问SDL2多窗口焦点

EN

"Window 1 lost focus"
"Window 2 gained focus."

"Window 2 lost focus."
"Window 1 gained focus."
"Window 1 lost focus."

#include <iostream>
#include <SDL2/SDL.h>
using namespace std;
int main(int argc, char **argv) {
  SDL_Init(SDL_INIT_VIDEO);
  SDL_Window* w1=SDL_CreateWindow("Window 1",
                                  SDL_WINDOWPOS_UNDEFINED,
                                  SDL_WINDOWPOS_UNDEFINED,
                                  250,200,SDL_WINDOW_SHOWN);
  SDL_Window* w2=SDL_CreateWindow("Window 2",
                                  SDL_WINDOWPOS_UNDEFINED,
                                  SDL_WINDOWPOS_UNDEFINED,
                                  200,250,SDL_WINDOW_SHOWN);
  bool quit=false;
  while(!quit){
    SDL_Event e;
    while(!quit && SDL_PollEvent(&e)){
      switch(e.type){
        case SDL_WINDOWEVENT :
          { // this block just scopes 'targetWindow' and 'title'
            SDL_Window* targetWindow=SDL_GetWindowFromID(e.window.windowID);
            const char* title=SDL_GetWindowTitle(targetWindow);
            switch(e.window.event){
              case SDL_WINDOWEVENT_FOCUS_GAINED :
                // tell which window gained focus
                cout << title << " gained focus!" << endl;
                break;
              case SDL_WINDOWEVENT_FOCUS_LOST :
                // tell which window lost focus
                cout << title << " lost focus!" << endl;
                break;
            }
          }
          break;
        case SDL_QUIT :
          quit=true;
          break;
      }
    }
  }
  SDL_Quit();
  return 0;
}