A MessageDialog is a modal message dialog, which means a popup that you have to respond to before you get back to what you were doing in the window that it's attached to. This one can cause the world to explode (or at least it says that it can). To make the popup appear when you run this sample, click on "Message" inside of its application menu -- that's the menu that appears when you click on an application's name in the upper-left screen corner, next to Activities.
The difference between a MessageDialog and a Dialog is that a Dialog can contain whatever widgets and content you want to put in it, whereas a MessageDialog is just a convenient way to make popups appear with a basic message and buttons.
These are the libraries we need to import for this application to run. Remember that the line which tells GNOME that we're using Gjs always needs to go at the start.
All the code for this sample goes in the MessageDialogExample class. The above code creates a Gtk.Application for our widgets and window to go in.
Before we call _buildUI to create the window and the widgets inside it, we need to call _initMenus, which tells GNOME to create the menu. We can put the actual code for _initMenus after the code for _buildUI, since it doesn't matter what order we put them in so long as _initMenus is called first in _onStartup.
The _buildUI function is where we put all the code to create the application's user interface. The first step is creating a new Gtk.ApplicationWindow to put all our widgets into.
For this example, all that we have in the window the popup comes out of is a silly warning Label.
To make our MessageDialog a popup attached to the main window, we set its modal property to true and set it to be "transient_for" _window. After that, we can set what kind of buttons it has and what kind of message it is (which determines what icon appears next to the message), and write out the text inside it, before connecting its "response" signal to the callback function which handles it.
Here are some resources for making your own MessageDialogs:
List of button types
List of message types
This function takes two parameters, the MessageDialog and its response_id, both of which are automatically supplied (you don't have to manually pass them to it for it to work). Here we use a simple switch to change the "warning label"'s text, depending on which option you select. The DELETE_EVENT occurs if you press Escape to cancel the MessageDialog, instead of clicking OK or Cancel. Whatever you select, the popup is destroyed afterwards.
Finally, we create a new instance of the finished MessageDialogExample class, and set the application running.
In this sample we used the following:
GMenu
GSimpleAction
Gtk.Application
Gtk.ApplicationWindow
Gtk.MessageDialog