Builder

A `GtkBuilder` reads XML descriptions of a user interface and instantiates the described objects.

To create a `GtkBuilder` from a user interface description, call [ctor@Gtk.Builder.new_from_file], [ctor@Gtk.Builder.new_from_resource] or [ ctor@Gtk.Builder.new_from_string].

In the (unusual) case that you want to add user interface descriptions from multiple sources to the same `GtkBuilder` you can call [ ctor@Gtk.Builder.new] to get an empty builder and populate it by (multiple) calls to [method@Gtk.Builder.add_from_file], [ method@Gtk.Builder.add_from_resource] or [method@Gtk.Builder.add_from_string].

A `GtkBuilder` holds a reference to all objects that it has constructed and drops these references when it is finalized. This finalization can cause the destruction of non-widget objects or widgets which are not contained in a toplevel window. For toplevel windows constructed by a builder, it is the responsibility of the user to call [method@Gtk.Window.destroy] to get rid of them and all the widgets they contain.

The functions [method@Gtk.Builder.get_object] and [method@Gtk.Builder.get_objects] can be used to access the widgets in the interface by the names assigned to them inside the UI description. Toplevel windows returned by these functions will stay around until the user explicitly destroys them with [method@Gtk.Window.destroy]. Other widgets will either be part of a larger hierarchy constructed by the builder (in which case you should not have to worry about their lifecycle), or without a parent, in which case they have to be added to some container to make use of them. Non-widget objects need to be reffed with @ref to keep them beyond the lifespan of the builder.

GtkBuilder UI Definitions

`GtkBuilder` parses textual descriptions of user interfaces which are specified in XML format. We refer to these descriptions as “GtkBuilder UI definitions” or just “UI definitions” if the context is clear.

### Structure of UI definitions

UI definition files are always encoded in UTF-8.

The toplevel element is `<interface>`. It optionally takes a “domain” attribute, which will make the builder look for translated strings using `dgettext()` in the domain specified. This can also be done by calling [method@Gtk.Builder.set_translation_domain] on the builder. For example:

```xml <?xml version="1.0" encoding="UTF-8"> <interface domain="your-app"> ... </interface> ```

### Requirements

The target toolkit version(s) are described by `<requires>` elements, the “lib” attribute specifies the widget library in question ( currently the only supported value is “gtk”) and the “version” attribute specifies the target version in the form “`<major>`.` <minor>`”. `GtkBuilder` will error out if the version requirements are not met. For example:

```xml <?xml version="1.0" encoding="UTF-8"> <interface domain="your-app"> <requires lib="gtk" version="4.0" /> < /interface> ```

### Objects

Objects are defined as children of the `<interface>` element.

Objects are described by `<object>` elements, which can contain `<property>` elements to set properties, `<signal>` elements which connect signals to handlers, and `<child>` elements, which describe child objects (most often widgets inside a container, but also e.g. actions in an action group, or columns in a tree model). A `<child>` element contains an `<object>` element which describes the child object.

Typically, the specific kind of object represented by an `<object>` element is specified by the “class” attribute. If the type has not been loaded yet, GTK tries to find the `get_type()` function from the class name by applying heuristics. This works in most cases, but if necessary, it is possible to specify the name of the `get_type()` function explicitly with the "type-func" attribute. If your UI definition is referencing internal types, you should make sure to call `g_type_ensure()` for each object type before parsing the UI definition.

Objects may be given a name with the “id” attribute, which allows the application to retrieve them from the builder with [ method@Gtk.Builder.get_object]. An id is also necessary to use the object as property value in other parts of the UI definition. GTK reserves ids starting and ending with `___` (three consecutive underscores) for its own purposes.

### Properties

Setting properties of objects is pretty straightforward with the `<property>` element: the “name” attribute specifies the name of the property, and the content of the element specifies the value:

```xml <object class="GtkButton"> <property name="label">Hello, world</property> </object> ```

If the “translatable” attribute is set to a true value, GTK uses `gettext()` (or `dgettext()` if the builder has a translation domain set) to find a translation for the value. This happens before the value is parsed, so it can be used for properties of any type, but it is probably most useful for string properties. It is also possible to specify a context to disambiguate short strings, and comments which may help the translators:

```xml <object class="GtkButton"> <property name="label" translatable="yes" context="button">Hello, world</property> < /object> ```

`GtkBuilder` can parse textual representations for the most common property types:

• characters
• strings
• integers
• floating-point numbers
• booleans (strings like “TRUE”, “t”, “yes”, “y”, “1” are interpreted as true values, strings like “FALSE”, “f”, “no”, “n”, “0” are interpreted as false values)
• enumeration types (can be specified by their full C identifier their short name used when registering the enumeration type, or their integer value)
• flag types (can be specified by their C identifier, short name, integer value, and optionally combined with “|” for bitwise OR, e.g. “GTK_INPUT_HINT_EMOJI|GTK_INPUT_HINT_LOWERCASE”, or “emoji|lowercase”)
• colors (in a format understood by [method@Gdk.RGBA.parse])
• `GVariant` (can be specified in the format understood by [func@GLib.Variant.parse])
• pixbufs (can be specified as a filename of an image file to load)

Objects can be referred to by their name and by default refer to objects declared in the local XML fragment and objects exposed via [ method@Gtk.Builder.expose_object]. In general, `GtkBuilder` allows forward references to objects declared in the local XML; an object doesn’t have to be constructed before it can be referred to. The exception to this rule is that an object has to be constructed before it can be used as the value of a construct-only property.

### Property bindings

It is also possible to bind a property value to another object's property value using the attributes "bind-source" to specify the source object of the binding, and optionally, "bind-property" and "bind-flags" to specify the source property and source binding flags respectively. Internally, `GtkBuilder` implements this using [class@GObject.Binding] objects.

For instance, in the example below the “label” property of the `bottom_label` widget is bound to the “label” property of the `top_button` widget:

```xml <object class="GtkBox"> <property name="orientation">vertical</property> <child> <object class="GtkButton" id="top_button"> <property name="label">Hello, world</property> </object> </child> <child> <object class="GtkLabel" id="bottom_label"> <property name="label" bind-source="top_button" bind-property="label" bind-flags="sync-create" / > </object> </child> </object> ```

For more information, see the documentation of the [method@GObject.Object.bind_property] method.

### Internal children

Sometimes it is necessary to refer to widgets which have implicitly been constructed by GTK as part of a composite widget, to set properties on them or to add further children (e.g. the content area of a `GtkDialog`). This can be achieved by setting the “internal-child” property of the `<child>` element to a true value. Note that `GtkBuilder` still requires an `<object>` element for the internal child, even if it has already been constructed.

### Specialized children

A number of widgets have different places where a child can be added (e.g. tabs vs. page content in notebooks). This can be reflected in a UI definition by specifying the “type” attribute on a `<child>` The possible values for the “type” attribute are described in the sections describing the widget-specific portions of UI definitions.

### Signal handlers and function pointers

Signal handlers are set up with the `<signal>` element. The “name” attribute specifies the name of the signal, and the “handler” attribute specifies the function to connect to the signal.

```xml <object class="GtkButton" id="hello_button"> <signal name="clicked" handler="hello_button__clicked" /> </object> ```

The remaining attributes, “after”, “swapped” and “object”, have the same meaning as the corresponding parameters of the [ func@GObject.signal_connect_object] or [func@GObject.signal_connect_data] functions:

• “after” matches the `G_CONNECT_AFTER` flag, and will ensure that the handler is called after the default class closure for the signal
• “swapped” matches the `G_CONNECT_SWAPPED` flag, and will swap the instance and closure arguments when invoking the signal handler
• “object” will bind the signal handler to the lifetime of the object referenced by the attribute

By default "swapped" will be set to "yes" if not specified otherwise, in the case where "object" is set, for convenience. A “last_modification_time” attribute is also allowed, but it does not have a meaning to the builder.

When compiling applications for Windows, you must declare signal callbacks with the `G_MODULE_EXPORT` decorator, or they will not be put in the symbol table:

```c G_MODULE_EXPORT void hello_button__clicked (GtkButton *button, gpointer data) { // ... } ```

On Linux and Unix, this is not necessary; applications should instead be compiled with the `-Wl,--export-dynamic` argument inside their compiler flags, and linked against `gmodule-export-2.0`.

Example UI Definition

```xml <interface> <object class="GtkDialog" id="dialog1"> <child internal-child="content_area"> <object class="GtkBox" > <child internal-child="action_area"> <object class="GtkBox"> <child> <object class="GtkButton" id="ok_button"> <property name="label" translatable="yes">_Ok</property> <property name="use-underline">True</property> <signal name="clicked" handler="ok_button_clicked"/> </object> </child> </object> </child> </object> </child > </object> </interface> ```

Using GtkBuildable for extending UI definitions

Objects can implement the [iface@Gtk.Buildable] interface to add custom elements and attributes to the XML. Typically, any extension will be documented in each type that implements the interface.

Templates

When describing a [class@Gtk.Widget], you can use the `<template>` tag to describe a UI bound to a specific widget type. GTK will automatically load the UI definition when instantiating the type, and bind children and signal handlers to instance fields and function symbols.

For more information, see the [`GtkWidget` documentation](class.Widget.html#building-composite-widgets-from-template-xml) for details.