Keep window active while being dragged (SDL on Win32)

Question

At first my code set up the SDL environment, and proceeded to update the OpenGL context, without performing any SDL_Event processing whatsoever. This causes the window, as long as it was open, to appear to Windows to be unresponsive. The window flickers a bit. The titlebar would get "(Not Responding)" appended to it, and upon clicking inside the window it becomes grayed out, as Windows does this by default on non responsive windows. However in this state (even as and after it becomes grayed out), the OpenGL display continues to update and animate, and here's the kicker, it even does so while the window is being dragged. Clearly in this case the application isn't handling events from windows correctly, causing windows to think that it is in a hanged state. But there is clear evidence that the opengl continues to render.

Now I make one single modification to the code, which is these three lines placed in an appropriate spot inside the loop (which also does the OpenGL draw):

SDL_Event event;
if (SDL_PollEvent(&event) && event.type == SDL_QUIT)
    break;

All this is doing is flushing the message queue using SDL.

Now the behavior is that Windows no longer thinks it is "Not Responding" and it does not get grayed out. No flicker. Everything seems to run swimmingly. But once I click and drag the title bar to drag the window, rendering gets blocked. I haven't debugged it to be sure, but I suspect that SDL_PollEvent blocks for the duration of the window drag.

Is there a way around this? This is interesting because part of the behavior exhibited by failing to handle events is proof that what I want is possible in theory.

Update: I found this thread: http://www.gamedev.net/topic/488074-win32-message-pump-and-opengl---rendering-pauses-while-draggingresizing/

The verdict seems to be that it comes down to certain choices that Microsoft made for us... It basically gets stuck in DefWindowProc() till the mouse is released. It would get very messy to hack a fix for this and I might be able to do a work around by rendering in another thread. But I don't even want to begin to think about juggling an OpenGL context from multiple threads, if that's even something that's possible.

Answer 1

Some workaround that works for me - add event filter for SDL_WINDOWEVENT_SIZE_CHANGED event and do additional SetViewport and draw frame.

int SDLApp::eventFilter(void* pthis, const SDL_Event *event)
{
    if (event->type == SDL_WINDOWEVENT &&
        event->window.event == SDL_WINDOWEVENT_SIZE_CHANGED)
    {
        SDLApp* app = (SDLApp*)pthis;
        // Note: NULL rectangle is the entire window
        SDL_RenderSetViewport(app->renderer_, NULL);
        app->DrawFrame();
    }
    return 1;
}

...
SDL_SetEventFilter((SDL_EventFilter)SDLApp::eventFilter, this);

Answer 2

This question is old, but the solution I'm using doesn't seem to be mentioned anywhere else, so here it is.

I got my inspiration from this answer, and it doesn't use additional threads.

#include <SDL.h>
#define WIN32_LEAN_AND_MEAN
#define NOMINMAX
#include <Windows.h>
#include <SDL_syswm.h>

#define SIZE_MOVE_TIMER_ID 1

bool sizeMoveTimerRunning = false;

int eventWatch(void*, SDL_Event* event) {
    if (event->type == SDL_SYSWMEVENT) {
        const auto& winMessage = event->syswm.msg->msg.win;
        if (winMessage.msg == WM_ENTERSIZEMOVE) {
            // the user started dragging, so create the timer (with the minimum timeout)
            // if you have vsync enabled, then this shouldn't render unnecessarily
            sizeMoveTimerRunning = SetTimer(GetActiveWindow(), SIZE_MOVE_TIMER_ID, USER_TIMER_MINIMUM, nullptr);
        }
        else if (winMessage.msg == WM_TIMER) {
            if (winMessage.wParam == SIZE_MOVE_TIMER_ID) {
                // call your render function
                render();
            }
        }
    }
    return 0;
}

// rendering function
void render() {
    /* do your rendering here */
}

// event loop - call this function after setting up your window to start the event loop
void eventLoop() {
    SDL_AddEventWatch(eventWatch, nullptr); // register the event watch function
    SDL_EventState(SDL_SYSWMEVENT, SDL_ENABLE); // we need the native Windows events, so we can listen to WM_ENTERSIZEMOVE and WM_TIMER
    while (true) {
        SDL_Event event;
        while (SDL_PollEvent(&event)) {
            if (sizeMoveTimerRunning) {
                // modal drag/size loop ended, so kill the timer
                KillTimer(GetActiveWindow(), SIZE_MOVE_TIMER_ID);
                sizeMoveTimerRunning = false;
            }
            /* handle the events here */
        }
        render();
    }
}

Of course, if your rendering function needs to keep additional state (e.g. if you're using OOP), use the void* parameter of eventWatch(void*, SDL_Event*) to pass the state.

Answer 3

I had a similar problem in which it would freeze video playback when the window was dragged or resized. The solution I found was to spawn a separate thread for rendering and use the main thread for input.

Example:

DWORD RenderThread(SDL_Window* window)
{
    //Rendering stuff here...
}

int main()
{
    SDL_Init(SDL_INIT_EVERYTHING);

    SDL_Window* window = SDL_CreateWindow("Title Here",
        SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, h, w, SDL_WINDOW_RESIZABLE);

    HANDLE hRenderThread = CreateThread(NULL, 0, (LPTHREAD_START_ROUTINE)RenderThread, window, 0, NULL);

    SDL_Event event;

    while (1)
    {
        SDL_PollEvent(&event);

        switch (event.type)
        {
            //Event handling here...
        }
    }
}

Keep in mind that you MUST create the window in the thread that does event handling. If not it won't work. You can create the window in your event handling thread then pass that window pointer to your rendering thread.

Answer 4

Many windows procedures run a separate message loop until a certain event occurs, so you shouldn't rely on your main loop to do the drawing. If possible, application logic and rendering should always be handled in a separate thread.

Your main thread (that only handles message processing) doesn't need GL context at all, so you wouldn't need to worry about sharing.

Answer 5

I propose you created 2 threads:

• Thread 1: loops calling SDL_PollEvent() (without rendering anything)
• Thread 2: does OpenGL rendering (without calling SDL_PollEvent())

This way, your OpenGL context would be manipulated from a single thread. The whole solution has a minimum impact the architecture of your application.