MessageDialog is an elementary OS styled dialog used to display a message to the user.
The class itself is similar to it's Gtk equivalent Gtk.MessageDialog but follows elementary OS design conventions.
See The Human Interface Guidelines for dialogs for more detailed disscussion on the dialog wording and design.
Example
var message_dialog = new Granite.MessageDialog.with_image_from_icon_name (
"This is a primary text",
"This is a secondary, multiline, long text. This text usually extends the primary text and prints e.g: the details of an error.",
"applications-development",
Gtk.ButtonsType.CLOSE
);
var custom_widget = new Gtk.CheckButton.with_label ("Custom widget");
custom_widget.show ();
message_dialog.custom_bin.add (custom_widget);
message_dialog.run ();
message_dialog.destroy ();