In this tutorial we will see how to use the advanced subclassing api available in the Unreal Engine 4 Python plugin, as well as how to build Slate GUIs

This tutorial has been tested with Unreal Engine 4.16 and Python 3.6 and assumes you are already familiar with installing the plugin into the engine and editing Python scripts.

If this is your first experience with the Unreal Engine Python plugin, this link is a good place to start: https://github.com/20tab/UnrealEnginePython/blob/master/tutorials/PlottingGraphsWithMatplotlibAndUnrealEnginePython.md#installing-the-python-plugin

The objective is to instruct the editor to import .dae (collada) files as static meshes, by building a new Factory. A Factory is a special class in Unreal Engine 4 allowing to create and import assets. There are factories for fbx's, wav's, images and all of the media files supported by UE4.

You generally create new factories by subclassing the UFactory class in C++, but in this tutorial we will see how to subclass UE4 C++ classes in pure python.

The subclassing api allows the developer to subclass any UClass (classes exposed to the UE4 reflection system). Unfortunately the UFactory class does not expose its methods to the reflection system, so an handy class named 'UPyFactory' (it ia simple subclass of UFactory) is exposed by the python plugin:

from unreal_engine.classes import PyFactory

import unreal_engine as ue

class ColladaFactory(PyFactory):

    def __init__(self):
        ue.log_error('Hello World, i am a useless factory')

save the file as collada_factory.py into the /Content/Scripts directory and run it rom the Unreal Engine Python Console:

Note that whenever you re-run the scripts, the internal Unreal Engine reflection system is updated, so you do not need to restart the editor (or launch slow compilations) when you make changes to the script code:

as you can see from the screenshot, the address of the UClass as well as its python UObject counterpart remains the same, while the __init__ call is updated. Remember that whenever you create a new UClass from python, the __init__ method is automatically called by the ClassDefaultObject (this information is useful only if you are into unreal engine internals, otherwise you can simply ignore it ;).

The collada format is based on XML, so we can easily parse it from python, but instead of doing the hard work by hand, we can use the amazing pycollada module (https://github.com/pycollada/pycollada). Just pip install it and update your class to import and use it:

from unreal_engine.classes import PyFactory, StaticMesh, Object, Class

import unreal_engine as ue

from collada import Collada

class ColladaFactory(PyFactory):

    def __init__(self):
        # inform the editor that this class is able to import assets
        self.bEditorImport = True
        # register the .dae extension as supported
        self.Formats = ['dae;Collada']
        # set the UClass this UFactory will generate
        self.SupportedClass = StaticMesh

    def PyFactoryCreateFile(self, uclass: Class, parent: Object, name: str, filename: str) -> Object:
        # load the collada file
        dae = Collada(filename)
        ue.log_warning(dae)
        # create a new UStaticMesh with the specified name and parent
        static_mesh = StaticMesh(name, parent)
        return static_mesh

running this new script will show that the ColladaFactory got a new method:

PyFactoryCreateFile, is the virtual C++ method exposed by UPyFactory and that we are overriding via python. Note that we need to use python3 annotations to inform unreal engine (that is obviously strongly typed) about the type of the functions. PyFactoryCreateFile is called by the editor whenever we try to import a file with the ColladaFactory

Now download the example .dae file from here:

https://github.com/20tab/UnrealEnginePython/raw/master/tutorials/WritingAColladaFactoryWithPython_Assets/duck_triangles.dae

(it is a simple duck)

once you re-run the collada_factory.py script you will be able to click on 'Import' in the Countent Browser and now you the 'dae' extension will show in the list of supported ones. Select the duck file and just look at the python console:

obviously we still have not added data to our StaticMesh, so in our Content Browser an empty Static Mesh will appear: