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Start a migration guide for GTK 5

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No need to panic, GTK 5 is still years away!
But it is good to write this material down while
it is fresh in mind.
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Matthias Clasen 2022-10-12 16:38:18 -04:00
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Title: Preparing for GTK 5
Slug: gtk-migrating-4-to-5
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 kepy 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
[method@Gtk.Widget.get_style_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.