# How to create a hook

Created by [HJHughJanus](https://github.com/HJHughJanus) on github, copied here for easier maintainability

## What is a hook?

### wrapMethod

Also called wrapper function, this piece of code **hooks** into existing game functions. 

{% hint style="info" %}
The only exception from that rule are functions and classes marked with the **`native`** keyword.
{% endhint %}

You define those wrappers outside of your own class at the root level.

To trigger the original callback, you need to call  `wrappedMethod(evt)` either before or after you did your own thing:

```swift
@wrapMethod(YourWrappingTarget)
protected cb func OnSomethingHappens(originalEventParam: ref<SomeRef>) -> Bool {
  wrappedMethod(originalEventParam);
  // do stuff here
}
```

### replaceMethod

Opposed to a wrapped method, a replaced method will stop the original game function from doing anything.

```swift
@replaceMethod(YourWrappingTarget)
protected cb func OnSomethingHappens(originalEventParam: ref<SomeRef>) -> Bool {
  return false;
}
```

## Things to hook

## **Example**

***Goal:*** Ragdoll any NPC hit by a bullet.\
\&#xNAN;***System:*** I need to find out how Cyberpunk 2077 handles bullets and damage → a look through the decompiled game scripts shows there is a class called „DamageSystem“. This class looked up on Cyberdoc shows there is a function wihtin that class called „ProcessLocalizedDamage“. Back in the decompiled game scripts the code of this function seems to handle a „hit event“ and checking if the bullet hit the head or any weak spots. That is very close to my goal, so I will use this function for the hook.\
\&#xNAN;***Functions:*** I want to ragdoll an NPC, so I will need a function to do that. I search on Cyberdoc for „ragdoll“ and find something called „ApplyRagdollImpulseEvent“. I search this function in the decompiled game scripts to find out how it is used.

\
First, I need to specify which system I want to use for injection, then specify the function.

```swift
@wrapMethod(DamageSystem)
private func ProcessLocalizedDamage(hitEvent: ref<gameHitEvent>) {
}
```

In this function I must now call the vanilla function first, so I don‘t overwrite any of the vanilla action – you can do this via the function `wrappedMethod(params)` (this one automatically calls the vanilla function).

```swift
@wrapMethod(DamageSystem)
private func ProcessLocalizedDamage(hitEvent: ref<gameHitEvent>) {
wrappedMethod(hitEvent);
}
```

Then I copy some code from the vanilla function to handle some errors and set up my variables.

```swift
@wrapMethod(DamageSystem)
private func ProcessLocalizedDamage(hitEvent: ref<gameHitEvent>) {
    wrappedMethod(hitEvent);
    
    //variables
    let hitUserData: ref<HitShapeUserDataBase>;
    let hitShapes: array<HitShapeData> = hitEvent.hitRepresentationResult.hitShapes;
    
    //error handling and setting up hitUserData (copy & paste from original)
    if !hitEvent.attackData.GetInstigator().IsPlayer() {
      return;
    };
    
    if AttackData.IsAreaOfEffect(hitEvent.attackData.GetAttackType()) {
      return;
    };
    
    if ArraySize(hitShapes) > 0 {
      hitUserData = DamageSystemHelper.GetHitShapeUserDataBase(hitShapes[0]);
    };
    
    if !IsDefined(hitUserData) {
      return;
    };
}

```

Now my actual code can take effect. Since I found the „ApplyRagdollImpulseEvent“ and looked up how it is used, I know this will not be done in one line. So, to keep the code in the wrapper function simple, I will write a separate function below which sets up the ragdolling for me.

```swift
@wrapMethod(DamageSystem)
private func RagdollNPC(target: ref<NPCPuppet>, pushForce: Float) -> Void {
  if IsDefined(target) {
    target.QueueEvent(CreateForceRagdollEvent(n"Debug Command"));
    if pushForce > 0.0 {
      GameInstance.GetDelaySystem(target.GetGame()).DelayEvent(target, CreateRagdollApplyImpulseEvent(target.GetWorldPosition(), Vector4.Normalize(target.GetWorldPosition()) * pushForce, 5.00), 0.10, false);
    };
  };
}
```

Now my actual code can take effect. Since I found the „ApplyRagdollImpulseEvent“ and looked up how it is used, I know this will not be done in one line. So, to keep the code in the wrapper function simple, I will write a separate function below which sets up the ragdolling for me.

<pre class="language-swift"><code class="lang-swift"><strong>@wrapMethod(DamageSystem)
</strong>private func ProcessLocalizedDamage(hitEvent: ref&#x3C;gameHitEvent>) {
    //start vanilla function, so everything at least runs normally (even if the mod doesnt work)
    wrappedMethod(hitEvent);
    
    //variables
    let hitUserData: ref&#x3C;HitShapeUserDataBase>;
    let hitShapes: array&#x3C;HitShapeData> = hitEvent.hitRepresentationResult.hitShapes;
    
    //error handling and setting up hitUserData (copy &#x26; paste from original)
    if !hitEvent.attackData.GetInstigator().IsPlayer() {
      return;
    };
    
    if AttackData.IsAreaOfEffect(hitEvent.attackData.GetAttackType()) {
      return;
    };
    
    if ArraySize(hitShapes) > 0 {
      hitUserData = DamageSystemHelper.GetHitShapeUserDataBase(hitShapes[0]);
    };
    
    if !IsDefined(hitUserData) {
      return;
    };

    //actual mod code
    let npc: ref&#x3C;NPCPuppet> = hitEvent.target as NPCPuppet;
    let npcID: StatsObjectID = Cast(npc.GetEntityID());
    if IsDefined(npc) {
      RagdollNPC(npc, 10.0);
    };
}
</code></pre>

The whole file now looks like this:

<figure><img src="https://4048438440-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F-McniwB8YOK2HnJ7SYg_%2Fuploads%2FlCJbcUTcLhPSVkbfVwzO%2Fimage.png?alt=media&#x26;token=851b6fd0-0215-4be5-8bea-207693a15473" alt=""><figcaption></figcaption></figure>

This file I save and then put into `D:\PATH\TO\CYBERPUNK2077\r6\scripts` so the game will load it.&#x20;

When I start the game, every NPC hit by a bullet should be ragdolled with a push.