This is a quick introduction to some common techniques you can use while making Fabric mods. To make mods for Minecraft, you'll often have to interact in non-standard ways with Minecraft's code. While Minecraft has increasingly become flexible to changes, it's still not inherently built to be modded.
Unlike other modding APIs, the Fabric Loader does not overwrite Minecraft's class files to add more functionality. Instead, code is injected using the Mixin library. However, most of the time you will not use this library directly.
Mixins can be fragile, and at times can cause conflicts. Therefore, some common functionality has already been implemented by the Fabric API for you. If it doesn't exist in the core Fabric API, often it will exist in a third-party library. In almost every situation it's preferable to use either the Fabric API or a third party library over implementing a mixin yourself.
Sometimes though, you don't need any of that. While Minecraft's not made for modding, it still contains a lot of features you can access without any additional changes at all.
This article will go over all the ways you can affect Minecraft, in order of preference.
If Minecraft already lets you do something, don't re-invent the wheel. A good example of this is the “Registry” class, which lets you add blocks and items without any modifications to Minecraft's code.
Minecraft also uses JSON data files for various data-driven features. You can add JSON files to your mod, which are then injected by the Fabric API. For example, block models and loot tables are implemented through JSON files.
Fabric itself, as installed into a client, is split up into two parts.
The API is intentionally kept relatively small, to make porting Fabric to newer Minecraft versions faster.
You can find out what's included in the Fabric API by looking over its source code on GitHub. The Fabric API contains a lot of common event hooks and general utilities for things like networking and rendering.
Because the Fabric API is intentionally kept small and focused, third party APIs exist to fill in the gaps. Mixins allow any third party library to affect Minecraft's code in the same way as the core Fabric API can. You should use these instead of writing your own mixins where possible to minimize the possibility of conflicts.
You can find an incomplete list of third party libraries on this wiki.
Finally, you can use mixins. Mixins are a powerful feature that lets you change any of Minecraft's own code. Some mixins can cause conflict but used responsibly these are key to adding unique behavior to your mod.
Mixins come in a variety of flavors, in rough order of preference:
This is not a complete list, but rather a quick overview. Some mixin types are omitted here.
This is probably one of the safest ways to use mixins. New interface implementations can be added to any Minecraft class. You can then access the interface by casting the class to it. This doesn't change anything about the class, it only adds new things, and is therefore very unlikely to conflict.
One caveat is that the function signature (name + parameter types) you inject must be unique. So if you use common parameter types, be sure to give it a very unique name.
Callback injectors let you add callback hooks to existing methods, as well as on specific method calls within that method. They also let you intercept and change the return value of a method, and return early. Callback injects can stack, and are therefore unlikely to cause conflicts between mods.
Redirects let you wrap method calls or variable access within a target method with your own code. Use these very sparingly, a target call or access can only be redirected once between all mods. If two mods redirect the same value, that will cause a conflict. Consider callback injects first.
Be very careful with how you use Overwrites. They replace a method entirely, removing all existing code and conflicting with any other types of mixins on the method. They are extremely likely to conflict not just with other mods, but also with changes to Minecraft itself. You most likely do not need an overwrite to do what you want to do.