[pygobject] docs: document Gtk.Template. Fixes #396

[Christoph Reiter <creiter src gnome org>]

commit a2bd7eb59b8a5963d27d58d58380c419ee00a472
Author: Chris Mayo <aklhfex gmail com>
Date:   Wed Feb 10 19:01:40 2021 +0000

    docs: document Gtk.Template. Fixes #396
    
    Examples derived from tests/test_gtk_template.py.

 docs/guide/gtk_template.rst | 90 +++++++++++++++++++++++++++++++++++++++++++++
 docs/guide/index.rst        |  1 +
 2 files changed, 91 insertions(+)
---
diff --git a/docs/guide/gtk_template.rst b/docs/guide/gtk_template.rst
new file mode 100644
index 00000000..1e80fddf
--- /dev/null
+++ b/docs/guide/gtk_template.rst
@@ -0,0 +1,90 @@
+============
+Gtk.Template
+============
+
+A GtkWidget subclass can use a
+`GtkBuilder UI Definition <https://developer.gnome.org/gtk3/stable/GtkBuilder.html#BUILDER-UI>`__
+XML document as a template to create child widgets and set its own
+properties, without creating a GtkBuilder instance. This is implemented
+for Python by PyGObject with Gtk.Template.
+
+The subclass uses a ``@Gtk.Template`` decorator and declares a class
+variable ``__gtype_name__`` with the value of the XML ``template``
+element ``class`` attribute.
+
+Child widgets are declared, typically with the same names as the XML
+``object`` element ``id`` attributes, at the class level as instances
+of ``Gtk.Template.Child``.
+
+Signal handler methods, typically with the same names as the XML ``signal``
+element ``handler`` attributes, use the ``@Gtk.Template.Callback`` decorator.
+
+``Gtk.Template()`` takes a mandatory keyword argument passing the XML document
+or its location, either ``string``, ``filename`` or ``resource_path``.
+
+``Gtk.Template.Child()`` and ``Gtk.Template.Callback()`` optionally take
+a ``name`` argument matching the value of the respective XML attribute,
+in which case the Python attribute can have a different name.
+
+Examples
+--------
+
+.. code-block:: python
+
+    xml = """\
+    <interface>
+      <template class="example1" parent="GtkBox">
+        <child>
+          <object class="GtkButton" id="hello_button">
+            <property name="label">Hello World</property>
+            <signal name="clicked" handler="hello_button_clicked" swapped="no" />
+          </object>
+        </child>
+      </template>
+    </interface>
+    """
+
+    @Gtk.Template(string=xml)
+    class Foo(Gtk.Box):
+        __gtype_name__ = "example1"
+
+        hello_button = Gtk.Template.Child()
+
+        @Gtk.Template.Callback()
+        def hello_button_clicked(self, *args):
+            pass
+
+Python attribute names that are different to the XML values:
+
+.. code-block:: python
+
+    @Gtk.Template(string=xml)
+    class Foo(Gtk.Box):
+        __gtype_name__ = "example1"
+
+        my_button = Gtk.Template.Child("hello_button")
+
+        @Gtk.Template.Callback("hello_button_clicked")
+        def bar(self, *args):
+            pass
+
+
+To add widgets to the built-in child of a parent, describe the built-in widget
+in the XML with its ``child`` element having an ``internal-child`` attribute set
+to the name of the built-in widget:
+
+.. code-block:: XML
+
+    <interface>
+      <template class="example2" parent="GtkDialog">
+        <child internal-child="vbox">
+          <object class="GtkBox">
+            <child>
+              <object class="GtkButton" id="hello_button">
+                <property name="label">Hello World</property>
+              </object>
+            </child>
+          </object>
+        </child>
+      </template>
+    </interface>
diff --git a/docs/guide/index.rst b/docs/guide/index.rst
index ac966d7e..e4dba05f 100644
--- a/docs/guide/index.rst
+++ b/docs/guide/index.rst
@@ -9,6 +9,7 @@ User Guide
 
     api/index
     cairo_integration
+    gtk_template
     threading
     debug_profile
     deploy