Custom Widgets
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## Custom Widgets

By deriving directly from `Gtk::Widget` you can do all the drawing for your widget directly, instead of just arranging child widgets. For instance, a Gtk::Label draws the text of the label, but does not do this by using other widgets.

### Example

This example implements a widget which draws a square.

Figure 21.2. Custom Widget

Source Code

File: examplewindow.h

    #ifndef GTKMM_EXAMPLEWINDOW_H
    #define GTKMM_EXAMPLEWINDOW_H

    #include <gtkmm.h>
    #include "mywidget.h"

    class ExampleWindow : public Gtk::Window
    {
    public:
      ExampleWindow();
      virtual ~ExampleWindow();

    protected:
      //Signal handlers:
      virtual void on_button_quit();

      //Child widgets:
      Gtk::VBox m_VBox;
      MyWidget m_MyWidget;
      Gtk::HButtonBox m_ButtonBox;
      Gtk::Button m_Button_Quit;
    };

    #endif //GTKMM_EXAMPLEWINDOW_H

File: mywidget.h

    #ifndef GTKMM_CUSTOM_WIDGET_MYWIDGET_H
    #define GTKMM_CUSTOM_WIDGET_MYWIDGET_H

    #include <gtkmm/widget.h>

    class MyWidget : public Gtk::Widget
    {
    public:
      MyWidget();
      virtual ~MyWidget();

    protected:

      //Overrides:
      virtual void on_size_request(Gtk::Requisition* requisition);
      virtual void on_size_allocate(Gtk::Allocation& allocation);
      virtual void on_map();
      virtual void on_unmap();
      virtual void on_realize();
      virtual void on_unrealize();
      virtual bool on_expose_event(GdkEventExpose* event);

      Glib::RefPtr<Gdk::Window> m_refGdkWindow;
      Glib::RefPtr<Gdk::GC> m_refGC;
    };

    #endif //GTKMM_CUSTOM_WIDGET_MYWIDGET_H

File: examplewindow.cc

    #include <iostream>
    #include "examplewindow.h"

    ExampleWindow::ExampleWindow()
    : m_Button_Quit("Quit")
    {
      set_title("Custom Widget example");
      set_border_width(6);
      set_default_size(400, 200);

      add(m_VBox);
      m_VBox.pack_start(m_MyWidget, Gtk::PACK_EXPAND_WIDGET);
      m_MyWidget.show();

      m_VBox.pack_start(m_ButtonBox, Gtk::PACK_SHRINK);

      m_ButtonBox.pack_start(m_Button_Quit, Gtk::PACK_SHRINK);
      m_ButtonBox.set_border_width(6);
      m_ButtonBox.set_layout(Gtk::BUTTONBOX_END);
      m_Button_Quit.signal_clicked().connect( sigc::mem_fun(*this, &ExampleWindow::on_button_quit) );

      show_all_children();
    }

    ExampleWindow::~ExampleWindow()
    {
    }

    void ExampleWindow::on_button_quit()
    {
      hide();
    }

File: main.cc

    #include <gtkmm/main.h>
    #include "examplewindow.h"

    int main(int argc, char *argv[])
    {
      Gtk::Main kit(argc, argv);

      ExampleWindow window;
      Gtk::Main::run(window); //Shows the window and returns when it is closed.

      return 0;
    }

File: mywidget.cc

    #include <iostream>
    #include "mywidget.h"

    MyWidget::MyWidget()
    {
      set_flags(Gtk::NO_WINDOW);
      //add_events(Gdk::EXPOSURE_MASK);
    }

    MyWidget::~MyWidget()
    {
    }

    void MyWidget::on_size_request(Gtk::Requisition* requisition)
    {
      //Initialize the output parameter:
      *requisition = Gtk::Requisition();

      //Discover the total amount of minimum space needed by this widget.

      //Let's make this simple example widget always need 50 by 50:
      requisition->height = 50;
      requisition->width = 50;
    }

    void MyWidget::on_size_allocate(Gtk::Allocation& allocation)
    {
      //Do something with the space that we have actually been given:
      //(We will not be given heights or widths less than we have requested, though we might get more)

      //Use the offered allocation for this container:
      set_allocation(allocation);

      if(m_refGdkWindow)
        m_refGdkWindow->move_resize( allocation.get_x(), allocation.get_y(), allocation.get_width(), allocation.get_height() );
    }

    void MyWidget::on_map()
    {
      //Call base class:
      Gtk::Widget::on_map();
    }

    void MyWidget::on_unmap()
    {
      //Call base class:
      Gtk::Widget::on_unmap();
    }

    void MyWidget::on_realize()
    {
      //Call base class:
      Gtk::Widget::on_realize();

      //TODO: Not sure whether this is necessary:
      set_flags(Gtk::REALIZED);
      set_window( get_parent_window() );
      GtkWidget* cWidget = (GtkWidget*)gobj();
      cWidget->style = gtk_style_attach(cWidget->style, get_window()->gobj());

      if(!m_refGdkWindow)
      {
        //Create the GdkWindow:

        GdkWindowAttr attributes;
        memset(&attributes, 0, sizeof(attributes));

        Gtk::Allocation allocation = get_allocation();

        //Set initial position and size of the Gdk::Window:
        attributes.x = allocation.get_x();
        attributes.y = allocation.get_y();
        attributes.width = allocation.get_width();
        attributes.height = allocation.get_height();

        attributes.window_type = GDK_WINDOW_CHILD;
        attributes.wclass = GDK_INPUT_OUTPUT;

        m_refGdkWindow = Gdk::Window::create(
          get_window() /* parent */,
          &attributes,
          GDK_WA_X | GDK_WA_Y);

        //Allocate a GC for use in on_expose_event():
        m_refGC = Gdk::GC::create(m_refGdkWindow);
      }
    }

    void MyWidget::on_unrealize()
    {
      m_refGdkWindow.clear();
      m_refGC.clear();

      //Call base class:
      Gtk::Widget::on_unrealize();
    }

    bool MyWidget::on_expose_event(GdkEventExpose* /* event */)
    {
      if(m_refGdkWindow)
      {
        //Draw on the Gdk::Window:

        Glib::RefPtr<Gdk::Colormap> colormap = get_default_colormap ();
        Gdk::Color color_blue("blue");
        Gdk::Color color_red("red");
        colormap->alloc_color(color_blue);
        colormap->alloc_color(color_red);

        m_refGdkWindow->set_background( color_red );
        m_refGdkWindow->clear();
        m_refGC->set_foreground( color_blue );
        m_refGdkWindow->draw_line(m_refGC, 1, 1, 100, 100);
      }

      return true;
    }

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