Window Events

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Last Updated 2/25/06Up until now we couldn't do much with our window, mainly because it's more work to deal with it being resizable. This tutorial will demonstrate how to deal with a resizable window.
//Our window class Window { private: //Whether the window is windowed or not bool windowed; //Whether the window is fine bool windowOK; public: //Constructor Window(); //Handle window events void handle_events(); //Turn fullscreen on/off void toggle_fullscreen(); //Check if anything's wrong with the window bool error(); };
Now we have a window class to manage all the things a resizable window can do.
For variables we have two flags. The "windowed" flag keeps track of whether the screen is windowed or fullscreen. "windowOK" is whether the window is operational.
Then we have a constructor, a function to handle events, a function to toggle windowed/fullscreen modes and an error checking function.
bool init() { //Initialize all SDL subsystems if( SDL_Init( SDL_INIT_EVERYTHING ) == -1 ) { return false; } //If eveything loads fine return true; }
Here's our init() function.
Because everything associated with the window is in a seperate class, all we do is initialize SDL.
Window::Window() { //Set up the screen screen = SDL_SetVideoMode( SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_BPP, SDL_SWSURFACE | SDL_RESIZABLE ); //If there's an error if( screen == NULL ) { windowOK = false; return; } else { windowOK = true; } //Set the window caption SDL_WM_SetCaption( "Window Event Test", NULL ); //Set window flag windowed = true; }
In the window class' constructor we create a window. Since we want it to be resizeable, we pass the SDL_RESIZABLE flag.
Then we check if the screen is NULL. If it is we set "windowOK" to false so we can tell that there was a problem.
If everything set up fine, we set "windowOK" to true, set the caption and since we're starting out windowed we set the "windowed" flag to true.
void Window::toggle_fullscreen() { //If the screen is windowed if( windowed == true ) { //Set the screen to fullscreen screen = SDL_SetVideoMode( SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_BPP, SDL_SWSURFACE | SDL_RESIZABLE | SDL_FULLSCREEN ); //If there's an error if( screen == NULL ) { windowOK = false; return; } //Set the window state flag windowed = false; } //If the screen is fullscreen else if( windowed == false ) { //Window the screen screen = SDL_SetVideoMode( SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_BPP, SDL_SWSURFACE | SDL_RESIZABLE ); //If there's an error if( screen == NULL ) { windowOK = false; return; } //Set the window state flag windowed = true; } }
In this program I added the ability to switch from windowed to fullscreen and back when you press enter, which you can only do with resizable window. Here's the function that toggles windowed/fullscreen modes.
When enter is pressed we check the if the screen is windowed or not. If it's windowed, we call SDL_SetVideoMode() with the SDL_FULLSCREEN flag to switch to fullscreen and then change "windowed" to false. If we're already fullscreen we call SDL_SetVideoMode() normally to switch to windowed mode and then change the "windowed" flag to true.
void Window::handle_events() { //If there's something wrong with the window if( windowOK == false ) { return; } //If the window resized if( event.type == SDL_VIDEORESIZE ) { //Resize the screen screen = SDL_SetVideoMode( event.resize.w, event.resize.h, SCREEN_BPP, SDL_SWSURFACE | SDL_RESIZABLE ); //If there's an error if( screen == NULL ) { windowOK = false; return; } }
Here we start our event handling and the first thing we do is check if the window is operational. Then we handle is a video resize event.
Whenever the window is resized you must set the video over again using SDL_SetVideoMode(). The dimensions of the new window is stored in our event structure in event.resize.w and event.resize.h so you can easily set the screen to the new size.
//If enter was pressed else if( ( event.type == SDL_KEYDOWN ) && ( event.key.keysym.sym == SDLK_RETURN ) ) { //Turn fullscreen on/off toggle_fullscreen(); }
When enter is pressed we toggle window/fullscreen modes.
//If the window focus changed else if( event.type == SDL_ACTIVEEVENT ) { //If the window was inconified or restored if( event.active.state & SDL_APPACTIVE ) { //If the application is no longer active if( event.active.gain == 0 ) { SDL_WM_SetCaption( "Window Event Test: Iconified", NULL ); } else { SDL_WM_SetCaption( "Window Event Test", NULL ); } }
Another type of event is SDL_ACTIVEEVENT. A SDL_ACTIVEEVENT happens whenever the screen, keyboard, or mouse becomes active or inactive.
Here we check if the screen became active or inactive by being iconified/minimized or restored. When the window is iconified/minimized, "gain" in our SDL_ActiveEvent structure is 0. When the window is restored, "gain" is 1.
In this program we change the caption to notify the user of change.
//If something happened to the keyboard focus else if( event.active.state & SDL_APPINPUTFOCUS ) { //If the application lost keyboard focus if( event.active.gain == 0 ) { SDL_WM_SetCaption( "Window Event Test: Keyboard focus lost", NULL ); } else { SDL_WM_SetCaption( "Window Event Test", NULL ); } } //If something happened to the mouse focus else if( event.active.state & SDL_APPMOUSEFOCUS ) { //If the application lost mouse focus if( event.active.gain == 0 ) { SDL_WM_SetCaption( "Window Event Test: Mouse Focus Lost", NULL ); } else { SDL_WM_SetCaption( "Window Event Test", NULL ); } } }
This where keyboard and mouse focus is handled.
SDL_APPINPUTFOCUS is change in keyboard focus. SDL_APPMOUSEFOCUS is change in mouse focus. As with SDL_APPACTIVE when focus is lost "gain" is 0, when focus is restored "gain" is 1.
Remember to check for SDL_APPACTIVE first because bitwise AND comparisons with "state" will report other active events.
//If the window's screen has been altered else if( event.type == SDL_VIDEOEXPOSE ) { //Update the screen if( SDL_Flip( screen ) == -1 ) { //If there's an error windowOK = false; return; } } }
Lastly, we handle the SDL_VIDEOEXPOSE event.
A SDL_VIDEOEXPOSE event happens whenever the screen is altered by something outside the program. We handle it by updating the screen.
bool Window::error() { return !windowOK; }
Here's the function we use to check if there's errors with our window.
We negate "windowOK" because when the window is ok there are no errors and when the window is not ok there is an error.
//Quit flag bool quit = false; //Initialize if( init() == false ) { return 1; } //Create a window Window myWindow; //If the window failed if( myWindow.error() == true ) { return 1; } //Load the files if( load_files() == false ) { return 1; }
At the top of our main function we create a Window object after we initialize. Then we check for errors in the window and continue normally.
//While the user hasn't quit while( quit == false ) { //While there's events to handle while( SDL_PollEvent( &event ) ) { //Handle window events myWindow.handle_events(); //If escape was pressed if( ( event.type == SDL_KEYDOWN ) && ( event.key.keysym.sym == SDLK_ESCAPE ) ) { //Quit the program quit = true; } //If the user has Xed out the window if( event.type == SDL_QUIT ) { //Quit the program quit = true; } } //If the window failed if( myWindow.error() == true ) { return 1; } //Fill the screen white SDL_FillRect( screen, &screen->clip_rect, SDL_MapRGB( screen->format, 0xFF, 0xFF, 0xFF ) ); //Show the image centered on the screen apply_surface( ( screen->w - testing->w ) / 2, ( screen->h - testing->h ) / 2, testing, screen ); //Update the screen if( SDL_Flip( screen ) == -1 ) { return 1; } }
Here's our main loop. Nothing much to point out other than we have to remember check for errors in the window before we render. There's no point to render to a screen that's non-operational.
Download the media and source code for this tutorial
here
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