SDL_WINDOWEVENT_SIZE_CHANGED does not appear when resize by dragging border?

Question

I am playing around with SDL2 on Windows. I am trying to get the resize event like this.

#include "SDL/SDL.h"
#include <iostream>
int main(int argc, char** argv) {
    SDL_Window* window = SDL_CreateWindow("title", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED,800, 500, SDL_WINDOW_OPENGL | SDL_WINDOW_SHOWN | SDL_WINDOW_RESIZABLE);
    while (true) {
        SDL_Event sdl_event;
        if (SDL_WaitEvent(&sdl_event)) {
            if (sdl_event.key.repeat == 0) { // no repeat event
                switch (sdl_event.type) {
                case SDL_QUIT:
                {
                    break;
                }
                case SDL_WINDOWEVENT:
                {
                    if (sdl_event.window.event == SDL_WINDOWEVENT_SIZE_CHANGED){
                        std::cout << sdl_event.window.data1 << " " << sdl_event.window.data2 << "\n";
                    }
                    break;
                }
                }
            }
        }
    }
    return 0;
}

But I am not getting the event when I try to resize the window by dragger the border. However, I do get the event when I use the maximize button. What is wrong?

Answer 1

ZhuYaDong's SDL_AddEventWatch() approach works on this Windows 10 box:

#include <SDL.h>
#include <iostream>
#include <thread>

int resizingEventWatcher( void* data, SDL_Event* event )
{
    SDL_Window* window = static_cast< SDL_Window* >( data );
    SDL_Event& ev = *event;

    if( ev.type == SDL_WINDOWEVENT &&
        ev.window.event == SDL_WINDOWEVENT_RESIZED )
    {
        if( SDL_GetWindowFromID( ev.window.windowID ) == window )
        {
            std::cout << std::this_thread::get_id() << ": " << ev.window.data1 << " " << ev.window.data2 << std::endl;
        }
    }
    return 0;
}

int main( int argc, char** argv )
{
    SDL_Init( SDL_INIT_EVERYTHING );

    SDL_Window* window = SDL_CreateWindow
        (
        "title",
        SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED,
        800, 500,
        SDL_WINDOW_RESIZABLE
        );
    SDL_AddEventWatch( resizingEventWatcher, window );

    bool running = true;
    while( running )
    {
        SDL_Event ev;
        while( SDL_PollEvent( &ev ) )
        {
            switch( ev.type )
            {
            case SDL_QUIT:
                running = false;
                break;
            case SDL_WINDOWEVENT:
                if( ev.window.event == SDL_WINDOWEVENT_SIZE_CHANGED )
                {
                    std::cout << std::this_thread::get_id() << ": "<< ev.window.data1 << " " << ev.window.data2 << std::endl;
                }
                break;
            }
        }
    }
    return 0;
}

If you want to get fancy you can create a user event in resizingEventWatcher() & post it to the main event queue with SDL_PushEvent(). EDIT: Sadly these user events seem to just get queued up during resizing and don't make it to the main event-handling loop until you stop sizing the window:

#include <SDL.h>
#include <iostream>
#include <thread>

Uint32 ResizingEvent = 0;
int resizingEventWatcher( void* data, SDL_Event* event )
{
    SDL_Event& ev = *event;
    if( ev.type == SDL_WINDOWEVENT &&
        ev.window.event == SDL_WINDOWEVENT_RESIZED )
    {
        SDL_Event newEvent = { 0 };
        newEvent.type = ResizingEvent;
        newEvent.user.windowID = ev.window.windowID;
        newEvent.user.data1 = reinterpret_cast< void* >( ev.window.data1 );
        newEvent.user.data2 = reinterpret_cast< void* >( ev.window.data2 );
        SDL_PushEvent( &newEvent );
    }
    return 0;
}

int main( int argc, char** argv )
{
    SDL_Init( SDL_INIT_EVERYTHING );
    ResizingEvent = SDL_RegisterEvents( 1 );

    SDL_Window* window = SDL_CreateWindow
        (
        "title",
        SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED,
        800, 500,
        SDL_WINDOW_RESIZABLE
        );
    SDL_AddEventWatch( resizingEventWatcher, window );

    bool running = true;
    while( running )
    {
        SDL_Event ev;
        while( SDL_PollEvent( &ev ) )
        {
            switch( ev.type )
            {
            case SDL_QUIT:
                running = false;
                break;
            case SDL_WINDOWEVENT:
                if( ev.window.event == SDL_WINDOWEVENT_SIZE_CHANGED )
                {
                    std::cout << std::this_thread::get_id() << ": "<< ev.window.data1 << " " << ev.window.data2 << std::endl;
                }
                break;
            case SDL_USEREVENT:
                if( ev.user.type == ResizingEvent )
                {
                    std::cout << std::this_thread::get_id() << ": "<< reinterpret_cast< Sint32 >( ev.user.data1 ) << " " << reinterpret_cast< Sint32 >( ev.user.data2 ) << std::endl;
                }
                break;
            }
        }
    }
    return 0;
}

(Using SDL's release-2.0.10 tag)