Differences between Gtk.jl and Gtk4.jl
Gtk4.jl builds on and is very similar to Gtk.jl. Here is a summary of what's different.
Properties
GObject properties can still be set and accessed using get_gtk_property and set_gtk_property!. However, properties are now mapped onto Julia properties, so one can set a window title using win.title = "My title". Also, the submodule GAccessor no longer exists. In Gtk4.jl, getter and setter methods are in the main Gtk4 module, but most are not exported. Whereas in Gtk.jl one uses GAccessor.title(win, "My title") to set the title, in Gtk4.jl the equivalent is Gtk4.title(win, "My title").
Constants, enums, and flags
GTK constants in Gtk4.jl are in the main module instead of a Constants submodule.
In Gtk.jl, GTK's enum and flags constants are turned into integers. In Gtk4.jl, these are now mapped onto Julia enums, specifically the implementations CEnum.jl for enums and BitFlags.jl for flags. This improves understandability when a function returns an enum or flag, but the downside is the sometimes extreme length of the enum's name. To mitigate this, convert methods are defined for commonly used enums so that shorter symbols can be used instead of the full enum name. For example, :h can be used instead of Gtk4.Orientation_HORIZONTAL in GtkBox(orientation, spacing).
G_ contains automatically generated methods
In Gtk.jl, the submodule Gtk.GAccessor contains getter and setter methods, which often correspond to object properties. In Gtk4.jl, the submodule Gtk4.G_ contains automatically generated methods, which include all methods in GAccessor and many more. These methods directly call the C functions in libgtk and thus use 0-based indexing. Where possible, they translate between Julia types and C types, for example converting nothing to C_NULL and vice versa.
For consistency, the getter and setter methods in G_ keep their full names, including "set" and "get". For example, to set the title of a window in Gtk4.jl use G_.set_title(w, "text") rather than GAccessor.title(w, "text") as in Gtk.jl.
GObject and struct names
The equivalent of Gtk.ShortNames doesn't exist. All GObject types are mapped onto Julia types with the same name. Leaving out the namespace, as is done in the Python pygobject bindings, would have led to name collisions between Gtk types and Julia types or between Gtk and other GObject libraries.
No showall
In GTK 4, widgets are shown by default, so showall does not exist, and calling show is no longer necessary in most situations. Exceptions include GtkDialogs and GtkApplicationWindows.
No GtkContainer
In GTK 4, GtkContainer has been removed and most widgets derive directly from GtkWidget. Each class that can contain child widgets has its own functions for adding and/or removing them. In Gtk4.jl, collection interface methods like push! have been defined for containers that hold many widgets, such as GtkBox. For widgets that have one child, such as GtkWindow, getindex and setindex! have been defined, so that one can set a child widget using window[] = child.
Events
Events such as button presses are handled through "event controllers" in GTK 4.
Dialogs
Dialogs no longer have a run method that takes over the GLib main loop while waiting for the user's response.
For developers
All uses of mutable from GLib.MutableTypes should be replaced by Julia's Ref. The type of a GValue can be set using settype! rather than setindex!.
More information
The GTK docs have a migration guide with detailed recommendations for migrating C code from GTK version 3 to version 4. Much of that advice applies to Julia code.