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Quick introduction to Hooks

Quick introduction to Hooks

This guide is intended for people new to PAYDAY 2 Lua modding and explains different methods of hooking functions and when to use them. I'll go over the most commonly used methods of function hooking and explain how they work and why we need them.

Why bother adding hooks?

You may ask yourself: Why all this complicated stuff? Can't I just take the original function, make my edits and use the edited function? Well yes, you can do a complete function override but you really shouldn't - unless it is absolutely unavoidable. Doing this will crash your game in the worst case and is more likely to break whenever the game is updated. You will also run into the risk of breaking mods that rely on the same function, as your mod would completely undo changes of other mods if it loaded afterwards. The major takeaway is therefore: Doing proper function hooks increases compatibility with other mods and game updates and is less likely to break over time.

With that out of the way, let's see what kind of hooking techniques exist.

Types of hooks

PostHook

Depending on what your mod is trying to achieve, you will have different needs when hooking functions. The most common scenario is probably adding additional data or code on top of an existing function after it executes. For this we generally use BLT's PostHook, which registers a custom function that is executed whenever the game executes the target function. Using a PostHook is simple and can be used if you don't care about the content of the original function and just want to add some custom code to it.

Let's look at an example: Let's say we want to change the HP of the regular street cops. We can do this after the cop's stats have been initialized, which is done in CharacterTweakData:_init_cop. Since we don't really care what the original code does and we just want to change the HP we can add code in the form of a PostHook. We would do that like this:

Hooks:PostHook(CharacterTweakData, "_init_cop", "our_unique_hook_id", function (self)
  self.cop.HEALTH_INIT = 10
end)

This would cause the cop to be initialized like normal, but directly afterwards our custom function would be executed, changing the cops health to our own supplied value.

Note that your function will be supplied with the same arguments that the original function will be called, so if you need them, you can specify them. In addition to the original function parameters, a function of the form function object:do_stuff(a, b) (: instead of . between object and function name) will provide the calling object itself as the original parameter (usually called self) so if you wanted to capture the parameters a and b of that function in a PostHook you would add an additional self in front of your parameter list.

PreHook

Very similar to a PostHook, the only difference is that it will be executed before the original function is called. Less commonly used but useful if you need to change some values right before a function call. For obvious reasons, you don't have access to any of the member variables that are created in the original function in a PreHook.

Function wrapping

The above mentioned hooking methods should cover a lot of usecases already, but there are some cases where they are not usable. Let's say you want to intercept the return value of a function and replace it with something else, or add additional checks depending on the return value. This simply can not be done with a PostHook since you don't have access to what the original function returned. In this case, you need to do a function wrap, often referred to as old_init. This involves saving the original function into a variable and then overriding the function with a new one. In the new function you would then call the original function manually and do whatever else you need to do.

Let's say you made a new custom enemy and for making it work properly you need to add it to the character map (a list of all enemies the game goes over and generates contour mappings for). The character map is returned by the function CharacterTweakData:character_map and the function itself creates and returns a local table with all characters in it, so we don't have access to this table via PostHook or PreHook. Adding your custom enemy to the list would mean you have to override the entire function, copy the list and add your enemy, right? While that works, I already went over why you shouldn't do this when it's avoidable, and in this case it is: We can simply let the original function run, and before it actually returns the value, add our own stuff to it. This is as simple as:

local character_map_original = CharacterTweakData.character_map
function CharacterTweakData:character_map(...)
  local char_map = character_map_original(self, ...)

  table.insert(char_map.basic.list, "custom_enemy_name")

  return char_map
end

Let's go over this code line by line:

  1. First we save the original function into a local variable, we're backing it up to still have access to it after we override it. We have to always use . as the connector between the class and the function name.
  2. Next, we are redefining the function, essentially overriding it with a completely new one.
    • Using ... in the function arguments simply means "take all arguments that are given when the function is called". We can use this to make sure we always supply the original function call with all the arguments it is called with, without actually listing them. If you need some of the actual arguments, you can simply list all arguments up to the ones you need and then follow them by .... It is suggested to always use ... at the end of the argument list, even if you use all of the original arguments to maintain compatibility with other mods and game updates that may add additional arguments.
  3. Then, we are calling the original function via the local variable we have created on line 1 and saving whatever it returns into char_map.
    • When calling the original function inside the new function, we have to supply self as the first argument if the function references an object (denoted by a : connection between class and function name). This basically restores the object reference that is "lost" when saving the function to a local variable.
  4. We are doing our changes to the return value, in this case the return value is a table and we are inserting our custom enemy into it.
  5. Finally we have to make sure to return whatever the original function is expected to return, as returning a wrong type or nothing at all will lead to crashes when the game expects a specific thing (in this case, a table).

Whenever the game calls the new CharacterTweakData:character_map now, it will first call its original function, then our custom enemy is added to the result of the original call. If another mod adds something to the character map in (hopefully) the same way, both changes will merge instead of override each other.

If all else fails

If none of the methods above can be applied to what you want to do, you will have to override the entire function. However, SuperBLT provides a way to do this in a way that at least keeps any hooks made by other mods intact. This looks like the following:

Hooks:OverrideFunction(GroupAIStateBesiege, "assign_enemy_to_group_ai", function (self)
  -- Your code here
end)

Note that if another mod used SuperBLTs PostHook or PreHook on the same function, they will still be called and you can maintain some compatibility with other mods.

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