Migrating from GTK+ 3.x to GTK+ 4

Preparation in GTK+ 3.x
Do not use deprecated symbols
Enable diagnostic warnings
Do not use widget style properties
Review your window creation flags
Stop using non-RGBA visuals
Stop using GtkBox:padding, GtkBox:fill and GtkBox:expand
Stop using the state argument of GtkStyleContext getters
Stop using gdk_pixbuf_get_from_window() and gdk_cairo_set_source_surface()
Stop using GtkButton's image-related API
Stop using GtkWidget event signals
Set a proper app_id
Changes that need to be done at the time of the switch
Stop using GdkScreen
Stop using the root window
Stop using GdkVisual
Stop using GdkDeviceManager
Adapt to GdkWindow API changes
Stop accessing GdkEvent fields
Stop using gdk_surface_set_event_compression
Adapt to GdkKeymap API changes
Stop using GtkEventBox
Adapt to GtkHeaderBar API changes
Adapt to GtkStyleContext API changes
Adapt to GtkCssProvider API changes
Stop using GtkContainer::border-width
Adapt to GtkWidget's size request changes
Adapt to GtkWidget's size allocation changes
Switch to GtkWidget's children APIs
Don't use -gtk-gradient in your CSS
Don't use -gtk-icon-effect in your CSS
Use gtk_widget_measure
Adapt to drawing model changes
Stop using APIs to query GdkSurfaces
Widgets are now visible by default
Adapt to changes in animated hiding and showing of widgets
Stop passing commandline arguments to gtk_init
GdkPixbuf is deemphasized
GtkWidget event signals are removed
Invalidation handling has changed
Stop using GtkWidget::draw
Window content observation has changed
The gtk_window_fullscreen_on_monitor API has changed
Adapt to cursor API changes
Adapt to icon size API changes

GTK+ 4 is a major new version of GTK+ that breaks both API and ABI compared to GTK+ 3.x. Thankfully, most of the changes are not hard to adapt to and there are a number of steps that you can take to prepare your GTK+ 3.x application for the switch to GTK+ 4. After that, there's a small number of adjustments that you may have to do when you actually switch your application to build against GTK+ 4.

Preparation in GTK+ 3.x

The steps outlined in the following sections assume that your application is working with GTK+ 3.24, which is the final stable release of GTK+ 3.x. It includes all the necessary APIs and tools to help you port your application to GTK+ 4. If you are still using an older version of GTK+ 3.x, you should first get your application to build and work with the latest minor release in the 3.24 series.

Do not use deprecated symbols

Over the years, a number of functions, and in some cases, entire widgets have been deprecated. These deprecations are clearly spelled out in the API reference, with hints about the recommended replacements. The API reference for GTK+ 3 also includes an index of all deprecated symbols.

To verify that your program does not use any deprecated symbols, you can use defines to remove deprecated symbols from the header files, as follows:

        make CFLAGS+="-DGDK_DISABLE_DEPRECATED -DGTK_DISABLE_DEPRECATED"
        

Note that some parts of our API, such as enumeration values, are not well covered by the deprecation warnings. In most cases, using them will require you to also use deprecated functions, which will trigger warnings.

Enable diagnostic warnings

Deprecations of properties and signals cannot be caught at compile time, as both properties and signals are installed and used after types have been instantiated. In order to catch deprecations and changes in the run time components, you should use the G_ENABLE_DIAGNOSTIC environment variable when running your application, e.g.:

          G_ENABLE_DIAGNOSTIC=1 ./your-app
        

Do not use widget style properties

Style properties do not exist in GTK+ 4. You should stop using them in your custom CSS and in your code.

Review your window creation flags

GTK+ 4 removes the GDK_WA_CURSOR flag. Instead, just use gdk_window_set_cursor() to set a cursor on the window after creating it.

GTK+ 4 also removes the GDK_WA_VISUAL flag, and always uses an RGBA visual for windows. To prepare your code for this, use gdk_window_set_visual (gdk_screen_get_rgba_visual()) after creating your window.

GTK+ 4 also removes the GDK_WA_WMCLASS flag. If you need this X11-specific functionality, use XSetClassHint() directly.

Stop using non-RGBA visuals

GTK+ 4 always uses RGBA visuals for its windows; you should make sure that your code works with that.

At the same time, you should stop using GdkVisual APIs, this object not longer exist in GTK+ 4. Most of its APIs are deprecated already and not useful when dealing with RGBA visuals.

Stop using GtkBox:padding, GtkBox:fill and GtkBox:expand

GTK+4 removes these GtkBox child properties, so you should not use them. You can replace GtkBox:padding using the “margin” properties on your GtkBox child widgets.

The fill child property can be replaced by setting appropriate values for the “halign” and “valign” properties of the child widgets. If you previously set the fill child property to TRUE, you can achieve the same effect by setting the halign or valign properties to GTK_ALIGN_FILL, depending on the parent box -- halign for a horizontal box, valign for a vertical one.

GtkBox also uses the expand child property. It can be replaced by setting “hexpand” or “vexpand” on the child widgets. To match the old behavior of the GtkBox's expand child property, you need to set “hexpand” on the child widgets of a horizontal GtkBox and “vexpand” on the child widgets of a vertical GtkBox.

Note that there's a subtle but important difference between GtkBox's expand and fill child properties and the ones in GtkWidget: setting “hexpand” or “vexpand” to TRUE will propagate up the widget hierarchy, so a pixel-perfect port might require you to reset the expansion flags to FALSE in a parent widget higher up the hierarchy.

Stop using the state argument of GtkStyleContext getters

The getters in the GtkStyleContext API, such as gtk_style_context_get_property(), gtk_style_context_get(), or gtk_style_context_get_color() only accept the context's current state for their state argument. You should update all callers to pass the current state.

Stop using gdk_pixbuf_get_from_window() and gdk_cairo_set_source_surface()

These functions are not supported in GTK+ 4. Instead, either use backend-specific APIs, or render your widgets using gtk_widget_render().

Stop using GtkButton's image-related API

The functions and properties related to automatically add a GtkImage to a GtkButton, and using a GtkSetting to control its visibility, are not supported in GTK+ 4. Instead, you can just pack a GtkImage inside a GtkButton, and control its visibility like you would for any other widget. If you only want to add a named icon to a GtkButton, you can use gtk_button_set_icon_name().

Stop using GtkWidget event signals

Event controllers and GtkGestures replace event signals in GTK+ 4. They have been backported to GTK+ 3.x so you can prepare for this change.

Set a proper app_id

In GTK+4 we want the application's GApplication 'application-id' (and therefore the D-Bus name), the desktop file basename and Wayland's xdg-shell app_id to match. In order to achieve this with GTK+3 call g_set_prgname() with the same application id you passed to GtkApplication. Rename your desktop files to match the application id if needed.

The call to g_set_prgname() can be removed once you fully migrated to GTK+4.