BLT Modding

Introduction

This guide will be a simple tutorial on how to make your first BLT mod with a suitable example to follow along.
The example mod will hide all (multiplayer) jobs from Crime.Net which are not stealthable.

Scope

Expect this:
DON'T expect this:

Setup

Beginner BLT modding

mod.txt

Every mod needs this file. This file is responsible for loading your mod with BLT. Below you will find the mod.txt used for this example. Copy the contents inside your own mod.txt. Most of the fields should be self explanatory expect for the hooks.

{
	"name": "Hide loud jobs",
	"description": "Hides jobs from crime.net which are not stealthable",
	"author": "Your name",
	"version": "1.0",
	"blt_version": 2,
	"hooks": [
		{
			"hook_id": "lib/managers/crimenetmanager",
			"script_path": "mod.lua"
		}
	]
}

Hooks

Finding the code

There's no magic trick to finding the right place in the source code. It takes practice and understanding of the codebase. We will not be covering that here but you are more than welcome to ask the community for help in the #payday-help-channel of our Discord on this topic.

You can still follow along by opening the file lib/managers/crimenetmanager(this is also the hook_id) inside the source code you downloaded earlier. Find the function CrimeNetGui:add_server_job (or view it here).

Editing the code

Copy the entire function in your mod.lua. Now we will add an if-statement around the code inside the function. This statement will check if it is indeed a stealthable mission. Only if that's the case will the game actually add it to the map. Luckily there is already such a function built into the game (as it's used to display the little ghost icon next to the job). You can find that part in the same file (or view it here). Let's have a look at it:

if data.job_id and managers.job:is_job_ghostable(data.job_id) then
	ghost_icon = icon_panel:bitmap({
		texture = "guis/textures/pd2/cn_minighost",
		name = "ghost_icon",
		blend_mode = "add",
		color = tweak_data.screen_colors.ghost_color
	})
  
	ghost_icon:set_top(side_icons_top)
	ghost_icon:set_right(next_icon_right)
  
	next_icon_right = next_icon_right - 12
end

From simply reading the code: If the job_id exists and the job with the given job_id is stealthable, we create a new icon with the texture of the blue ghost and set its position. Now let's try to apply that to our code. Our function is also given the parameter data. This is in fact the same object for this case. Therefore we can safely wrap our code in the exact same if-statement like this:

mod.lua

Here's how your finished mod.lua should look like:

function CrimeNetGui:add_server_job(data)
	if data.job_id and managers.job:is_job_ghostable(data.job_id) then
		local gui_data = self:_create_job_gui(data, "server")
		gui_data.server = true
		gui_data.host_name = data.host_name
		self._jobs[data.id] = gui_data
	end
end

That's it - Try it out!

► End of basic tutorial ◄

Continuation: Finding the code

I will explain to you my thought process of how I was able to find the correct function in case it may help you:

For this example I was specifically looking for crimenet which brought me directly to the CrimeNetManager. I knew that stealth is mostly referred to as ghost in the code - so I quickly picked up on the idea of the ghost_icon which was used in the function _create_job_gui but that one is difficult to grasp and gets called through the more accessible function add_server_job. Now I only had to go back to the ghost_icon to figure out how the game determined if it should be displayed or not.

Different methods:

Let's review some other methods we could have used to get this mod working.

[Hard Overwrite] (as shown above)

function CrimeNetGui:add_server_job(data)
	if data.job_id and managers.job:is_job_ghostable(data.job_id) then
		local gui_data = self:_create_job_gui(data, "server")
		gui_data.server = true
		gui_data.host_name = data.host_name
		self._jobs[data.id] = gui_data
	end
end
Explanation:

This method is the most straightforward: It is the exact copy from the source code but with our adjustments.

[Soft Overwrite]

local old_add_server_job = CrimeNetGui.add_server_job

function CrimeNetGui:add_server_job(data)
	if data.job_id and managers.job:is_job_ghostable(data.job_id) then
		old_add_server_job(self, data)
	end
end
Explanation:

This method works by keeping a copy to the original function. This function is then overwritten with our implementation where we call the reference to the original function.

[Hook]

Hooks:PostHook(CrimeNetGui, 'add_server_job', 'hideloudjobs_crimenetgui',
function(self, data)
	if data.job_id and not managers.job:is_job_ghostable(data.job_id) then
		self:remove_job(data.id, true)
	end
end)
Explanation:

Hooks don't touch the original function. They get executed just before (Pre) or after (Post) the function.

So which one should you use?

Generally speaking: Hooks > Soft Overwrite > Hard Overwrite

Use Hooks wherever you can. Use Soft Overwrites when dealing with function where you need to change the returning value and only use the Hard Overwrite method if you really have to.

Resources

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 using hooks?

You may ask yourself: Why all this complicated stuff? Can't I just copy and paste the original function and make my edits to it? While yes, you could do a complete function override, you really shouldn't unless it is absolutely unavoidable. In the worst case this will crash the game when an update changes some things in that function and you will have to copy and change the new function again to fix it. 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 SuperBLT'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 need to change the functionality of the original function and just want to add some additional code.

Let's look at an example: We want to change the HP of the regular street cops. We can do this after the cop's stats have been initialized, which happens in the function CharacterTweakData:_init_cop.
Looking at this function in the game code we can see that the cop's health is set here. Since we just want to change the HP and still want all the other things to be initialized like usual 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)

When the game runs the original CharacterTweakData:_init_cop function, all the cop data will be initialized as usual, but immediately afterwards, BLT will execute our hook function, changing the cop's health to the value we supplied.

The function you specify for the PostHook will be supplied with the same arguments that the original function will be called, so if you need them, you can specify them.
Note that CharacterTweakData:_init_cop(presets) is functionally identical to CharacterTweakData._init_cop(self, presets) (specifically note . instead of : between CharacterTweakData and _init_cop). This is important for your hook function if you plan to make use of the function arguments and the reason why you usually see self as the first argument in a hook function even if the original function doesn't have it.
If you are unsure about wether to include self in the list of arguments for your hook, just remember that if the function is defined with a : in the game code, the first argument to it in a hook should be self.

If you need access to what the original function of your hook returned you can use Hooks:GetReturn() which will return any values that have been returned by hook functions and the original function call that ran before your hook function. 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.

Hooks:PostHook(CharacterTweakData, "character_map", "our_other_unique_hook_id", function (self)
	local char_map = Hooks:GetReturn()
	table.insert(char_map.basic.list, "custom_enemy_name")
	return char_map
end)

You can see that it easy to access the return value and change it (in this case the return value is a table and we insert an entry into it. Another thing that you should notice is that you can return values from a hook, this will override the return value of the original function (and any function hooks on the same function that came before yours).

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 or add additional code right before a function call. Note that setting fields that are created or set in the function in the original code will have no effect since your code runs before the original function and the original call will just override any values that it sets.

Another niche use case for a PreHook is changing function arguments that are of table type, as tables are passed by reference and you can therefore change the content of the table which will then be passed to the original function. As other data types are passed by value this only works for tables.

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 change the arguments the original function is called with or stop the function from being called based on some condition. This simply can not be done with a PostHook or PreHook since all function arguments except tables are passed by value and PostHook or PreHook will always run the original function.
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 then call the original function manually and do whatever else you need to do.

local build_suppression_original = CopDamage.build_suppression
function CopDamage:build_suppression(amount, ...)
	if amount == "max" then
		amount = 2
	end
  
	return build_suppression_original(self, amount, ...)
end

Some notes on this code:

So you chose to override after all

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.
An example could be 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. Redefining the function without making use of Hooks:OverrideFunction will remove all hooks that were made by mods that ran before yours which could lead to unexpected behavior.