Add documentation for godot-cpp hook system

tutorials/scripting/cpp/build_system/cmake.rst

@@ -102,6 +102,12 @@ are the notable differences:
     will set this variable if it isn't already set. So, include it before other
     dependencies to have the value propagate across the projects.
 
+Additionally, the :ref:`hook system <doc_godot_cpp_modifying_generated_files>` works a
+bit differently with CMake:
+
+* Your subclass has to be named ``CustomBindingGeneratorHooks``.
+* Pass the filepath of the python file containing your subclass with the ``GODOTCPP_BINDING_HOOK_FILE`` option to CMake.
+
 Basic Walk-Through
 ------------------
 
@@ -179,7 +185,9 @@ See setting-build-variables_ and build-configurations_ for more information.
 A non-exhaustive list of options:
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-.. code-block:: text
+
+.. marked as glsl to highlight comments
+.. code-block:: glsl
 
     // Path to a custom GDExtension API JSON file.
     // (takes precedence over GODOTCPP_GDEXTENSION_DIR)
@@ -199,6 +207,10 @@ A non-exhaustive list of options:
     // Enable the extra accounting required to support hot reload. (ON|OFF)
     GODOTCPP_USE_HOT_RELOAD:BOOL=
 
+    // Use the binding generator's hook system to modify generated files.
+    // ( /path/to/custom_generator_file.py)
+    GODOTCPP_BINDING_HOOK_FILE:FILEPATH=
+
 Compiling
 ~~~~~~~~~
 

tutorials/scripting/cpp/build_system/scons.rst

@@ -107,3 +107,64 @@ the following line to your SConstruct file:
 .. code-block:: python
 
     localEnv["custom_api_file"] = "extension_api.json"
+
+.. _doc_godot_cpp_modifying_generated_files:
+
+Modifying generated files
+-------------------------
+
+If you have to modify the files generated by godot-cpp, its *hook system* is the right
+way of doing so. We provide the ``BindingGeneratorHooks`` class, located at ``godot-cpp/tools/binding_generator_hooks.py``.
+Your custom hooks are a python class that extends ``BindingGeneratorHooks`` and overrides
+(some of) its methods. After defining your subclass, you should export an instance of it
+from your ``SConstruct`` with the key ``binding_hooks`` to godot-cpp's ``SConstruct``.
+This way you let godot-cpp's ``SConstruct`` know about your class, see the example below for
+one way of doing this.
+
+Example
+~~~~~~~
+
+This example adds a string constant of every signal in a class to its class header.
+For example, in ``base_button.hpp`` we will add ``static constexpr char SIGNAL_PRESSED[] = "pressed";``
+for the ``pressed`` signal. We will use the ``SConstruct`` file from the `godot-cpp template <https://github.com/godotengine/godot-cpp-template>`__.
+We start by creating `custom_generator.py` at the root of our project. It only overrides
+``alter_engine_class_header``, as that is all we need.
+
+.. code-block:: python
+    :caption: custom_generator.py
+
+    import sys
+
+    sys.path.insert(0, "godot-cpp")
+    from tools.binding_generator_hooks import BindingGeneratorHooks
+
+    class CustomBindingGeneratorHooks(BindingGeneratorHooks):
+        def alter_engine_class_header(self, class_api, lines):
+            signals = []
+            if "signals" in class_api:
+                for signal_api in class_api["signals"]:
+                    name = signal_api["name"]
+                    signal_constant = "\tstatic constexpr char SIGNAL_" + name.upper() + '[] = "' + name + '";'
+                    signals.append(signal_constant)
+                try:
+                    idx = lines.index("public:") + 1
+                    for signal_const in signals:
+                        lines.insert(idx, signal_const)
+                        idx += 1
+                except ValueError:
+                    print("no public keyword found, not adding signals")
+            return lines
+
+Next, we import the class in our ``SConstruct`` file:
+
+.. code-block:: python
+
+    from custom_generator import CustomBindingGeneratorHooks
+
+Additionally, we edit the line that exports variables to ``godot-cpp/SConstruct`` to include an instance of our class, like so:
+
+.. code-block:: python
+
+   env = SConscript("godot-cpp/SConstruct", {"env": env, "customs": customs, "binding_hooks": CustomBindingGeneratorHooks()})
+
+This is everything you need, when you regenerate the files they should contain signal name constants for all signals in the API file.