Window Events

Last Updated: May 28th, 2024

SDL also supports resizable windows. When you have resizable windows there are additional events to handle, which is what we'll be doing here.

class LWindow
{
    public:
        //Intializes internals
        LWindow();

        //Creates window
        bool init();

        //Creates renderer from internal window
        SDL_Renderer* createRenderer();

        //Handles window events
        void handleEvent( SDL_Event& e );

        //Deallocates internals
        void free();

        //Window dimensions
        int getWidth();
        int getHeight();

        //Window focii
        bool hasMouseFocus();
        bool hasKeyboardFocus();
        bool isMinimized();

    private:
        //Window data
        SDL_Window* mWindow;

        //Window dimensions
        int mWidth;
        int mHeight;

        //Window focus
        bool mMouseFocus;
        bool mKeyboardFocus;
        bool mFullScreen;
        bool mMinimized;
};

Here is our window class we'll be using as a wrapper for the SDL_Window. It has a constructor, an initializer that creates the window, a function to create a renderer from the window, an event handler, a deallocator, and some accessor functions to get various attributes from the window.

In terms of data members, we have the window we're wrapping, the dimensions of the window, and flags for the types of focus the windows has. We'll go into more detail further in the program.

//Our custom window
LWindow gWindow;

//The window renderer
SDL_Renderer* gRenderer = NULL;

//Scene textures
LTexture gSceneTexture;

We'll be using our window as a global object.

LWindow::LWindow()
{
    //Initialize non-existant window
    mWindow = NULL;
    mMouseFocus = false;
    mKeyboardFocus = false;
    mFullScreen = false;
    mMinimized = false;
    mWidth = 0;
    mHeight = 0;
}

In the constructor we initialize all our variables.

bool LWindow::init()
{
    //Create window
    mWindow = SDL_CreateWindow( "SDL Tutorial", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, SCREEN_WIDTH, SCREEN_HEIGHT, SDL_WINDOW_SHOWN | SDL_WINDOW_RESIZABLE );
    if( mWindow != NULL )
    {
        mMouseFocus = true;
        mKeyboardFocus = true;
        mWidth = SCREEN_WIDTH;
        mHeight = SCREEN_HEIGHT;
    }

    return mWindow != NULL;
}

Our initialization function creates the window with the SDL_WINDOW_RESIZABLE flag which allows for our window to be resizable. If the function succeeds we set the corresponding flags and dimensions. Then we return whether the window is null or not.

SDL_Renderer* LWindow::createRenderer()
{
    return SDL_CreateRenderer( mWindow, -1, SDL_RENDERER_ACCELERATED | SDL_RENDERER_PRESENTVSYNC );
}

Here we're handling the creation of a renderer from the member window. We're returning the created renderer because rendering will be handled outside of the class.

void LWindow::handleEvent( SDL_Event& e )
{
    //Window event occured
    if( e.type == SDL_WINDOWEVENT )
    {
        //Caption update flag
        bool updateCaption = false;

In our window's event handler we'll be looking for events of type SDL_WINDOWEVENT. SDL_WindowEvents are actually a family of events. Depending on the event we may have to update the caption of the window, so we have a flag that keeps track of that.

        switch( e.window.event )
        {
            //Get new dimensions and repaint on window size change
            case SDL_WINDOWEVENT_SIZE_CHANGED:
            mWidth = e.window.data1;
            mHeight = e.window.data2;
            SDL_RenderPresent( gRenderer );
            break;

            //Repaint on exposure
            case SDL_WINDOWEVENT_EXPOSED:
            SDL_RenderPresent( gRenderer );
            break;

When we have a window event we then want to check the SDL_WindowEventID to see what type of event it is. An SDL_WINDOWEVENT_SIZE_CHANGED is a resize event, so we get the new dimensions and refresh the image on the screen.

An SDL_WINDOWEVENT_EXPOSED just means that window was obscured in some way and now is not obscured so we want to repaint the window.

            //Mouse entered window
            case SDL_WINDOWEVENT_ENTER:
            mMouseFocus = true;
            updateCaption = true;
            break;
            
            //Mouse left window
            case SDL_WINDOWEVENT_LEAVE:
            mMouseFocus = false;
            updateCaption = true;
            break;

            //Window has keyboard focus
            case SDL_WINDOWEVENT_FOCUS_GAINED:
            mKeyboardFocus = true;
            updateCaption = true;
            break;

            //Window lost keyboard focus
            case SDL_WINDOWEVENT_FOCUS_LOST:
            mKeyboardFocus = false;
            updateCaption = true;
            break;

SDL_WINDOWEVENT_ENTER/SDL_WINDOWEVENT_LEAVE handles when the mouse moves into and out of the window. SDL_WINDOWEVENT_FOCUS_GAINED/SDL_WINDOWEVENT_FOCUS_LOST have to do when the window is getting input from the keyboard. Since our caption keeps track of mouse/keyboard focus, we set the update caption flag when any of these events happen.

            //Window minimized
            case SDL_WINDOWEVENT_MINIMIZED:
            mMinimized = true;
            break;

            //Window maximized
            case SDL_WINDOWEVENT_MAXIMIZED:
            mMinimized = false;
            break;
            
            //Window restored
            case SDL_WINDOWEVENT_RESTORED:
            mMinimized = false;
            break;
        }

Finally here we handle when the window was minimized, maximized, or restored from being minimized.

        //Update window caption with new data
        if( updateCaption )
        {
            std::stringstream caption;
            caption << "SDL Tutorial - MouseFocus:" << ( ( mMouseFocus ) ? "On" : "Off" ) << " KeyboardFocus:" << ( ( mKeyboardFocus ) ? "On" : "Off" );
            SDL_SetWindowTitle( mWindow, caption.str().c_str() );
        }
    }

If the caption needs to be updated, we load a string stream with the updated data and update the caption with SDL_SetWindowTitle.

    //Enter exit full screen on return key
    else if( e.type == SDL_KEYDOWN && e.key.keysym.sym == SDLK_RETURN )
    {
        if( mFullScreen )
        {
            SDL_SetWindowFullscreen( mWindow, 0 );
            mFullScreen = false;
        }
        else
        {
            SDL_SetWindowFullscreen( mWindow, SDL_WINDOW_FULLSCREEN_DESKTOP );
            mFullScreen = true;
            mMinimized = false;
        }
    }
}

For this demo we'll be toggling fullscreen with the return key. We can set fullscreen mode desktop mode using SDL_SetWindowFullscreen. Full screen desktop mode is essentially a "fake" fullscreen mode that resizes the window to the size of the screen. You can also use true fullscreen but there will be a slight delay in the transition. Each has their costs and benefits so pick the fullscreen mode that will work for your application.

int LWindow::getWidth()
{
    return mWidth;
}

int LWindow::getHeight()
{
    return mHeight;
}

bool LWindow::hasMouseFocus()
{
    return mMouseFocus;
}

bool LWindow::hasKeyboardFocus()
{
    return mKeyboardFocus;
}

bool LWindow::isMinimized()
{
    return mMinimized;
}

Here is a quick rundown of the accessors we use.

        //Create window
        if( !gWindow.init() )
        {
            printf( "Window could not be created! SDL Error: %s\n", SDL_GetError() );
            success = false;
        }
        else
        {
            //Create renderer for window
            gRenderer = gWindow.createRenderer();
            if( gRenderer == NULL )
            {
                printf( "Renderer could not be created! SDL Error: %s\n", SDL_GetError() );
                success = false;
            }

In our initialization function we create our window and renderer only this time with our window wrapper.

void close()
{
    //Free loaded images
    gSceneTexture.free();

    //Destroy window    
    SDL_DestroyRenderer( gRenderer );
    gWindow.free();

    //Quit SDL subsystems
    IMG_Quit();
    SDL_Quit();
}

In our clean up function we still deallocate our window and renderer.

            //While application is running
            while( !quit )
            {
                //Handle events on queue
                while( SDL_PollEvent( &e ) != 0 )
                {
                    //User requests quit
                    if( e.type == SDL_QUIT )
                    {
                        quit = true;
                    }

                    //Handle window events
                    gWindow.handleEvent( e );
                }

                //Only draw when not minimized
                if( !gWindow.isMinimized() )
                {
                    //Clear screen
                    SDL_SetRenderDrawColor( gRenderer, 0xFF, 0xFF, 0xFF, 0xFF );
                    SDL_RenderClear( gRenderer );

                    //Render text textures
                    gSceneTexture.render( ( gWindow.getWidth() - gSceneTexture.getWidth() ) / 2, ( gWindow.getHeight() - gSceneTexture.getHeight() ) / 2 );

                    //Update screen
                    SDL_RenderPresent( gRenderer );
                }
            }

In the main loop we make sure to pass events to the window wrapper to handle resize events and in the rendering part of our code we make sure to only render when the window is not minimized because this can cause some bugs when we try to render to a minimized window.

Download the media and source code for this demo here.

Download SDL3 port of this demo (graciously provided by Sam Lantinga, creator of SDL) here.

Back to Index