JUCE/modules/juce_opengl/opengl/juce_OpenGLContext.cpp

/*
  ==============================================================================

   This file is part of the JUCE library.
   Copyright (c) 2022 - Raw Material Software Limited

   JUCE is an open source library subject to commercial or open-source
   licensing.

   By using JUCE, you agree to the terms of both the JUCE 7 End-User License
   Agreement and JUCE Privacy Policy.

   End User License Agreement: www.juce.com/juce-7-licence
   Privacy Policy: www.juce.com/juce-privacy-policy

   Or: You may also use this code under the terms of the GPL v3 (see
   www.gnu.org/licenses).

   JUCE IS PROVIDED "AS IS" WITHOUT ANY WARRANTY, AND ALL WARRANTIES, WHETHER
   EXPRESSED OR IMPLIED, INCLUDING MERCHANTABILITY AND FITNESS FOR PURPOSE, ARE
   DISCLAIMED.

  ==============================================================================
*/

#if JUCE_MAC
 #include <juce_gui_basics/native/juce_PerScreenDisplayLinks_mac.h>
#endif

namespace juce
{

#if JUCE_IOS
struct AppInactivityCallback // NB: this is a duplicate of an internal declaration in juce_core
{
    virtual ~AppInactivityCallback() {}
    virtual void appBecomingInactive() = 0;
};

extern Array<AppInactivityCallback*> appBecomingInactiveCallbacks;

// On iOS, all GL calls will crash when the app is running in the background, so
// this prevents them from happening (which some messy locking behaviour)
struct iOSBackgroundProcessCheck  : public AppInactivityCallback
{
    iOSBackgroundProcessCheck()              { isBackgroundProcess(); appBecomingInactiveCallbacks.add (this); }
    ~iOSBackgroundProcessCheck() override    { appBecomingInactiveCallbacks.removeAllInstancesOf (this); }

    bool isBackgroundProcess()
    {
        const bool b = Process::isForegroundProcess();
        isForeground.set (b ? 1 : 0);
        return ! b;
    }

    void appBecomingInactive() override
    {
        int counter = 2000;

        while (--counter > 0 && isForeground.get() != 0)
            Thread::sleep (1);
    }

private:
    Atomic<int> isForeground;

    JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR (iOSBackgroundProcessCheck)
};

#endif

#if JUCE_WINDOWS && JUCE_WIN_PER_MONITOR_DPI_AWARE
 extern JUCE_API double getScaleFactorForWindow (HWND);
#endif

static bool contextHasTextureNpotFeature()
{
    if (getOpenGLVersion() >= Version (2))
        return true;

    // If the version is < 2, we can't use the newer extension-checking API
    // so we have to use glGetString
    const auto* extensionsBegin = glGetString (GL_EXTENSIONS);

    if (extensionsBegin == nullptr)
        return false;

    const auto* extensionsEnd = findNullTerminator (extensionsBegin);
    const std::string extensionsString (extensionsBegin, extensionsEnd);
    const auto stringTokens = StringArray::fromTokens (extensionsString.c_str(), false);
    return stringTokens.contains ("GL_ARB_texture_non_power_of_two");
}

//==============================================================================
class OpenGLContext::CachedImage  : public CachedComponentImage
{
    template <typename T, typename U>
    static constexpr bool isFlagSet (const T& t, const U& u) { return (t & u) != 0; }

    struct AreaAndScale
    {
        Rectangle<int> area;
        double scale;

        auto tie() const { return std::tie (area, scale); }

        auto operator== (const AreaAndScale& other) const { return tie() == other.tie(); }
        auto operator!= (const AreaAndScale& other) const { return tie() != other.tie(); }
    };

    class LockedAreaAndScale
    {
    public:
        auto get() const
        {
            const ScopedLock lock (mutex);
            return data;
        }

        template <typename Fn>
        void set (const AreaAndScale& d, Fn&& ifDifferent)
        {
            const auto old = [&]
            {
                const ScopedLock lock (mutex);
                return std::exchange (data, d);
            }();

            if (old != d)
                ifDifferent();
        }

    private:
        CriticalSection mutex;
        AreaAndScale data { {}, 1.0 };
    };

public:
    CachedImage (OpenGLContext& c, Component& comp,
                 const OpenGLPixelFormat& pixFormat, void* contextToShare)
        : context (c),
          component (comp)
    {
        nativeContext.reset (new NativeContext (component, pixFormat, contextToShare,
                                                c.useMultisampling, c.versionRequired));

        if (nativeContext->createdOk())
            context.nativeContext = nativeContext.get();
        else
            nativeContext.reset();

        refreshDisplayLinkConnection();
    }

    ~CachedImage() override
    {
        stop();
    }

    //==============================================================================
    void start()
    {
        if (nativeContext != nullptr)
            resume();
    }

    void stop()
    {
        // make sure everything has finished executing
        state |= StateFlags::pendingDestruction;

        if (workQueue.size() > 0)
        {
            if (! renderThread->contains (this))
                resume();

            while (workQueue.size() != 0)
                Thread::sleep (20);
        }

        pause();
    }

    //==============================================================================
    void pause()
    {
        renderThread->remove (this);

        if ((state.fetch_and (~StateFlags::initialised) & StateFlags::initialised) == 0)
            return;

        ScopedContextActivator activator;
        activator.activate (context);

        if (context.renderer != nullptr)
            context.renderer->openGLContextClosing();

        associatedObjectNames.clear();
        associatedObjects.clear();
        cachedImageFrameBuffer.release();
        nativeContext->shutdownOnRenderThread();
    }

    void resume()
    {
        renderThread->add (this);
    }

    //==============================================================================
    void paint (Graphics&) override
    {
        if (MessageManager::getInstance()->isThisTheMessageThread())
        {
            updateViewportSize();
        }
        else
        {
            // If you hit this assertion, it's because paint has been called from a thread other
            // than the message thread. This commonly happens when nesting OpenGL contexts, because
            // the 'outer' OpenGL renderer will attempt to call paint on the 'inner' context's
            // component from the OpenGL thread.
            // Nesting OpenGL contexts is not directly supported, however there is a workaround:
            // https://forum.juce.com/t/opengl-how-do-3d-with-custom-shaders-and-2d-with-juce-paint-methods-work-together/28026/7
            jassertfalse;
        }
    }

    bool invalidateAll() override
    {
        validArea.clear();
        triggerRepaint();
        return false;
    }

    bool invalidate (const Rectangle<int>& area) override
    {
        validArea.subtract (area.toFloat().transformedBy (transform).getSmallestIntegerContainer());
        triggerRepaint();
        return false;
    }

    void releaseResources() override
    {
        stop();
    }

    void triggerRepaint()
    {
        state |= (StateFlags::pendingRender | StateFlags::paintComponents);
        renderThread->triggerRepaint();
    }

    //==============================================================================
    bool ensureFrameBufferSize (Rectangle<int> viewportArea)
    {
        JUCE_ASSERT_MESSAGE_MANAGER_IS_LOCKED

        auto fbW = cachedImageFrameBuffer.getWidth();
        auto fbH = cachedImageFrameBuffer.getHeight();

        if (fbW != viewportArea.getWidth() || fbH != viewportArea.getHeight() || ! cachedImageFrameBuffer.isValid())
        {
            if (! cachedImageFrameBuffer.initialise (context, viewportArea.getWidth(), viewportArea.getHeight()))
                return false;

            validArea.clear();
            JUCE_CHECK_OPENGL_ERROR
        }

        return true;
    }

    void clearRegionInFrameBuffer (const RectangleList<int>& list)
    {
        glClearColor (0, 0, 0, 0);
        glEnable (GL_SCISSOR_TEST);

        auto previousFrameBufferTarget = OpenGLFrameBuffer::getCurrentFrameBufferTarget();
        cachedImageFrameBuffer.makeCurrentRenderingTarget();
        auto imageH = cachedImageFrameBuffer.getHeight();

        for (auto& r : list)
        {
            glScissor (r.getX(), imageH - r.getBottom(), r.getWidth(), r.getHeight());
            glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
        }

        glDisable (GL_SCISSOR_TEST);
        context.extensions.glBindFramebuffer (GL_FRAMEBUFFER, previousFrameBufferTarget);
        JUCE_CHECK_OPENGL_ERROR
    }

    struct ScopedContextActivator
    {
        bool activate (OpenGLContext& ctx)
        {
            if (! active)
                active = ctx.makeActive();

            return active;
        }

        ~ScopedContextActivator()
        {
            if (active)
                OpenGLContext::deactivateCurrentContext();
        }

    private:
        bool active = false;
    };

    enum class RenderStatus
    {
        nominal,
        messageThreadAborted,
        noWork,
    };

    RenderStatus renderFrame (MessageManager::Lock& mmLock)
    {
       if (! isFlagSet (state, StateFlags::initialised))
       {
            switch (initialiseOnThread())
            {
                case InitResult::fatal:
                case InitResult::retry: return RenderStatus::noWork;
                case InitResult::success: break;
            }
        }

        state |= StateFlags::initialised;

       #if JUCE_IOS
        if (backgroundProcessCheck.isBackgroundProcess())
            return RenderStatus::noWork;
       #endif

        std::optional<MessageManager::Lock::ScopedTryLockType> scopedLock;
        ScopedContextActivator contextActivator;

        const auto stateToUse = state.fetch_and (StateFlags::persistent);

       #if JUCE_MAC
        // On macOS, we use a display link callback to trigger repaints, rather than
        // letting them run at full throttle
        const auto noAutomaticRepaint = true;
       #else
        const auto noAutomaticRepaint = ! context.continuousRepaint;
       #endif

        if (! isFlagSet (stateToUse, StateFlags::pendingRender) && noAutomaticRepaint)
            return RenderStatus::noWork;

        const auto isUpdating = isFlagSet (stateToUse, StateFlags::paintComponents);

        if (context.renderComponents && isUpdating)
        {
            bool abortScope = false;
            // If we early-exit here, we need to restore these flags so that the render is
            // attempted again in the next time slice.
            const ScopeGuard scope { [&] { if (! abortScope) state |= stateToUse; } };

            // This avoids hogging the message thread when doing intensive rendering.
            std::this_thread::sleep_until (lastMMLockReleaseTime + std::chrono::milliseconds { 2 });

            if (renderThread->isListChanging())
                return RenderStatus::messageThreadAborted;

            doWorkWhileWaitingForLock (contextActivator);

            scopedLock.emplace (mmLock);

            // If we can't get the lock here, it's probably because a context has been removed
            // on the main thread.
            // We return, just in case this renderer needs to be removed from the rendering thread.
            // If another renderer is being removed instead, then we should be able to get the lock
            // next time round.
            if (! scopedLock->isLocked())
                return RenderStatus::messageThreadAborted;

            abortScope = true;
        }

        {
            NativeContext::Locker locker (*nativeContext);

            if (! contextActivator.activate (context))
                return RenderStatus::noWork;

            JUCE_CHECK_OPENGL_ERROR

            doWorkWhileWaitingForLock (contextActivator);

            const auto currentAreaAndScale = areaAndScale.get();
            const auto viewportArea = currentAreaAndScale.area;

            if (context.renderer != nullptr)
            {
                OpenGLRendering::SavedBinding<OpenGLRendering::TraitsVAO> vaoBinding;

                glViewport (0, 0, viewportArea.getWidth(), viewportArea.getHeight());
                context.currentRenderScale = currentAreaAndScale.scale;
                context.renderer->renderOpenGL();
                clearGLError();
            }

            if (context.renderComponents)
            {
                if (isUpdating)
                {
                    paintComponent (currentAreaAndScale);

                    if (! isFlagSet (state, StateFlags::initialised))
                        return RenderStatus::noWork;

                    scopedLock.reset();
                    lastMMLockReleaseTime = std::chrono::steady_clock::now();
                }

                glViewport (0, 0, viewportArea.getWidth(), viewportArea.getHeight());
                drawComponentBuffer();
            }
        }

        bufferSwapper.swap();
        return RenderStatus::nominal;
    }

    void updateViewportSize()
    {
        JUCE_ASSERT_MESSAGE_THREAD

        if (auto* peer = component.getPeer())
        {
           #if JUCE_MAC
            updateScreen();

            const auto displayScale = Desktop::getInstance().getGlobalScaleFactor() * [this]
            {
                if (auto* view = getCurrentView())
                {
                    if ([view respondsToSelector: @selector (backingScaleFactor)])
                        return [(id) view backingScaleFactor];

                    if (auto* window = [view window])
                        return [window backingScaleFactor];
                }

                return areaAndScale.get().scale;
            }();
           #else
            const auto displayScale = Desktop::getInstance().getDisplays()
                                                            .getDisplayForRect (component.getTopLevelComponent()
                                                                                        ->getScreenBounds())
                                                           ->scale;
           #endif

            const auto localBounds = component.getLocalBounds();
            const auto newArea = peer->getComponent().getLocalArea (&component, localBounds).withZeroOrigin() * displayScale;

           #if JUCE_WINDOWS && JUCE_WIN_PER_MONITOR_DPI_AWARE
            // Some hosts (Pro Tools 2022.7) do not take the current DPI into account when sizing
            // plugin editor windows. Instead of querying the OS for the DPI of the editor window,
            // we approximate based on the physical size of the window that was actually provided
            // for the context to draw into. This may break if the OpenGL context's component is
            // scaled differently in its width and height - but in this case, a single scale factor
            // isn't that helpful anyway.
            const auto newScale = (float) newArea.getWidth() / (float) localBounds.getWidth();
           #else
            const auto newScale = (float) displayScale;
           #endif

            areaAndScale.set ({ newArea, newScale }, [&]
            {
                // Transform is only accessed when the message manager is locked
                transform = AffineTransform::scale ((float) newArea.getWidth()  / (float) localBounds.getWidth(),
                                                    (float) newArea.getHeight() / (float) localBounds.getHeight());

                nativeContext->updateWindowPosition (peer->getAreaCoveredBy (component));
                invalidateAll();
            });
        }
    }

    void checkViewportBounds()
    {
        auto screenBounds = component.getTopLevelComponent()->getScreenBounds();

        if (lastScreenBounds != screenBounds)
        {
            updateViewportSize();
            lastScreenBounds = screenBounds;
        }
    }

    void paintComponent (const AreaAndScale& currentAreaAndScale)
    {
        JUCE_ASSERT_MESSAGE_MANAGER_IS_LOCKED

        // you mustn't set your own cached image object when attaching a GL context!
        jassert (get (component) == this);

        if (! ensureFrameBufferSize (currentAreaAndScale.area))
            return;

        RectangleList<int> invalid (currentAreaAndScale.area);
        invalid.subtract (validArea);
        validArea = currentAreaAndScale.area;

        if (! invalid.isEmpty())
        {
            clearRegionInFrameBuffer (invalid);

            {
                std::unique_ptr<LowLevelGraphicsContext> g (createOpenGLGraphicsContext (context, cachedImageFrameBuffer));
                g->clipToRectangleList (invalid);
                g->addTransform (transform);

                paintOwner (*g);
                JUCE_CHECK_OPENGL_ERROR
            }
        }

        JUCE_CHECK_OPENGL_ERROR
    }

    void drawComponentBuffer()
    {
        if (! OpenGLRendering::TraitsVAO::isCoreProfile())
            glEnable (GL_TEXTURE_2D);

       #if JUCE_WINDOWS
        // some stupidly old drivers are missing this function, so try to at least avoid a crash here,
        // but if you hit this assertion you may want to have your own version check before using the
        // component rendering stuff on such old drivers.
        jassert (context.extensions.glActiveTexture != nullptr);
        if (context.extensions.glActiveTexture != nullptr)
       #endif
        {
            context.extensions.glActiveTexture (GL_TEXTURE0);
        }

        glBindTexture (GL_TEXTURE_2D, cachedImageFrameBuffer.getTextureID());

        const Rectangle<int> cacheBounds (cachedImageFrameBuffer.getWidth(), cachedImageFrameBuffer.getHeight());
        context.copyTexture (cacheBounds, cacheBounds, cacheBounds.getWidth(), cacheBounds.getHeight(), false);
        glBindTexture (GL_TEXTURE_2D, 0);
        JUCE_CHECK_OPENGL_ERROR
    }

    void paintOwner (LowLevelGraphicsContext& llgc)
    {
        Graphics g (llgc);

      #if JUCE_ENABLE_REPAINT_DEBUGGING
       #ifdef JUCE_IS_REPAINT_DEBUGGING_ACTIVE
        if (JUCE_IS_REPAINT_DEBUGGING_ACTIVE)
       #endif
        {
            g.saveState();
        }
       #endif

        JUCE_TRY
        {
            component.paintEntireComponent (g, false);
        }
        JUCE_CATCH_EXCEPTION

      #if JUCE_ENABLE_REPAINT_DEBUGGING
       #ifdef JUCE_IS_REPAINT_DEBUGGING_ACTIVE
        if (JUCE_IS_REPAINT_DEBUGGING_ACTIVE)
       #endif
        {
            // enabling this code will fill all areas that get repainted with a colour overlay, to show
            // clearly when things are being repainted.
            g.restoreState();

            static Random rng;
            g.fillAll (Colour ((uint8) rng.nextInt (255),
                               (uint8) rng.nextInt (255),
                               (uint8) rng.nextInt (255),
                               (uint8) 0x50));
        }
       #endif
    }

    void handleResize()
    {
        updateViewportSize();

       #if JUCE_MAC
        if (isFlagSet (state, StateFlags::initialised))
        {
            [nativeContext->view update];

            // We're already on the message thread, no need to lock it again.
            MessageManager::Lock mml;
            renderFrame (mml);
        }
       #endif
    }

    //==============================================================================
    InitResult initialiseOnThread()
    {
        // On android, this can get called twice, so drop any previous state.
        associatedObjectNames.clear();
        associatedObjects.clear();
        cachedImageFrameBuffer.release();

        context.makeActive();

        if (const auto nativeResult = nativeContext->initialiseOnRenderThread (context); nativeResult != InitResult::success)
            return nativeResult;

       #if JUCE_ANDROID
        // On android the context may be created in initialiseOnRenderThread
        // and we therefore need to call makeActive again
        context.makeActive();
       #endif

        gl::loadFunctions();

       #if JUCE_DEBUG
        if (getOpenGLVersion() >= Version { 4, 3 } && glDebugMessageCallback != nullptr)
        {
            glEnable (GL_DEBUG_OUTPUT);
            glEnable (GL_DEBUG_OUTPUT_SYNCHRONOUS);
            glDebugMessageCallback ([] (GLenum, GLenum type, GLuint, GLenum severity, GLsizei, const GLchar* message, const void*)
            {
                // This may reiterate issues that are also flagged by JUCE_CHECK_OPENGL_ERROR.
                // The advantage of this callback is that it will catch *all* errors, even if we
                // forget to check manually.
                DBG ("OpenGL DBG message: " << message);
                jassert (type != GL_DEBUG_TYPE_ERROR && severity != GL_DEBUG_SEVERITY_HIGH);
            }, nullptr);
        }
       #endif

        const auto currentViewportArea = areaAndScale.get().area;
        glViewport (0, 0, currentViewportArea.getWidth(), currentViewportArea.getHeight());

        nativeContext->setSwapInterval (1);

       #if ! JUCE_OPENGL_ES
        JUCE_CHECK_OPENGL_ERROR
        shadersAvailable = OpenGLShaderProgram::getLanguageVersion() > 0;
        clearGLError();
       #endif

        textureNpotSupported = contextHasTextureNpotFeature();

        if (context.renderer != nullptr)
            context.renderer->newOpenGLContextCreated();

        return InitResult::success;
    }

    //==============================================================================
    struct BlockingWorker  : public OpenGLContext::AsyncWorker
    {
        BlockingWorker (OpenGLContext::AsyncWorker::Ptr && workerToUse)
            : originalWorker (std::move (workerToUse))
        {}

        void operator() (OpenGLContext& calleeContext)
        {
            if (originalWorker != nullptr)
                (*originalWorker) (calleeContext);

            finishedSignal.signal();
        }

        void block() noexcept  { finishedSignal.wait(); }

        OpenGLContext::AsyncWorker::Ptr originalWorker;
        WaitableEvent finishedSignal;
    };

    void doWorkWhileWaitingForLock (ScopedContextActivator& contextActivator)
    {
        while (const auto work = workQueue.removeAndReturn (0))
        {
            if (renderThread->isListChanging() || ! contextActivator.activate (context))
                break;

            NativeContext::Locker locker (*nativeContext);

            (*work) (context);
            clearGLError();
        }
    }

    void execute (OpenGLContext::AsyncWorker::Ptr workerToUse, bool shouldBlock)
    {
        if (! isFlagSet (state, StateFlags::pendingDestruction))
        {
            if (shouldBlock)
            {
                auto blocker = new BlockingWorker (std::move (workerToUse));
                OpenGLContext::AsyncWorker::Ptr worker (*blocker);
                workQueue.add (worker);

                renderThread->abortLock();
                context.triggerRepaint();

                blocker->block();
            }
            else
            {
                workQueue.add (std::move (workerToUse));

                renderThread->abortLock();
                context.triggerRepaint();
            }
        }
        else
        {
            jassertfalse; // you called execute AFTER you detached your OpenGLContext
        }
    }

    //==============================================================================
    static CachedImage* get (Component& c) noexcept
    {
        return dynamic_cast<CachedImage*> (c.getCachedComponentImage());
    }

    class RenderThread
    {
    public:
        RenderThread() = default;

        ~RenderThread()
        {
            flags.setDestructing();
            thread.join();
        }

        void add (CachedImage* x)
        {
            const std::scoped_lock lock { listMutex };
            images.push_back (x);
        }

        void remove (CachedImage* x)
        {
            JUCE_ASSERT_MESSAGE_THREAD;

            flags.setSafe (false);
            abortLock();

            {
                const std::scoped_lock lock { callbackMutex, listMutex };
                images.remove (x);
            }

            flags.setSafe (true);
        }

        bool contains (CachedImage* x)
        {
            const std::scoped_lock lock { listMutex };
            return std::find (images.cbegin(), images.cend(), x) != images.cend();
        }

        void triggerRepaint()   { flags.setRenderRequested(); }

        void abortLock()        { messageManagerLock.abort(); }

        bool isListChanging()   { return ! flags.isSafe(); }

    private:
        RenderStatus renderAll()
        {
            auto result = RenderStatus::noWork;

            const std::scoped_lock lock { callbackMutex, listMutex };

            for (auto* x : images)
            {
                listMutex.unlock();
                const ScopeGuard scope { [&] { listMutex.lock(); } };

                const auto status = x->renderFrame (messageManagerLock);

                switch (status)
                {
                    case RenderStatus::noWork: break;
                    case RenderStatus::nominal: result = RenderStatus::nominal; break;
                    case RenderStatus::messageThreadAborted: return RenderStatus::messageThreadAborted;
                }
            }

            return result;
        }

        /*  Allows the main thread to communicate changes to the render thread.

            When the render thread needs to change in some way (asked to resume rendering,
            a renderer is added/removed, or the thread needs to stop prior to destruction),
            the main thread can set the appropriate flag on this structure. The render thread
            will call waitForWork() repeatedly, pausing when the render thread has no work to do,
            and resuming when requested by the main thread.
        */
        class Flags
        {
        public:
            void setDestructing()       { update ([] (auto& f) { f |= destructorCalled; }); }
            void setRenderRequested()   { update ([] (auto& f) { f |= renderRequested;  }); }

            void setSafe (const bool safe)
            {
                update ([safe] (auto& f)
                {
                    if (safe)
                        f |= listSafe;
                    else
                        f &= ~listSafe;
                });
            }

            bool isSafe()
            {
                const std::scoped_lock lock { mutex };
                return (flags & listSafe) != 0;
            }

            /*  Blocks until the 'safe' flag is set, and at least one other flag is set.
                After returning, the renderRequested flag will be unset.
                Returns true if rendering should continue.
            */
            bool waitForWork (bool requestRender)
            {
                std::unique_lock lock { mutex };
                flags |= (requestRender ? renderRequested : 0);
                condvar.wait (lock, [this] { return flags > listSafe; });
                flags &= ~renderRequested;
                return ((flags & destructorCalled) == 0);
            }

        private:
            template <typename Fn>
            void update (Fn fn)
            {
                {
                    const std::scoped_lock lock { mutex };
                    fn (flags);
                }

                condvar.notify_one();
            }

            enum
            {
                renderRequested  = 1 << 0,
                destructorCalled = 1 << 1,
                listSafe         = 1 << 2
            };

            std::mutex mutex;
            std::condition_variable condvar;
            int flags = listSafe;
        };

        MessageManager::Lock messageManagerLock;
        std::mutex listMutex, callbackMutex;
        std::list<CachedImage*> images;
        Flags flags;

        std::thread thread { [this]
        {
            Thread::setCurrentThreadName ("OpenGL Renderer");
            while (flags.waitForWork (renderAll() != RenderStatus::noWork)) {}
        } };
    };

    void refreshDisplayLinkConnection()
    {
       #if JUCE_MAC
        if (context.continuousRepaint)
        {
            connection.emplace (sharedDisplayLinks->registerFactory ([this] (CGDirectDisplayID display)
            {
                return [this, display]
                {
                    if (display == lastDisplay)
                        triggerRepaint();
                };
            }));
        }
        else
        {
            connection.reset();
        }
       #endif
    }

    //==============================================================================
    class BufferSwapper : private AsyncUpdater
    {
    public:
        explicit BufferSwapper (CachedImage& img)
            : image (img) {}

        ~BufferSwapper() override
        {
            cancelPendingUpdate();
        }

        void swap()
        {
            static const auto swapBuffersOnMainThread = []
            {
                const auto os = SystemStats::getOperatingSystemType();

                if ((os & SystemStats::MacOSX) != 0)
                    return (os != SystemStats::MacOSX && os < SystemStats::MacOSX_10_14);

                return false;
            }();

            if (swapBuffersOnMainThread && ! MessageManager::getInstance()->isThisTheMessageThread())
                triggerAsyncUpdate();
            else
                image.nativeContext->swapBuffers();
        }

    private:
        void handleAsyncUpdate() override
        {
            ScopedContextActivator activator;
            activator.activate (image.context);

            NativeContext::Locker locker (*image.nativeContext);
            image.nativeContext->swapBuffers();
        }

        CachedImage& image;
    };

    //==============================================================================
    friend class NativeContext;
    std::unique_ptr<NativeContext> nativeContext;

    OpenGLContext& context;
    Component& component;

    SharedResourcePointer<RenderThread> renderThread;

    OpenGLFrameBuffer cachedImageFrameBuffer;
    RectangleList<int> validArea;
    Rectangle<int> lastScreenBounds;
    AffineTransform transform;
    LockedAreaAndScale areaAndScale;

    StringArray associatedObjectNames;
    ReferenceCountedArray<ReferenceCountedObject> associatedObjects;

    WaitableEvent canPaintNowFlag, finishedPaintingFlag;
   #if JUCE_OPENGL_ES
    bool shadersAvailable = true;
   #else
    bool shadersAvailable = false;
   #endif
    bool textureNpotSupported = false;
    std::chrono::steady_clock::time_point lastMMLockReleaseTime{};
    BufferSwapper bufferSwapper { *this };

   #if JUCE_MAC
    NSView* getCurrentView() const
    {
        JUCE_ASSERT_MESSAGE_THREAD;

        if (auto* peer = component.getPeer())
            return static_cast<NSView*> (peer->getNativeHandle());

        return nullptr;
    }

    NSWindow* getCurrentWindow() const
    {
        JUCE_ASSERT_MESSAGE_THREAD;

        if (auto* view = getCurrentView())
            return [view window];

        return nullptr;
    }

    NSScreen* getCurrentScreen() const
    {
        JUCE_ASSERT_MESSAGE_THREAD;

        if (auto* window = getCurrentWindow())
            return [window screen];

        return nullptr;
    }

    void updateScreen()
    {
        const auto screen = getCurrentScreen();
        const auto display = ScopedDisplayLink::getDisplayIdForScreen (screen);

        if (lastDisplay.exchange (display) == display)
            return;

        const auto newRefreshPeriod = sharedDisplayLinks->getNominalVideoRefreshPeriodSForScreen (display);

        if (newRefreshPeriod != 0.0 && ! approximatelyEqual (std::exchange (refreshPeriod, newRefreshPeriod), newRefreshPeriod))
            nativeContext->setNominalVideoRefreshPeriodS (newRefreshPeriod);

        updateColourSpace();
    }

    void updateColourSpace()
    {
        if (auto* view = nativeContext->getNSView())
            if (auto* window = [view window])
                [window setColorSpace: [NSColorSpace sRGBColorSpace]];
    }

    std::atomic<CGDirectDisplayID> lastDisplay { 0 };
    double refreshPeriod = 0.0;

    FunctionNotificationCenterObserver observer { NSWindowDidChangeScreenNotification,
                                                  getCurrentWindow(),
                                                  [this] { updateScreen(); } };

    // Note: the NSViewComponentPeer also has a SharedResourcePointer<PerScreenDisplayLinks> to
    // avoid unnecessarily duplicating display-link threads.
    SharedResourcePointer<PerScreenDisplayLinks> sharedDisplayLinks;

    // On macOS, rather than letting swapBuffers block as appropriate, we use a display link
    // callback to mark the view as needing to repaint.
    std::optional<PerScreenDisplayLinks::Connection> connection;
   #endif

    enum StateFlags
    {
        pendingRender           = 1 << 0,
        paintComponents         = 1 << 1,
        pendingDestruction      = 1 << 2,
        initialised             = 1 << 3,

        // Flags that should retain their state after each frame
        persistent              = initialised | pendingDestruction
    };

    std::atomic<int> state { 0 };
    ReferenceCountedArray<OpenGLContext::AsyncWorker, CriticalSection> workQueue;

   #if JUCE_IOS
    iOSBackgroundProcessCheck backgroundProcessCheck;
   #endif

    JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR (CachedImage)
};

//==============================================================================
class OpenGLContext::Attachment  : public ComponentMovementWatcher,
                                   private Timer
{
public:
    Attachment (OpenGLContext& c, Component& comp)
       : ComponentMovementWatcher (&comp), context (c)
    {
        if (canBeAttached (comp))
            attach();
    }

    ~Attachment() override
    {
        detach();
    }

    void detach()
    {
        auto& comp = *getComponent();
        stop();
        comp.setCachedComponentImage (nullptr);
        context.nativeContext = nullptr;
    }

    void componentMovedOrResized (bool /*wasMoved*/, bool /*wasResized*/) override
    {
        auto& comp = *getComponent();

        if (isAttached (comp) != canBeAttached (comp))
            componentVisibilityChanged();

        if (comp.getWidth() > 0 && comp.getHeight() > 0
             && context.nativeContext != nullptr)
        {
            if (auto* c = CachedImage::get (comp))
                c->handleResize();

            if (auto* peer = comp.getTopLevelComponent()->getPeer())
                context.nativeContext->updateWindowPosition (peer->getAreaCoveredBy (comp));
        }
    }

    using ComponentMovementWatcher::componentMovedOrResized;

    void componentPeerChanged() override
    {
        detach();
        componentVisibilityChanged();
    }

    void componentVisibilityChanged() override
    {
        auto& comp = *getComponent();

        if (canBeAttached (comp))
        {
            if (isAttached (comp))
                comp.repaint(); // (needed when windows are un-minimised)
            else
                attach();
        }
        else
        {
            detach();
        }
    }

    using ComponentMovementWatcher::componentVisibilityChanged;

   #if JUCE_DEBUG || JUCE_LOG_ASSERTIONS
    void componentBeingDeleted (Component& c) override
    {
        /* You must call detach() or delete your OpenGLContext to remove it
           from a component BEFORE deleting the component that it is using!
        */
        jassertfalse;

        ComponentMovementWatcher::componentBeingDeleted (c);
    }
   #endif

private:
    OpenGLContext& context;

    bool canBeAttached (const Component& comp) noexcept
    {
        return (! context.overrideCanAttach) && comp.getWidth() > 0 && comp.getHeight() > 0 && isShowingOrMinimised (comp);
    }

    static bool isShowingOrMinimised (const Component& c)
    {
        if (! c.isVisible())
            return false;

        if (auto* p = c.getParentComponent())
            return isShowingOrMinimised (*p);

        return c.getPeer() != nullptr;
    }

    static bool isAttached (const Component& comp) noexcept
    {
        return comp.getCachedComponentImage() != nullptr;
    }

    void attach()
    {
        auto& comp = *getComponent();
        auto* newCachedImage = new CachedImage (context, comp,
                                                context.openGLPixelFormat,
                                                context.contextToShareWith);
        comp.setCachedComponentImage (newCachedImage);

        start();
    }

    void stop()
    {
        stopTimer();

        auto& comp = *getComponent();

       #if JUCE_MAC
        [[(NSView*) comp.getWindowHandle() window] disableScreenUpdatesUntilFlush];
       #endif

        if (auto* oldCachedImage = CachedImage::get (comp))
            oldCachedImage->stop(); // (must stop this before detaching it from the component)
    }

    void start()
    {
        auto& comp = *getComponent();

        if (auto* cachedImage = CachedImage::get (comp))
        {
            cachedImage->start(); // (must wait until this is attached before starting its thread)
            cachedImage->updateViewportSize();

            startTimer (400);
        }
    }

    void timerCallback() override
    {
        if (auto* cachedImage = CachedImage::get (*getComponent()))
            cachedImage->checkViewportBounds();
    }
};

//==============================================================================
OpenGLContext::OpenGLContext()
{
}

OpenGLContext::~OpenGLContext()
{
    detach();
}

void OpenGLContext::setRenderer (OpenGLRenderer* rendererToUse) noexcept
{
    // This method must not be called when the context has already been attached!
    // Call it before attaching your context, or use detach() first, before calling this!
    jassert (nativeContext == nullptr);

    renderer = rendererToUse;
}

void OpenGLContext::setComponentPaintingEnabled (bool shouldPaintComponent) noexcept
{
    // This method must not be called when the context has already been attached!
    // Call it before attaching your context, or use detach() first, before calling this!
    jassert (nativeContext == nullptr);

    renderComponents = shouldPaintComponent;
}

void OpenGLContext::setContinuousRepainting (bool shouldContinuouslyRepaint) noexcept
{
    continuousRepaint = shouldContinuouslyRepaint;

   #if JUCE_MAC
    if (auto* component = getTargetComponent())
    {
        detach();
        attachment.reset (new Attachment (*this, *component));
    }

    if (auto* cachedImage = getCachedImage())
        cachedImage->refreshDisplayLinkConnection();
   #endif

    triggerRepaint();
}

void OpenGLContext::setPixelFormat (const OpenGLPixelFormat& preferredPixelFormat) noexcept
{
    // This method must not be called when the context has already been attached!
    // Call it before attaching your context, or use detach() first, before calling this!
    jassert (nativeContext == nullptr);

    openGLPixelFormat = preferredPixelFormat;
}

void OpenGLContext::setTextureMagnificationFilter (OpenGLContext::TextureMagnificationFilter magFilterMode) noexcept
{
    texMagFilter = magFilterMode;
}

void OpenGLContext::setNativeSharedContext (void* nativeContextToShareWith) noexcept
{
    // This method must not be called when the context has already been attached!
    // Call it before attaching your context, or use detach() first, before calling this!
    jassert (nativeContext == nullptr);

    contextToShareWith = nativeContextToShareWith;
}

void OpenGLContext::setMultisamplingEnabled (bool b) noexcept
{
    // This method must not be called when the context has already been attached!
    // Call it before attaching your context, or use detach() first, before calling this!
    jassert (nativeContext == nullptr);

    useMultisampling = b;
}

void OpenGLContext::setOpenGLVersionRequired (OpenGLVersion v) noexcept
{
    versionRequired = v;
}

void OpenGLContext::attachTo (Component& component)
{
    component.repaint();

    if (getTargetComponent() != &component)
    {
        detach();
        attachment.reset (new Attachment (*this, component));
    }
}

void OpenGLContext::detach()
{
    if (auto* a = attachment.get())
    {
        a->detach(); // must detach before nulling our pointer
        attachment.reset();
    }

    nativeContext = nullptr;
}

bool OpenGLContext::isAttached() const noexcept
{
    return nativeContext != nullptr;
}

Component* OpenGLContext::getTargetComponent() const noexcept
{
    return attachment != nullptr ? attachment->getComponent() : nullptr;
}

OpenGLContext* OpenGLContext::getContextAttachedTo (Component& c) noexcept
{
    if (auto* ci = CachedImage::get (c))
        return &(ci->context);

    return nullptr;
}

static ThreadLocalValue<OpenGLContext*> currentThreadActiveContext;

OpenGLContext* OpenGLContext::getCurrentContext()
{
    return currentThreadActiveContext.get();
}

bool OpenGLContext::makeActive() const noexcept
{
    auto& current = currentThreadActiveContext.get();

    if (nativeContext != nullptr && nativeContext->makeActive())
    {
        current = const_cast<OpenGLContext*> (this);
        return true;
    }

    current = nullptr;
    return false;
}

bool OpenGLContext::isActive() const noexcept
{
    return nativeContext != nullptr && nativeContext->isActive();
}

void OpenGLContext::deactivateCurrentContext()
{
    NativeContext::deactivateCurrentContext();
    currentThreadActiveContext.get() = nullptr;
}

void OpenGLContext::triggerRepaint()
{
    if (auto* cachedImage = getCachedImage())
        cachedImage->triggerRepaint();
}

void OpenGLContext::swapBuffers()
{
    if (nativeContext != nullptr)
        nativeContext->swapBuffers();
}

unsigned int OpenGLContext::getFrameBufferID() const noexcept
{
    return nativeContext != nullptr ? nativeContext->getFrameBufferID() : 0;
}

bool OpenGLContext::setSwapInterval (int numFramesPerSwap)
{
    return nativeContext != nullptr && nativeContext->setSwapInterval (numFramesPerSwap);
}

int OpenGLContext::getSwapInterval() const
{
    return nativeContext != nullptr ? nativeContext->getSwapInterval() : 0;
}

void* OpenGLContext::getRawContext() const noexcept
{
    return nativeContext != nullptr ? nativeContext->getRawContext() : nullptr;
}

bool OpenGLContext::isCoreProfile() const
{
    auto* c = getCachedImage();
    return c != nullptr && OpenGLRendering::TraitsVAO::isCoreProfile();
}

OpenGLContext::CachedImage* OpenGLContext::getCachedImage() const noexcept
{
    if (auto* comp = getTargetComponent())
        return CachedImage::get (*comp);

    return nullptr;
}

bool OpenGLContext::areShadersAvailable() const
{
    auto* c = getCachedImage();
    return c != nullptr && c->shadersAvailable;
}

bool OpenGLContext::isTextureNpotSupported() const
{
    auto* c = getCachedImage();
    return c != nullptr && c->textureNpotSupported;
}

ReferenceCountedObject* OpenGLContext::getAssociatedObject (const char* name) const
{
    jassert (name != nullptr);

    auto* c = getCachedImage();

    // This method must only be called from an openGL rendering callback.
    jassert (c != nullptr && nativeContext != nullptr);
    jassert (getCurrentContext() != nullptr);

    auto index = c->associatedObjectNames.indexOf (name);
    return index >= 0 ? c->associatedObjects.getUnchecked (index).get() : nullptr;
}

void OpenGLContext::setAssociatedObject (const char* name, ReferenceCountedObject* newObject)
{
    jassert (name != nullptr);

    if (auto* c = getCachedImage())
    {
        // This method must only be called from an openGL rendering callback.
        jassert (nativeContext != nullptr);
        jassert (getCurrentContext() != nullptr);

        const int index = c->associatedObjectNames.indexOf (name);

        if (index >= 0)
        {
            if (newObject != nullptr)
            {
                c->associatedObjects.set (index, newObject);
            }
            else
            {
                c->associatedObjectNames.remove (index);
                c->associatedObjects.remove (index);
            }
        }
        else if (newObject != nullptr)
        {
            c->associatedObjectNames.add (name);
            c->associatedObjects.add (newObject);
        }
    }
}

void OpenGLContext::setImageCacheSize (size_t newSize) noexcept     { imageCacheMaxSize = newSize; }
size_t OpenGLContext::getImageCacheSize() const noexcept            { return imageCacheMaxSize; }

void OpenGLContext::execute (OpenGLContext::AsyncWorker::Ptr workerToUse, bool shouldBlock)
{
    if (auto* c = getCachedImage())
        c->execute (std::move (workerToUse), shouldBlock);
    else
        jassertfalse; // You must have attached the context to a component
}

//==============================================================================
struct DepthTestDisabler
{
    DepthTestDisabler() noexcept
    {
        glGetBooleanv (GL_DEPTH_TEST, &wasEnabled);

        if (wasEnabled)
            glDisable (GL_DEPTH_TEST);
    }

    ~DepthTestDisabler() noexcept
    {
        if (wasEnabled)
            glEnable (GL_DEPTH_TEST);
    }

    GLboolean wasEnabled;
};

//==============================================================================
void OpenGLContext::copyTexture (const Rectangle<int>& targetClipArea,
                                 const Rectangle<int>& anchorPosAndTextureSize,
                                 const int contextWidth, const int contextHeight,
                                 bool flippedVertically)
{
    if (contextWidth <= 0 || contextHeight <= 0)
        return;

    JUCE_CHECK_OPENGL_ERROR
    glBlendFunc (GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
    glEnable (GL_BLEND);

    DepthTestDisabler depthDisabler;

    if (areShadersAvailable())
    {
        OpenGLRendering::SavedBinding<OpenGLRendering::TraitsVAO> vaoBinding;

        struct OverlayShaderProgram  : public ReferenceCountedObject
        {
            OverlayShaderProgram (OpenGLContext& context)
                : program (context), params (program)
            {}

            static const OverlayShaderProgram& select (OpenGLContext& context)
            {
                static const char programValueID[] = "juceGLComponentOverlayShader";
                OverlayShaderProgram* program = static_cast<OverlayShaderProgram*> (context.getAssociatedObject (programValueID));

                if (program == nullptr)
                {
                    program = new OverlayShaderProgram (context);
                    context.setAssociatedObject (programValueID, program);
                }

                program->program.use();
                return *program;
            }

            struct BuiltProgram : public OpenGLShaderProgram
            {
                explicit BuiltProgram (OpenGLContext& ctx)
                    : OpenGLShaderProgram (ctx)
                {
                    addVertexShader (OpenGLHelpers::translateVertexShaderToV3 (
                        "attribute " JUCE_HIGHP " vec2 position;"
                        "uniform " JUCE_HIGHP " vec2 screenSize;"
                        "uniform " JUCE_HIGHP " float textureBounds[4];"
                        "uniform " JUCE_HIGHP " vec2 vOffsetAndScale;"
                        "varying " JUCE_HIGHP " vec2 texturePos;"
                        "void main()"
                        "{"
                          JUCE_HIGHP " vec2 scaled = position / (0.5 * screenSize.xy);"
                          "gl_Position = vec4 (scaled.x - 1.0, 1.0 - scaled.y, 0, 1.0);"
                          "texturePos = (position - vec2 (textureBounds[0], textureBounds[1])) / vec2 (textureBounds[2], textureBounds[3]);"
                          "texturePos = vec2 (texturePos.x, vOffsetAndScale.x + vOffsetAndScale.y * texturePos.y);"
                        "}"));

                    addFragmentShader (OpenGLHelpers::translateFragmentShaderToV3 (
                        "uniform sampler2D imageTexture;"
                        "varying " JUCE_HIGHP " vec2 texturePos;"
                        "void main()"
                        "{"
                          "gl_FragColor = texture2D (imageTexture, texturePos);"
                        "}"));

                    link();
                }
            };

            struct Params
            {
                Params (OpenGLShaderProgram& prog)
                    : positionAttribute (prog, "position"),
                      screenSize (prog, "screenSize"),
                      imageTexture (prog, "imageTexture"),
                      textureBounds (prog, "textureBounds"),
                      vOffsetAndScale (prog, "vOffsetAndScale")
                {}

                void set (const float targetWidth, const float targetHeight, const Rectangle<float>& bounds, bool flipVertically) const
                {
                    const GLfloat m[] = { bounds.getX(), bounds.getY(), bounds.getWidth(), bounds.getHeight() };
                    textureBounds.set (m, 4);
                    imageTexture.set (0);
                    screenSize.set (targetWidth, targetHeight);

                    vOffsetAndScale.set (flipVertically ? 0.0f : 1.0f,
                                         flipVertically ? 1.0f : -1.0f);
                }

                OpenGLShaderProgram::Attribute positionAttribute;
                OpenGLShaderProgram::Uniform screenSize, imageTexture, textureBounds, vOffsetAndScale;
            };

            BuiltProgram program;
            Params params;
        };

        auto left   = (GLshort) targetClipArea.getX();
        auto top    = (GLshort) targetClipArea.getY();
        auto right  = (GLshort) targetClipArea.getRight();
        auto bottom = (GLshort) targetClipArea.getBottom();
        const GLshort vertices[] = { left, bottom, right, bottom, left, top, right, top };

        auto& program = OverlayShaderProgram::select (*this);
        program.params.set ((float) contextWidth, (float) contextHeight, anchorPosAndTextureSize.toFloat(), flippedVertically);

        OpenGLRendering::SavedBinding<OpenGLRendering::TraitsArrayBuffer> savedArrayBuffer;
        extensions.glBufferData (GL_ARRAY_BUFFER, sizeof (vertices), vertices, GL_STATIC_DRAW);

        auto index = (GLuint) program.params.positionAttribute.attributeID;
        extensions.glVertexAttribPointer (index, 2, GL_SHORT, GL_FALSE, 4, nullptr);
        extensions.glEnableVertexAttribArray (index);
        JUCE_CHECK_OPENGL_ERROR

        if (extensions.glCheckFramebufferStatus (GL_FRAMEBUFFER) == GL_FRAMEBUFFER_COMPLETE)
        {
            glDrawArrays (GL_TRIANGLE_STRIP, 0, 4);
            extensions.glDisableVertexAttribArray (index);
        }
        else
        {
            clearGLError();
        }
    }
    else
    {
        jassert (attachment == nullptr); // Running on an old graphics card!
    }

    JUCE_CHECK_OPENGL_ERROR
}

#if JUCE_ANDROID

void OpenGLContext::NativeContext::surfaceCreated (LocalRef<jobject>)
{
    {
        const std::lock_guard lock { nativeHandleMutex };

        jassert (hasInitialised);

        // has the context already attached?
        jassert (surface.get() == EGL_NO_SURFACE && context.get() == EGL_NO_CONTEXT);

        const auto window = getNativeWindow();

        if (window == nullptr)
        {
            // failed to get a pointer to the native window so bail out
            jassertfalse;
            return;
        }

        // create the surface
        surface.reset (eglCreateWindowSurface (display, config, window.get(), nullptr));
        jassert (surface.get() != EGL_NO_SURFACE);

        // create the OpenGL context
        EGLint contextAttribs[] = { EGL_CONTEXT_CLIENT_VERSION, 2, EGL_NONE };
        context.reset (eglCreateContext (display, config, EGL_NO_CONTEXT, contextAttribs));
        jassert (context.get() != EGL_NO_CONTEXT);
    }

    if (auto* cached = CachedImage::get (component))
    {
        cached->resume();
        cached->triggerRepaint();
    }
}

void OpenGLContext::NativeContext::surfaceDestroyed (LocalRef<jobject>)
{
    if (auto* cached = CachedImage::get (component))
        cached->pause();

    {
        const std::lock_guard lock { nativeHandleMutex };

        context.reset (EGL_NO_CONTEXT);
        surface.reset (EGL_NO_SURFACE);
    }
}

#endif

} // namespace juce