GTK 5 will be a major new version of GTK that breaks both API and ABI compared to GTK 4.x. GTK 5 does not exist yet, so we cannot be entirely sure what will be involved in a migration from GTK 4 to GTK 5. But we can already give some preliminary hints about the likely changes, and how to prepare for them in code that is using GTK 4.

Do not use deprecated symbols

As always, functions and types that are known to go away in the next major version of GTK are being marked as deprecated in GTK 4.

Removing the use of deprecated APIs is the most important step to prepare your code for the next major version of GTK. Often, deprecation notes will include hints about replacement APIs to help you with this.

Sometimes, it is helpful to have some background information about the motivation and goals of larger API changes.

Cell renderers are going away

Cell renderers were introduced in GTK 2 to support rendering of “big data” UIs, in particular treeviews. Over the years, more “data-like” widgets have started to use them, and cell renderers have grown into a shadowy, alternative rendering infrastructure that duplicates much of what widgets do, while duplicating the code and adding their own dose of bugs.

In GTK 4, replacement widgets for GtkTreeView, GtkIconView and GtkComboBox have appeared: GtkListView, GtkColumnView, GtkGridView and GtkDropDown. For GTK 5, we will take the next step and remove all cell renderer-based widgets.

Themed rendering APIs are going away

The old GTK 2 era rendering APIs for theme components like gtk_render_frame() or gtk_render_check() have not been used by GTK itself even in later GTK 3, but they have been kept around for the benefit of “external drawing” users - applications that want their controls to look like GTK without using widgets.

Supporting this is increasingly getting in the way of making the GTK CSS machinery fast and correct. One notable problem is that temporary style changes (using gtk_style_context_save()) is breaking animations. Therefore, these APIs will be going away in GTK 5, together with their more modern GtkSnapshot variants like gtk_snapshot_render_background() or gtk_snapshot_render_focus().

The best way to render parts of your widget using CSS styling is to use subwidgets. For example, to show a piece of text with fonts, effects and shadows according to the current CSS style, use a GtkLabel.

If you have a need for custom drawing that fits into the current (dark or light) theme, e.g. for rendering a graph, you can still get the current style foreground color, using gtk_widget_get_color().

Local stylesheets are going away

The cascading part of GTK’s CSS implementation is complicated by the existence of local stylesheets (i.e. those added with gtk_style_context_add_provider()). And local stylesheets are unintuitive in that they do not apply to the whole subtree of widgets, but just to the one widget where the stylesheet was added.

GTK 5 will no longer provide this functionality. The recommendations is to use a global stylesheet (i.e. gtk_style_context_add_provider_for_display()) and rely on style classes to make your CSS apply only where desired.

Non-standard CSS extensions are going away

GTK’s CSS machinery has a some non-standard extensions around colors: named colors with `define`-color and color functions: lighter(), darker(), shade(), alpha(), mix().

GTK now implements equivalent functionality from the CSS specs.

`define`-color is going away

`define`-color should be replaced by custom properties in the :root scope.

Instead of

@define-color fg_color #2e3436

...

box {
  color: @fg_color;
}

use

:root {
  --fg-color: #2e3436;
}

...

box {
  color: var(--fg-color);
}

For more information about custom CSS properties and variables, see the CSS Custom Properties for Cascading Variables spec.

Color expressions are going away

The color functions can all be replaced by combinations of calc() and color-mix().

ligher(c) and darker(c) are just shade(c, 1.3) or shade(c, 0.7), respectively, and thus can be handled the same way as shade in the examples below.

Replace

a {
  color: mix(red, green, 0.8);
}

b {
  color: alpha(green, 0.6);
}

c {
  color: shade(red, 1.3);
}

d {
  color: shade(red, 0.7);
}

with

a {
  color: color-mix(in srgb, red, green 80%);
}

b {
  color: rgb(from green, r g b / calc(alpha * 0.6));
}

c {
  color: hsl(from red, h calc(s * 1.3) calc(l * 1.3));
}

d {
  color: hsl(from red, h calc(s * 0.7) calc(l * 0.7));
}

Variations of these replacements are possible.

Note that GTK has historically computed mix() and shade() values in the SRGB and HSL colorspaces, but using OKLAB instead might yield slightly better results.

For more information about color-mix(), see the CSS Color spec.

Chooser interfaces are going away

The GtkColorChooser, GtkFontChooser, GtkFileChooser and GtkAppChooser interfaces and their implementations as dialogs, buttons and widgets are phased out. The are being replaced by a new family of async APIs that will be more convenient to use from language bindings, in particular for languages that have concepts like promises. The new APIs are GtkColorDialog, GtkFontDialog and GtkFileDialog, There are also equivalents for some of the ‘button’ widgets: GtkColorDialogButton, GtkFontDialogButton.

GtkMessageDialog is going away

Like the Chooser interfaces, GtkMessageDialog has been replaced by a new async API that will be more convenient, in particular for language binding. The new API is GtkAlertDialog.

GtkDialog is going away

After gtk_dialog_run() was removed, the usefulness of GtkDialog is much reduced, and it has awkward, archaice APIs. Therefore, it is dropped. The recommended replacement is to just create your own window and add buttons as required, either in the header or elsewhere.

GtkInfoBar is going away

GtkInfoBar had a dialog API, and with dialogs going away, it was time to retire it. If you need such a widget, it is relatively trivial to create one using a GtkRevealer with labels and buttons.

Other libraries, such as libadwaita, may provide replacements as well.

gtk_show_uri is being replaced

Instead of gtk_show_uri(), you should use GtkUriLauncher or GtkFileLauncher.

GtkStatusbar is going away

This is an oldfashioned widget that does not do all that much anymore, since it no longer has a resize handle for the window.

GtkLockButton and GtkVolumeButton are going away

These are very specialized widgets that should better live with the application where they are used.

Widget size api changes

The functions gtk_widget_get_allocated_width() and gtk_widget_get_allocated_height() are going away. In most cases, gtk_widget_get_width() and gtk_widget_get_height() are suitable replacements. Note that the semantics are slightly different though: the old functions return the size of the CSS border area, while the new functions return the size of the widgets content area. In places where this difference matters, you can use gtk_widget_compute_bounds (widget, widget, &bounds) instead.

The function gtk_widget_get_allocation() is also going away. It does not have a direct replacement, but the previously mentioned alternatives can be used for it too.

The function gtk_widget_get_allocated_baseline() has been renamed to gtk_widget_get_baseline().

Stop using GdkPixbuf

GTK is moving away from GdkPixbuf as the primary API for transporting image data, in favor of GdkTexture. APIs that are accepting or returning GdkPixbufs are being replaced by equivalent APIs using GdkTexture or GdkPaintable objects.