Migrating from GTK+ 3.x to GTK+ 4: GTK+ 4 Reference Manual

## Migrating from GTK+ 3.x to GTK+ 4

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+ 3.22, 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 3.22.

### 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.

### Do not use widget style properties

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

### 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.

### 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 vertial 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 migth require a reset of 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. Update all callers to pass the current state.

### Stop using `gdk_pixbuf_get_from_window()` and `gdk_cairo_set_source_window()`

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