ladybird/Libraries/LibWeb/HTML/RenderingThread.cpp

/*
 * Copyright (c) 2025-2026, Aliaksandr Kalenik <kalenik.aliaksandr@gmail.com>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include <LibCore/EventLoop.h>
#include <LibThreading/Thread.h>
#include <LibWeb/HTML/RenderingThread.h>
#include <LibWeb/HTML/TraversableNavigable.h>
#include <LibWeb/Painting/DisplayListPlayerSkia.h>

namespace Web::HTML {

struct BackingStoreState {
    RefPtr<Gfx::PaintingSurface> front_store;
    RefPtr<Gfx::PaintingSurface> back_store;
    i32 front_bitmap_id { -1 };
    i32 back_bitmap_id { -1 };

    void swap()
    {
        AK::swap(front_store, back_store);
        AK::swap(front_bitmap_id, back_bitmap_id);
    }

    bool is_valid() const { return front_store && back_store; }
};

struct UpdateDisplayListCommand {
    NonnullRefPtr<Painting::DisplayList> display_list;
    Painting::ScrollStateSnapshotByDisplayList scroll_state_snapshot;
};

struct UpdateBackingStoresCommand {
    RefPtr<Gfx::PaintingSurface> front_store;
    RefPtr<Gfx::PaintingSurface> back_store;
    i32 front_bitmap_id;
    i32 back_bitmap_id;
};

struct PresentFrameCommand {
    Function<void(i32)> callback;
};

struct ScreenshotCommand {
    NonnullRefPtr<Gfx::PaintingSurface> target_surface;
    Function<void()> callback;
};

using CompositorCommand = Variant<UpdateDisplayListCommand, UpdateBackingStoresCommand, PresentFrameCommand, ScreenshotCommand>;

class RenderingThread::ThreadData final : public AtomicRefCounted<ThreadData> {
public:
    ThreadData(Core::EventLoop& main_thread_event_loop)
        : m_main_thread_event_loop(main_thread_event_loop)
    {
    }

    ~ThreadData() = default;

    void set_skia_player(OwnPtr<Painting::DisplayListPlayerSkia>&& player)
    {
        m_skia_player = move(player);
    }

    bool has_skia_player() const { return m_skia_player != nullptr; }

    void exit()
    {
        Threading::MutexLocker const locker { m_mutex };
        m_exit = true;
        m_command_ready.signal();
    }

    void enqueue_command(CompositorCommand&& command)
    {
        Threading::MutexLocker const locker { m_mutex };
        m_command_queue.enqueue(move(command));
        m_command_ready.signal();
    }

    void compositor_loop()
    {
        while (true) {
            auto command = [this]() -> Optional<CompositorCommand> {
                Threading::MutexLocker const locker { m_mutex };
                while (m_command_queue.is_empty() && !m_exit) {
                    m_command_ready.wait();
                }
                if (m_exit)
                    return {};
                return m_command_queue.dequeue();
            }();

            if (!command.has_value()) {
                VERIFY(m_exit);
                break;
            }

            command->visit(
                [this](UpdateDisplayListCommand& cmd) {
                    m_cached_display_list = move(cmd.display_list);
                    m_cached_scroll_state_snapshot = move(cmd.scroll_state_snapshot);
                },
                [this](UpdateBackingStoresCommand& cmd) {
                    m_backing_stores.front_store = move(cmd.front_store);
                    m_backing_stores.back_store = move(cmd.back_store);
                    m_backing_stores.front_bitmap_id = cmd.front_bitmap_id;
                    m_backing_stores.back_bitmap_id = cmd.back_bitmap_id;
                },
                [this](PresentFrameCommand& cmd) {
                    if (!m_cached_display_list || !m_backing_stores.is_valid())
                        return;

                    m_skia_player->execute(*m_cached_display_list, Painting::ScrollStateSnapshotByDisplayList(m_cached_scroll_state_snapshot), *m_backing_stores.back_store);
                    i32 rendered_bitmap_id = m_backing_stores.back_bitmap_id;
                    m_backing_stores.swap();

                    if (cmd.callback) {
                        invoke_on_main_thread([callback = move(cmd.callback), rendered_bitmap_id]() mutable {
                            callback(rendered_bitmap_id);
                        });
                    }
                },
                [this](ScreenshotCommand& cmd) {
                    if (!m_cached_display_list)
                        return;
                    m_skia_player->execute(*m_cached_display_list, Painting::ScrollStateSnapshotByDisplayList(m_cached_scroll_state_snapshot), *cmd.target_surface);
                    if (cmd.callback) {
                        invoke_on_main_thread([callback = move(cmd.callback)]() mutable {
                            callback();
                        });
                    }
                });

            if (m_exit)
                break;
        }
    }

private:
    template<typename Invokee>
    void invoke_on_main_thread(Invokee invokee)
    {
        if (m_exit)
            return;
        m_main_thread_event_loop.deferred_invoke([self = NonnullRefPtr(*this), invokee = move(invokee)]() mutable {
            invokee();
        });
    }

    Core::EventLoop& m_main_thread_event_loop;

    mutable Threading::Mutex m_mutex;
    mutable Threading::ConditionVariable m_command_ready { m_mutex };
    Atomic<bool> m_exit { false };

    Queue<CompositorCommand> m_command_queue;

    OwnPtr<Painting::DisplayListPlayerSkia> m_skia_player;
    RefPtr<Painting::DisplayList> m_cached_display_list;
    Painting::ScrollStateSnapshotByDisplayList m_cached_scroll_state_snapshot;
    BackingStoreState m_backing_stores;
};

RenderingThread::RenderingThread()
    : m_thread_data(adopt_ref(*new ThreadData(Core::EventLoop::current())))
    , m_main_thread_exit_promise(Core::Promise<NonnullRefPtr<Core::EventReceiver>>::construct())
{
    // FIXME: Come up with a better "event loop exited" notification mechanism.
    m_main_thread_exit_promise->on_rejection = [thread_data = m_thread_data](Error const&) -> void {
        thread_data->exit();
    };
    Core::EventLoop::current().add_job(m_main_thread_exit_promise);
}

RenderingThread::~RenderingThread()
{
    // Note: Promise rejection is expected to signal the thread to exit.
    m_main_thread_exit_promise->reject(Error::from_errno(ECANCELED));
    if (m_thread) {
        (void)m_thread->join();
    }
}

void RenderingThread::start(DisplayListPlayerType)
{
    VERIFY(m_thread_data->has_skia_player());
    m_thread = Threading::Thread::construct([thread_data = m_thread_data] {
        thread_data->compositor_loop();
        return static_cast<intptr_t>(0);
    });
    m_thread->start();
}

void RenderingThread::set_skia_player(OwnPtr<Painting::DisplayListPlayerSkia>&& player)
{
    m_thread_data->set_skia_player(move(player));
}

void RenderingThread::update_display_list(NonnullRefPtr<Painting::DisplayList> display_list, Painting::ScrollStateSnapshotByDisplayList&& scroll_state_snapshot)
{
    m_thread_data->enqueue_command(UpdateDisplayListCommand { move(display_list), move(scroll_state_snapshot) });
}

void RenderingThread::update_backing_stores(RefPtr<Gfx::PaintingSurface> front, RefPtr<Gfx::PaintingSurface> back, i32 front_id, i32 back_id)
{
    m_thread_data->enqueue_command(UpdateBackingStoresCommand { move(front), move(back), front_id, back_id });
}

void RenderingThread::present_frame(Function<void(i32)>&& callback)
{
    m_thread_data->enqueue_command(PresentFrameCommand { move(callback) });
}

void RenderingThread::request_screenshot(NonnullRefPtr<Gfx::PaintingSurface> target_surface, Function<void()>&& callback)
{
    m_thread_data->enqueue_command(ScreenshotCommand { move(target_surface), move(callback) });
}

}