OpenGLFrameBuffer: Automatically save/restore content when the context is paused/resumed

This addresses an issue where OpenGL-backed image content could get lost
when putting and Android app into an inactive state. This happens
because the GL context gets destroyed, freeing all associated resources.

The workaround introduced here will listen for OpenGL state-change
events, and attempt to save and restore framebuffer content into main
memory, so that the content can be restored once the app is reactivated.

modules/juce_opengl/native/juce_OpenGL_android.h

@@ -252,6 +252,26 @@ public:
         const ScopedLock lock;
     };
 
+    void addListener (NativeContextListener& l)
+    {
+        listeners.add (&l);
+    }
+
+    void removeListener (NativeContextListener& l)
+    {
+        listeners.remove (&l);
+    }
+
+    void notifyWillPause()
+    {
+        listeners.call ([&] (auto& l) { l.contextWillPause(); });
+    }
+
+    void notifyDidResume()
+    {
+        listeners.call ([&] (auto& l) { l.contextDidResume(); });
+    }
+
     Component& component;
 
 private:
@@ -395,6 +415,7 @@ private:
 
     mutable std::mutex nativeHandleMutex;
     OpenGLContext* juceContext = nullptr;
+    ListenerList<NativeContextListener> listeners;
     std::unique_ptr<std::remove_pointer_t<EGLSurface>, SurfaceDestructor> surface { EGL_NO_SURFACE };
     std::unique_ptr<std::remove_pointer_t<EGLContext>, ContextDestructor> context { EGL_NO_CONTEXT };
 

modules/juce_opengl/native/juce_OpenGL_ios.h

@@ -213,6 +213,9 @@ public:
         const ScopedLock lock;
     };
 
+    void addListener (NativeContextListener&) {}
+    void removeListener (NativeContextListener&) {}
+
 private:
     CriticalSection mutex;
     Component& component;

modules/juce_opengl/native/juce_OpenGL_linux.h

@@ -404,6 +404,9 @@ public:
         const ScopedLock lock;
     };
 
+    void addListener (NativeContextListener&) {}
+    void removeListener (NativeContextListener&) {}
+
 private:
     bool tryChooseVisual (const OpenGLPixelFormat& format, const std::vector<GLint>& optionalAttribs)
     {

modules/juce_opengl/native/juce_OpenGL_mac.h

@@ -295,6 +295,9 @@ public:
         double videoRefreshPeriodS = 1.0 / 60.0;
     };
 
+    void addListener (NativeContextListener&) {}
+    void removeListener (NativeContextListener&) {}
+
     Component& owner;
     NSOpenGLContext* renderContext = nil;
     NSOpenGLView* view = nil;

modules/juce_opengl/native/juce_OpenGL_windows.h

@@ -196,6 +196,9 @@ public:
         return nullptr;
     }
 
+    void addListener (NativeContextListener&) {}
+    void removeListener (NativeContextListener&) {}
+
 private:
     //==============================================================================
     void handleAsyncUpdate() override

modules/juce_opengl/opengl/juce_OpenGLContext.cpp

@@ -193,6 +193,10 @@ public:
     //==============================================================================
     void pause()
     {
+       #if JUCE_ANDROID
+        context.executeOnGLThread ([this] (auto&) { nativeContext->notifyWillPause(); }, true);
+       #endif
+
         renderThread->remove (this);
 
         if ((state.fetch_and (~StateFlags::initialised) & StateFlags::initialised) == 0)
@@ -670,6 +674,10 @@ public:
         if (context.renderer != nullptr)
             context.renderer->newOpenGLContextCreated();
 
+       #if JUCE_ANDROID
+        nativeContext->notifyDidResume();
+       #endif
+
         return InitResult::success;
     }
 
@@ -1669,6 +1677,16 @@ void OpenGLContext::copyTexture (const Rectangle<int>& targetClipArea,
     JUCE_CHECK_OPENGL_ERROR
 }
 
+void OpenGLContext::NativeContextListener::addListener (OpenGLContext& ctx, NativeContextListener& l)
+{
+    ctx.nativeContext->addListener (l);
+}
+
+void OpenGLContext::NativeContextListener::removeListener (OpenGLContext& ctx, NativeContextListener& l)
+{
+    ctx.nativeContext->removeListener (l);
+}
+
 #if JUCE_ANDROID
 
 void OpenGLContext::NativeContext::surfaceCreated (LocalRef<jobject>)

modules/juce_opengl/opengl/juce_OpenGLContext.h

@@ -348,6 +348,7 @@ public:
     //==============================================================================
    #ifndef DOXYGEN
     class NativeContext;
+    class NativeContextListener;
    #endif
 
 private:

modules/juce_opengl/opengl/juce_OpenGLFrameBuffer.cpp

@@ -35,7 +35,23 @@
 namespace juce
 {
 
-class OpenGLFrameBuffer::Pimpl
+/*
+    Used on Android to detect when the GL context and associated resources (textures, framebuffers,
+    etc.) need to be destroyed/created due to the Surface changing state.
+*/
+class OpenGLContext::NativeContextListener
+{
+public:
+    virtual ~NativeContextListener() = default;
+
+    virtual void contextWillPause() = 0;
+    virtual void contextDidResume() = 0;
+
+    static void addListener (OpenGLContext& ctx, NativeContextListener& l);
+    static void removeListener (OpenGLContext& ctx, NativeContextListener& l);
+};
+
+class OpenGLFrameBuffer::Pimpl : private OpenGLContext::NativeContextListener
 {
 public:
     struct SavedState
@@ -44,6 +60,11 @@ public:
         std::vector<PixelARGB> data;
     };
 
+    ~Pimpl() override
+    {
+        release();
+    }
+
     bool isValid() const noexcept
     {
         return std::holds_alternative<TransientState> (state);
@@ -63,6 +84,8 @@ public:
             return false;
 
         associatedContext = &context;
+        NativeContextListener::addListener (*associatedContext, *this);
+
         return true;
     }
 
@@ -115,7 +138,9 @@ public:
 
     void release()
     {
-        associatedContext = nullptr;
+        if (auto* prev = std::exchange (associatedContext, nullptr))
+            NativeContextListener::removeListener (*prev, *this);
+
         state.emplace<std::monostate>();
     }
 
@@ -353,7 +378,7 @@ private:
             if (OpenGLHelpers::isContextActive())
             {
                 if (textureID != 0)
-                    glDeleteTextures (1, &textureID);
+                    gl::glDeleteTextures (1, &textureID);
 
                 if (depthOrStencilBuffer != 0)
                     gl::glDeleteRenderbuffers (1, &depthOrStencilBuffer);
@@ -428,6 +453,17 @@ private:
         return nullptr;
     }
 
+    void contextWillPause() override
+    {
+        saveAndRelease();
+    }
+
+    void contextDidResume() override
+    {
+        if (associatedContext != nullptr)
+            reloadSavedCopy (*associatedContext);
+    }
+
     OpenGLContext* associatedContext = nullptr;
     std::variant<std::monostate, TransientState, SavedState> state;
 };