By Arjan Molenaar (2022-12-10)
GTK is a cross-platform toolkit. It’s a key component for the GNOME desktop, GIMP, Inkscape, and many other high quality open source desktop applications. GTK 4 is the most recent major version, the long awaited successor of GTK+ 3. For desktop applications there’s a lot to gain by porting to GTK 4. First and foremost because GTK 4 takes full support of your GPU.
For Gaphor we provide binaries for Linux, Windows and macOS.
GTK 4 no longer maps its keyboard shortcuts to macOS native keybindings. While Ctrl is used on Linux and Windows, Command is used on macOS as primary modifier key. Although that’s inconvenient, it’s not as bad as it sounds.
Let’s take a step back. Why should we even care about macOS as a target platform?
For our project it is a must to run on all platforms. Gaphor is a tool that requires collaboration, and we do not want to leave out Windows and macOS users. After all, macOS is a popular platform for software development.
We want to let as many people as possible use and experience open source software. Therefore, we can’t simply build an application only for Linux: it would exclude a lot of potential users. By bringing the software to their platform, they can experience what it is to use open source software1.
For Gaphor there are many benefits upgrading the macOS binary to GTK 4: the responsiveness of the application improves a lot. A whole lot. GTK 4 runs as fast as a native macOS application.
For shortcuts that we define in our application, we’re in control.
In GTK the Command key is represented by the Meta keyboard modifier.
GTK+ 3 has a keyboard modifier called <Primary>. It maps to Command
on macOS and Ctrl on Linux and Windows. In GTK 4, <Primary> is an
alias for <Control>. There’s good reason the GTK developers want to drop the
Primary key modifier: the translation from Linux to macOS key bindings is not
as simple as switching Ctrl for Command. For example:
where Linux uses Home to move the cursor to the beginning of a line,
macOS uses Command+←.
There are still a lot of common shortcuts, for example for saving
(Ctrl+s) and opening files (Ctrl+o).
For those cases we can simply reinstate the old Primary behavior:
def platform_specific_modifier(shortcut):
"""Shortcuts are written like `<Primary>s`.
"""
return shortcut.replace(
"<Primary>",
"<Meta>" if sys.platform == "darwin" else "<Control>"
)
def new_shortcut(shortcut, detailed_name):
return Gtk.Shortcut.new(
trigger=Gtk.ShortcutTrigger.parse_string(platform_specific_modifier(shortcut)),
action=Gtk.NamedAction.new(detailed_name),
)
...
new_shortcut("<Primary>s", "win.save-file")
For built-in widget, we have to use a different strategy. The keyboard shortcuts are defined on a class level. GTK’s own classes. Subclassing does not help us.
We do not try to map all shortcuts, only the important ones: copy/paste, undo/redo, cursor navigation.
Thanks to the modular design of GTK 4, only two widgets need some extra shortcuts: Gtk.Text and Gtk.TextView. The existing shortcuts are not in the way, so they can remain as is.
Although it’s recommended to add keyboard shortcuts during class construction, you can also add shortcuts on class level after the class has been created.
def new_shortcut_with_args(shortcut, signal, *args):
shortcut = Gtk.Shortcut.new(
trigger=Gtk.ShortcutTrigger.parse_string(shortcut),
action=Gtk.SignalAction.new(signal),
)
if args:
shortcut.set_arguments(GLib.Variant.new_tuple(*args))
return shortcut
Gtk.TextView.add_shortcut(
new_shortcut_with_args(
"<Meta>a", "select-all",
GLib.Variant.new_boolean(True)
)
)
To find the right shortcuts to map, I had a look at the GTK source code and the documentation on Mac keyboard shortcuts. The translation to macOS shortcuts is really straight forward.
If you want to know how it’s implemented in Gaphor, have a look at our macOS shim module.
When you port an application to another platform, there is always a bit of custom code involved. That’s not a problem. It’s a chance to tailor the user experience to the target platform. Providing a good user experience is crucial to attract and retain users.