SDL_SetWindowSize resizes window, but SDL_GetWindowSize reports old size — in Unix

问题

I can't use SDL_GetWindowSize cross-platform, because in Unix it doesn't know that the window size changed (though I can see it did). Visual Studio and MinGW have no problem. (I keep running into cases where Unix is an outlier, but it's always been that I was doing something wrong and the other 2 platforms didn't notice.)

I was running SDL2's version 2.0.8+dfsg1-4ubuntu on 64-bit Ubuntu 4.18.0 on VirtualBox; have since upgraded to SDL2 2.0.9+dfsg1-1ubuntu1 on Ubuntu 5.0.0, but behavior is the same. It's using GNOME Shell with X11.

The interaction with SDL_PollEvent is strange. But I do think it's possible and reasonable to call it sometimes!

So how can I get SDL_GetWindowSize to accurately report the size?

MCVE:

#include <cassert>
#include <SDL.h>

using namespace std;

int main (int argc, char** argv)
{
    //set things up
    if (SDL_Init(SDL_INIT_EVERYTHING) < 0) return -1;

    SDL_Window* window = SDL_CreateWindow("",0, 0,640, 480,0);      
    if (!window) return -1;

    SDL_Renderer* renderer = SDL_CreateRenderer(window, -1, 0);

    if (!renderer) return -1;

    //I need to be able to resize at times, so this is in the MCVE
    SDL_SetWindowSize(window, 600, 100);

    //Without this line, no problem
    SDL_PollEvent(nullptr); 

    //Assertions fail -- in Unix, not Visual Studio
    int w, h; SDL_GetWindowSize (window, &w, &h);
    assert (w == 600);
    assert (h == 100);

    return 0;
}

EDIT: trying a new test at commenter's suggestion. It does resolve the problem with SDL_GetWindowSize reporting the wrong dimensions.

#include <cassert>
#include <SDL.h>
#include <stdio.h>

using namespace std;

static void flush_event_queue(void) {
    SDL_Event ev;
    while(SDL_PollEvent(&ev)) {
        printf("ev.type=%d\n", ev.type);
        if(ev.type == SDL_WINDOWEVENT) {
            printf("window event %d\n", ev.window.event);
            if(ev.window.event == SDL_WINDOWEVENT_RESIZED || ev.window.event == SDL_WINDOWEVENT_SIZE_CHANGED) {
                printf("resized %d %dx%d\n", ev.window.event, ev.window.data1, ev.window.data2);
            }
        }
    }
}

int main (int argc, char** argv) {
    if (SDL_Init(SDL_INIT_EVERYTHING) < 0) return -1;

    SDL_Window* window = SDL_CreateWindow("",0, 0,640, 480,0);      
    if (!window) return -1;

    SDL_Renderer* renderer = SDL_CreateRenderer(window, -1, 0);

    if (!renderer) return -1;

    flush_event_queue();

    SDL_SetWindowSize(window, 600, 100);
    printf("SetWindowSize\n");

    flush_event_queue();

    int w, h; SDL_GetWindowSize (window, &w, &h);
    printf("%dx%d\n", w, h);
    assert (w == 600);
    assert (h == 100);

    //Now let's draw something and see if it shows up. No--
    SDL_SetRenderDrawColor(renderer, 255, 255, 255, 255);
    SDL_Rect rect1 = {0, 0, 100, 100};
    SDL_RenderFillRect(renderer, &rect1);
    SDL_RenderPresent (renderer);

    SDL_Event sdlEvent; //Wait for key hit, so we can see rect
    do
        SDL_WaitEvent (&sdlEvent);
    while (sdlEvent.type != SDL_KEYDOWN);


    return 0;
}

The resulting output is

ev.type=4352
ev.type=4352
ev.type=512
window event 1
ev.type=512
window event 2
ev.type=512
window event 1
ev.type=512
window event 4
ev.type=512
window event 3
ev.type=512
window event 12
ev.type=512
window event 15
ev.type=770
SetWindowSize
ev.type=512
window event 6
resized 6 600x100
600x100

If I comment out that first flush_event_queue() call in the new test, I don't ever get to the output -- it crashes on my asserts.

来源：https://stackoverflow.com/questions/57051494/sdl-setwindowsize-resizes-window-but-sdl-getwindowsize-reports-old-size-in-u