecorationShadow supports the concept of sharing shadows between
multiple Decorations and this is supported by our Shadow class. But
still the created GLTextures were not shared.

This change introduces a sharing mechanismn for all GLTextures created
for a DecorationShadow. Thus if two decorations share the same
DecorationShadow they will also share the same GLTexture.

scene_opengl.cpp

@@ -1930,16 +1930,88 @@ void SceneOpenGL::EffectFrame::cleanup()
 //****************************************
 // SceneOpenGL::Shadow
 //****************************************
+class DecorationShadowTextureCache
+{
+public:
+    ~DecorationShadowTextureCache();
+    DecorationShadowTextureCache(const DecorationShadowTextureCache&) = delete;
+    static DecorationShadowTextureCache &instance();
+
+    void unregister(SceneOpenGLShadow *shadow);
+    QSharedPointer<GLTexture> getTexture(SceneOpenGLShadow *shadow);
+
+private:
+    DecorationShadowTextureCache() = default;
+    struct Data {
+        QSharedPointer<GLTexture> texture;
+        QVector<SceneOpenGLShadow*> shadows;
+    };
+    QHash<KDecoration2::DecorationShadow*, Data> m_cache;
+};
+
+DecorationShadowTextureCache &DecorationShadowTextureCache::instance()
+{
+    static DecorationShadowTextureCache s_instance;
+    return s_instance;
+}
+
+DecorationShadowTextureCache::~DecorationShadowTextureCache()
+{
+    Q_ASSERT(m_cache.isEmpty());
+}
+
+void DecorationShadowTextureCache::unregister(SceneOpenGLShadow *shadow)
+{
+    auto it = m_cache.begin();
+    while (it != m_cache.end()) {
+        auto &d = it.value();
+        // check whether the Vector of Shadows contains our shadow and remove all of them
+        auto glIt = d.shadows.begin();
+        while (glIt != d.shadows.end()) {
+            if (*glIt == shadow) {
+                glIt = d.shadows.erase(glIt);
+            } else {
+                glIt++;
+            }
+        }
+        // if there are no shadows any more we can erase the cache entry
+        if (d.shadows.isEmpty()) {
+            it = m_cache.erase(it);
+        } else {
+            it++;
+        }
+    }
+}
+
+QSharedPointer<GLTexture> DecorationShadowTextureCache::getTexture(SceneOpenGLShadow *shadow)
+{
+    Q_ASSERT(shadow->hasDecorationShadow());
+    unregister(shadow);
+    const auto &decoShadow = shadow->decorationShadow();
+    Q_ASSERT(!decoShadow.isNull());
+    auto it = m_cache.find(decoShadow.data());
+    if (it != m_cache.end()) {
+        Q_ASSERT(!it.value().shadows.contains(shadow));
+        it.value().shadows << shadow;
+        return it.value().texture;
+    }
+    Data d;
+    d.shadows << shadow;
+    d.texture = QSharedPointer<GLTexture>::create(shadow->decorationShadowImage());
+    m_cache.insert(decoShadow.data(), d);
+    return d.texture;
+}
+
 SceneOpenGLShadow::SceneOpenGLShadow(Toplevel *toplevel)
     : Shadow(toplevel)
-    , m_texture(NULL)
 {
 }
 
 SceneOpenGLShadow::~SceneOpenGLShadow()
 {
     effects->makeOpenGLContextCurrent();
-    delete m_texture;
+    DecorationShadowTextureCache::instance().unregister(this);
+    m_texture.reset();
 }
 
 void SceneOpenGLShadow::buildQuads()
@@ -2056,8 +2128,7 @@ bool SceneOpenGLShadow::prepareBackend()
     if (hasDecorationShadow()) {
         // simplifies a lot by going directly to
         effects->makeOpenGLContextCurrent();
-        delete m_texture;
-        m_texture = new GLTexture(decorationShadowImage());
+        m_texture = DecorationShadowTextureCache::instance().getTexture(this);
 
         return true;
     }
@@ -2087,8 +2158,7 @@ bool SceneOpenGLShadow::prepareBackend()
     p.end();
 
     effects->makeOpenGLContextCurrent();
-    delete m_texture;
-    m_texture = new GLTexture(image);
+    m_texture = QSharedPointer<GLTexture>::create(image);
 
     return true;
 }

scene_opengl.h

@@ -317,13 +317,13 @@ public:
     virtual ~SceneOpenGLShadow();
 
     GLTexture *shadowTexture() {
-        return m_texture;
+        return m_texture.data();
     }
 protected:
     virtual void buildQuads();
     virtual bool prepareBackend();
 private:
-    GLTexture *m_texture;
+    QSharedPointer<GLTexture> m_texture;
 };
 
 /**

shadow.h

@@ -107,6 +107,10 @@ public:
     }
     QImage decorationShadowImage() const;
 
+    QWeakPointer<KDecoration2::DecorationShadow> decorationShadow() const {
+        return m_decorationShadow.toWeakRef();
+    }
+
 public Q_SLOTS:
     void geometryChanged();