The user can provide input by typing keys and manipulating the mouse. An application's window handles keyboard activity by overriding the key_pressed method inherited from Window. The SGL event manager directs mouse events to one of four mouse processing methods: mouse_pressed, mouse_released, mouse_moved, and mouse_dragged. If the user presses a key on the keyboard, the event manager calls the window's key_pressed method. Clients override these methods to provide behavior appropriate for the application.

Example 3.3 allows a user to change the color of a rectangular box by clicking a mouse button or pressing the T (for toggle) key on the keyboard.

#include <GL/sgl.hpp>

class ColorBox: public sgl::Window {
protected:
    bool is_red;
public:
    ColorBox(): sgl::Window("Click Me", 100, 100, 400, 300, 
                            0.0, 100.0, 0.0, 100.0), is_red(true) {}

    void paint() override {
        if (is_red)       
            sgl::set_color(sgl::RED);
        else
            sgl::set_color(sgl::BLUE);
        sgl::fill_rectangle(30.0, 30.0, 40.0, 40.0);
    }

    void mouse_pressed(double, double, sgl::MouseButton) override {
        is_red = !is_red;  //  Toggle the color
        repaint();         //  Request a repaint
    }

    void key_pressed(int key, double x, double y) override {
        //  Did the user press the T key?
        if (key == 'T' || key == 't') {
            is_red = !is_red;  //  Toggle the color
            repaint();         //  Request a repaint
        }
        //  Call base class method in case user pressed
        //  Alt-F4 to close the window
        sgl::Window::key_pressed(key, x, y);
    }
};

int main() {
    //  Create a window instance and execute it
    sgl::run<ColorBox>();
}

Each input handling method will usually include a call to repaint. This is because the user's input generally affects what the program displays. In the case of Example 3.3, the user's mouse click is supposed to change the color of the box, but the window is repainted only when the event manager calls the window object's paint method. The repaint call notifies the event manager that the contents of the window need to be redrawn. Client code should not call paint directly.