UE4 Plugins

Following on from the previous post on UE4 modules, I wanted to briefly cover plugins as another form of encapsulation available in UE4.

Plugins vs modules

In UE4, plugins are simply a higher level unit of encapsulation than modules, rather than an alternative approach. Like projects, plugins can contain any number of code modules within them. They can also optionally contain content.

UE4 plugins are not strictly plugins

UE4's concept of a plugin deviates a little from the norm. A true plugin conforms to and implements a pre-existing interface, thereby extending an application in a predefined way. While UE4 plugins can take this approach, they can also export new types and be used as libraries, with project code having a direct dependency on the plugin. See this excellent AnswerHub post for an overview of this distinction.

Another way in which the UE4 approach differs is that, by default at least, its plugins are linked and their content processed when packaging a release build of a project. The underlying architecture may support delaying this process and loading them dynamically as true plugins (I'd be interested to know if anyone has tried), however it certainly doesn't seem to be the intended usage.

Engine level vs project level plugins

Plugins can exist at either the engine level ([UE4 Installation]/Engine/Plugins), or the project level ([Project Folder]/Plugins). In a launcher installation, plugins at the engine level must be prebuilt. Project level plugins containing source code will always be compiled as part of the project build process, regardless of engine installation type.

Creating a plugin

While the editor has a plugin wizard to automate the process, I'm going to go over the manual approach to detail all the elements of a UE4 plugin. In the following example, the plugin is named 'MyPlugin'.

Here is the basic structure of a plugin. The following would be placed in either /Engine/Plugins or [Project Folder]/Plugins.

- MyPlugin
    - MyPlugin.uplugin
    - Source [Optional]
        [C++ modules here]
    - Content [Optional]
    - Config [Optional]
        - FilterPlugin.ini

The plugin descriptor

The plugin descriptor file lives in the root folder of the plugin, and is named [PluginName].uplugin. In this example, 'MyPlugin.uplugin'. Here is a basic example.

{
    "FileVersion": 3,
    "Version": 1,
    "VersionName": "1.0",
    "FriendlyName": "My Plugin",
    "Description": "An example plugin.",
    "Category": "Kantan Dev",
    "Modules": [
        {
            "Name": "MyPluginMainModule",
            "Type": "Runtime",
            "WhitelistPlatforms": [ "Win64", "Win32", "Android", "Mac", "IOS", "Linux" ]
        },
        {
            "Name": "MyPluginEditorModule",
            "Type": "Editor",
            "WhitelistPlatforms": [ "Win64", "Mac", "Linux" ]
        }
    ],
    "EnabledByDefault": false,
    "CanContainContent": true
}

See the official documentation for more details.

Plugin source

The Source folder contains C++ module code, in exactly the same structure as for a project's Source folder. See my earlier post for a rundown of modules.

In the above example, there would be subfolders for two modules, MyPluginMainModule and MyPluginEditorModule.

Plugin content

If you want to include content in your plugin, the easiest way is to create it in the editor in the context of a project. Make sure you have "CanContainContent" in the plugin descriptor set to true. I generally create a dedicated development project for each plugin I make and do all coding and development in the context of that project. In the editor, ensure you have Show Plugin Content enabled in the content browser view options, and you should see a content folder visible for your plugin. You can then create whatever assets you need there.

Note that the editor's content migration does not support moving assets into plugin content folders, and that copying assets between plugin and project content folders in the content browser is also unreliable when it comes to asset references.

Building a plugin

As noted above, at project level, plugin code will be compiled along with project code. However if you intend to distribute a plugin, the recommended way to do so is with the engine's automation tool. On Windows, run the RunUAT batch file located at [UE4 Installation]/Engine/Build/BatchFiles as follows:

RunUAT.bat BuildPlugin -plugin=[path/to/pluginname.uplugin] -package=[path/to/output/directory]

This will compile the plugin code for its whitelisted platforms (for Development, DevelopmentEditor and Shipping configurations), and output the full plugin ready for distribution, including source, intermediate, binaries and content, along with anything specified by the plugin filter (see below).

FilterPlugin.ini

This optional config file specifies any additional files that should be included when building a plugin for distribution using the automation tool as described above. Note that this is not the same as packaging a project which uses a plugin - that is a later step, and any additional files that should also be copied at that stage must be specified separately via the RuntimeDependencies array inside the Build.cs file of one of your plugin's modules.

For example syntax of the FilterPlugin.ini, just run the BuildPlugin command above. An example FilterPlugin.ini file will be generated in /Config directory of the source plugin.

Conclusion

The vast majority of my UE4 programming is done in plugin form. Just like with modules, the encapsulation encourages good design and less dependencies. Plugins can have content bundled with them when necessary, and can be easily shared and reused. They also offer a way of reducing project recompilation time, by putting infrequently changing code into a binary-only or engine level plugin.

UE4 Code Modules

What are UE4 code modules?

In UE4, a module is a distinct unit of C++ code, with an accompanying C# build file. A game project can be made up of one or more modules, as can a plugin. Under the hood, a module corresponds to a dynamic library, although by default in shipping builds all code is linked together into a single executable.

Why use multiple modules?

It's possible to create a game (or plugin) that is composed of a single module, and for smaller projects that's fine. There are a number of reasons to consider splitting up your code however.

• Encapsulation and organisation. It's always a good idea to encapsulate code as much as possible. Building a component or system in its own module encourages you to keep dependencies down.
• Code reuse. A module is a natural unit of code for reusing across multiple projects. Separating logically distinct systems at the code level makes it easier to reuse something, even if initially you didn't envisage it being useful outside the project for which you originally wrote it. One effective approach, if you use git, is to put your reusable module(s) into their own git repository, and then incorporate that as a git submodule in any project repository.
• Configuration-specific code. If you want to write editor extension code for your custom classes and game systems, you should put it into a dedicated Editor module. While preprocessor definitions (#if WITH_EDITOR) can be used in some cases, any non-trivial amount of editor-specific code should go into an Editor module. It's also possible to create Development only modules, so you can have, for example, debugging code which gets automatically compiled out of shipping builds. The same goes for server/client-only code.
• Platform-specific code. Again, preprocessor macros for platform-specific code should be kept to a minimum. It's possible to provide platform-specific implementations of project components, each in their own module, and selectively build and package based on the target.

Adding a module

For the remainder of this article, wherever you see 'YourModuleName', regardless of case, be it as part of a filename or in code, replace it with whatever you want to name your module.

Adding an extra module is essentially the same whether you're adding it to a project or a plugin. Inside the source directory, create a new folder named YourModuleName. Within that, you first need the module build file, named YourModuleName.Build.cs. A basic one will look something like this:

// YourModuleName.Build.cs

using UnrealBuildTool;

public class YourModuleName : ModuleRules
{
    public YourModuleName(ReadOnlyTargetRules Target) : base(Target)
    {
        PCHUsage = PCHUsageMode.UseExplicitOrSharedPCHs;

        PublicDependencyModuleNames.AddRange(new string[] {
            "Core",
            "CoreUObject",
            "Engine",
            });
    }
}

Then you will want to add two folders, named Public and Private. The source (.cpp) files always go in the Private folder. Header files can go in either. If they define types or functions that you want to use from code inside other modules, put them in Public. Otherwise, put them in Private for maximum encapsulation.

A bare minimum compilable module requires a source file (YourModuleNameModule.cpp is a good standard) in the Private folder containing the following:

#include "ModuleManager.h"
IMPLEMENT_MODULE(FDefaultModuleImpl, YourModuleName);

To ensure your module gets built with your project, you can add a reference to it in your target file (YourProjectName.Target.cs/YourProjectNameEditor.Target.cs) as follows:

ExtraModuleNames.Add("YourModule");

However, in practice you will generally have a dependency chain connecting your module to the main project module (through one or more PublicDependencyModuleNames additions in .Build.cs files), in which case this step is not actually necessary.

Finally, you need to add a module reference to the .uproject (or .uplugin) descriptor, in the "Modules" array:

...
"Modules": [
    {
        ...,
        {
            "Name": YourModuleName,
            "Type": "Runtime",
            "LoadingPhase": "Default"
        }
    }
],
...

See the engine documentation for information on options for module type and loading phase. The linked page is written for plugins, but is the most up-to-date and also applies for adding modules at the project level.

Exposing code to other modules

In some cases, your module might not need to expose anything at all. It could just define some AActor/UObject types that will be picked up by the engine for use in the editor only, or it might just be registering some editor extensions. Often, however, you'll write a module that acts as a library, providing types and functionality to be used by other module code. In that case, you need to explicitly provide access to those elements.

I'll just detail the standard approach here. There is an alternative that has some benefits as well as restrictions, but I'll leave that for a later article.

In your public headers, add the YOURMODULENAME_API macro to declarations of types or functions that should be exposed.

// Exposed code from module 'YourModuleName', for use in other modules.

UCLASS()
class YOURMODULENAME_API AMyActor: public AActor
{...};

struct YOURMODULENAME_API FMyStruct
{...};

YOURMODULENAME_API void MyFunction();

Then these specific types/functions will be accessible from within the code of other modules. Your other module will need to add a static dependency in its build file. For example, if SomeOtherModule needs to use types from YourModuleName, add the following to SomeOtherModule.Build.cs:

PublicDependencyModuleNames.Add("YourModuleName");

Examples

The repository containing source code examples for other articles on this site has been written in modular form, so is a good reference. Kantan Charts, also on Github, is another example of using multiple modules, this time in the context of a plugin.

Conclusion

Reducing dependencies and keeping code organized can save you some major headaches down the line. Modules are great, use them!

I'll go into some more details, and also touch on plugins, in another article soon.