Custom Widgets
| Chapter 28. Custom Widgets |

## 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.

When deriving from `Gtk::Widget`, you should override the following virtual methods. The methods marked (optional) need not be overridden in all custom widgets. The base class's methods may be appropriate.

- `get_request_mode_vfunc()`: (optional) Return what `Gtk::SizeRequestMode` is preferred by the widget.

- `get_preferred_width_vfunc()`: Calculate the minimum and natural width of the widget.

- `get_preferred_height_vfunc()`: Calculate the minimum and natural height of the widget.

- `get_preferred_width_for_height_vfunc()`: Calculate the minimum and natural width of the widget, if it would be given the specified height.

- `get_preferred_height_for_width_vfunc()`: Calculate the minimum and natural height of the widget, if it would be given the specified width.

- `on_size_allocate()`: Position the widget, given the height and width that it has actually been given.

- `on_realize()`: Associate a `Gdk::Window` with the widget.

- `on_unrealize()`: (optional) Break the association with the `Gdk::Window`.

- `on_map()`: (optional)

- `on_unmap()`: (optional)

- `on_draw()`: Draw on the supplied `Cairo::Context`.

The first 6 methods in the previous table are also overridden in custom containers. They are briefly described in the Custom Containers section.

Most custom widgets need their own `Gdk::Window` to draw on. Then you can call `Gtk::Widget::set_has_window(true)` in your constructor. (This is the default value.) If you do not call `set_has_window(false)`, you must override `on_realize()` and call `Gtk::Widget::set_realized()` and `Gtk::Widget::set_window()` from there.

### Example

This example implements a widget which draws a Penrose triangle.

Figure 28.2. Custom Widget

Source Code

File: `mywidget.h` (For use with gtkmm 3, not gtkmm 2)

    #ifndef GTKMM_CUSTOM_WIDGET_MYWIDGET_H
    #define GTKMM_CUSTOM_WIDGET_MYWIDGET_H

    #include <gtkmm/widget.h>
    #include <gtkmm/cssprovider.h>

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

    protected:

      //Overrides:
      virtual Gtk::SizeRequestMode get_request_mode_vfunc() const;
      virtual void get_preferred_width_vfunc(int& minimum_width, int& natural_width) const;
      virtual void get_preferred_height_for_width_vfunc(int width, int& minimum_height, int& natural_height) const;
      virtual void get_preferred_height_vfunc(int& minimum_height, int& natural_height) const;
      virtual void get_preferred_width_for_height_vfunc(int height, int& minimum_width, int& natural_width) const;
      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_draw(const Cairo::RefPtr<Cairo::Context>& cr);

      Glib::RefPtr<Gdk::Window> m_refGdkWindow;
      Glib::RefPtr<Gtk::CssProvider> m_refStyleProvider;

      int m_scale;
    };

    #endif //GTKMM_CUSTOM_WIDGET_MYWIDGET_H

File: `examplewindow.h` (For use with gtkmm 3, not gtkmm 2)

    #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:
      void on_button_quit();

      //Child widgets:
      Gtk::Box m_VBox;
      MyWidget m_MyWidget;
      Gtk::ButtonBox m_ButtonBox;
      Gtk::Button m_Button_Quit;
    };

    #endif //GTKMM_EXAMPLEWINDOW_H

File: `examplewindow.cc` (For use with gtkmm 3, not gtkmm 2)

    #include "examplewindow.h"

    ExampleWindow::ExampleWindow()
    : m_VBox(Gtk::ORIENTATION_VERTICAL),
      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` (For use with gtkmm 3, not gtkmm 2)

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

    int main(int argc, char *argv[])
    {
      Glib::RefPtr<Gtk::Application> app = Gtk::Application::create(argc, argv, "org.gtkmm.example");

      ExampleWindow window;

      //Shows the window and returns when it is closed.
      return app->run(window);
    }

File: `mywidget.cc` (For use with gtkmm 3, not gtkmm 2)

    #include "mywidget.h"
    #include <gdkmm/general.h>  // for cairo helper functions
    #include <iostream>
    //#include <gtk/gtkwidget.h> //For GTK_IS_WIDGET()
    #include <cstring>

    MyWidget::MyWidget() :
      //The GType name will actually be gtkmm__CustomObject_mywidget
      Glib::ObjectBase("mywidget"),
      Gtk::Widget(),
      m_scale(1000)
    {
      set_has_window(true);

      //This shows the GType name, which must be used in the CSS file.
      std::cout << "GType name: " << G_OBJECT_TYPE_NAME(gobj()) << std::endl;

      //This shows that the GType still derives from GtkWidget:
      //std::cout << "Gtype is a GtkWidget?:" << GTK_IS_WIDGET(gobj()) << std::endl;

      //Install a style so that an aspect of this widget may be themed via a CSS
      //style sheet file:
      gtk_widget_class_install_style_property(GTK_WIDGET_CLASS(
                  G_OBJECT_GET_CLASS(gobj())),
          g_param_spec_int("example_scale",
            "Scale of Example Drawing",
            "The scale to use when drawing. This is just a silly example.",
            G_MININT,
            G_MAXINT,
            500,
            G_PARAM_READABLE) );

      m_refStyleProvider = Gtk::CssProvider::create();
      Glib::RefPtr<Gtk::StyleContext> refStyleContext = get_style_context();
      refStyleContext->add_provider(m_refStyleProvider,
        GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);

      try
      {
        m_refStyleProvider->load_from_path("custom_gtk.css");
      }
      catch(const Glib::Error& ex)
      {
        std::cerr << "Gtk::CssProvider::load_from_path() failed: " << ex.what() << std::endl;
      }
    }

    MyWidget::~MyWidget()
    {
    }

    Gtk::SizeRequestMode MyWidget::get_request_mode_vfunc() const
    {
      //Accept the default value supplied by the base class.
      return Gtk::Widget::get_request_mode_vfunc();
    }

    //Discover the total amount of minimum space and natural space needed by
    //this widget.
    //Let's make this simple example widget always need minimum 60 by 50 and
    //natural 100 by 70.
    void MyWidget::get_preferred_width_vfunc(int& minimum_width, int& natural_width) const
    {
      minimum_width = 60;
      natural_width = 100;
    }

    void MyWidget::get_preferred_height_for_width_vfunc(int /* width */,
       int& minimum_height, int& natural_height) const
    {
      minimum_height = 50;
      natural_height = 70;
    }

    void MyWidget::get_preferred_height_vfunc(int& minimum_height, int& natural_height) const
    {
      minimum_height = 50;
      natural_height = 70;
    }

    void MyWidget::get_preferred_width_for_height_vfunc(int /* height */,
       int& minimum_width, int& natural_width) const
    {
      minimum_width = 60;
      natural_width = 100;
    }

    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()
    {
      //Do not call base class Gtk::Widget::on_realize().
      //It's intended only for widgets that set_has_window(false).

      set_realized();

      //Get the themed style from the CSS file:
      get_style_property("example_scale", m_scale);
      std::cout << "m_scale (example_scale from the theme/css-file) is: "
          << m_scale << std::endl;

      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.event_mask = get_events () | Gdk::EXPOSURE_MASK;
        attributes.window_type = GDK_WINDOW_CHILD;
        attributes.wclass = GDK_INPUT_OUTPUT;

        m_refGdkWindow = Gdk::Window::create(get_parent_window(), &attributes,
                GDK_WA_X | GDK_WA_Y);
        set_window(m_refGdkWindow);

        //set colors
        override_background_color(Gdk::RGBA("red"));
        override_color(Gdk::RGBA("blue"));

        //make the widget receive expose events
        m_refGdkWindow->set_user_data(gobj());
      }
    }

    void MyWidget::on_unrealize()
    {
      m_refGdkWindow.reset();

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

    bool MyWidget::on_draw(const Cairo::RefPtr<Cairo::Context>& cr)
    {
      const double scale_x = (double)get_allocation().get_width() / m_scale;
      const double scale_y = (double)get_allocation().get_height() / m_scale;

      // paint the background
      Gdk::Cairo::set_source_rgba(cr, get_style_context()->get_background_color());
      cr->paint();

      // draw the foreground
      Gdk::Cairo::set_source_rgba(cr, get_style_context()->get_color());
      cr->move_to(155.*scale_x, 165.*scale_y);
      cr->line_to(155.*scale_x, 838.*scale_y);
      cr->line_to(265.*scale_x, 900.*scale_y);
      cr->line_to(849.*scale_x, 564.*scale_y);
      cr->line_to(849.*scale_x, 438.*scale_y);
      cr->line_to(265.*scale_x, 100.*scale_y);
      cr->line_to(155.*scale_x, 165.*scale_y);
      cr->move_to(265.*scale_x, 100.*scale_y);
      cr->line_to(265.*scale_x, 652.*scale_y);
      cr->line_to(526.*scale_x, 502.*scale_y);
      cr->move_to(369.*scale_x, 411.*scale_y);
      cr->line_to(633.*scale_x, 564.*scale_y);
      cr->move_to(369.*scale_x, 286.*scale_y);
      cr->line_to(369.*scale_x, 592.*scale_y);
      cr->move_to(369.*scale_x, 286.*scale_y);
      cr->line_to(849.*scale_x, 564.*scale_y);
      cr->move_to(633.*scale_x, 564.*scale_y);
      cr->line_to(155.*scale_x, 838.*scale_y);
      cr->stroke();

      return true;
    }

Chapter 28. Custom Widgets |  | Chapter 29. Multi-threaded programs