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UnityEvent

Type	Class
Namespace	`UnityEngine.Events`
Variants	`UnityEvent<T0>` up to `UnityEvent<T0, T1, T2, T3>`
Seen in	Button "On Click ()", Slider "On Value Changed"

UnityEvent

A UnityEvent is a list of methods to call when something happens. The special part is that you can fill that list in the Inspector by dragging objects and picking methods from a dropdown, the same way a UI Button's "On Click" works. You don't need code to connect them.

A doorbell. The button (your script) doesn't know or care what the bell is wired to. It could be a chime, a light, or a message to your phone. It just rings (Invoke()). Whoever installs it (you, in the Inspector) decides what's connected. This means you can reuse the same button in many places and connect it to different things each time.

Methods

Method	Returns	Description
`Invoke()`	`void`	Calls every connected method (Inspector ones first, then code ones). For `UnityEvent<T>`: `Invoke(value)`.
`AddListener(UnityAction call)`	`void`	Connect a method from code.
`RemoveListener(UnityAction call)`	`void`	Disconnect a method added with `AddListener`.
`RemoveAllListeners()`	`void`	Removes all code-added listeners. Does not remove the ones set in the Inspector.
`GetPersistentEventCount()`	`int`	How many listeners were set up in the Inspector.

The plate only knows its own event. Which methods run is stored in the Inspector (dashed), so adding a fourth reaction never touches PressurePlate.cs.

What the Inspector can call

In the dropdown, Unity lists public methods and property setters of every component on the object you dragged in, as long as they take zero or one parameter of a simple type (int, float, string, bool, or a Unity Object type).

Static parameters: you type a fixed value in the Inspector, e.g. always call SetActive(true).
Dynamic parameters (only for UnityEvent<T>): the value passed to Invoke(value) is forwarded. They appear at the top of the dropdown under "Dynamic float" (etc.).

Code Samples

A reusable pressure plate

using UnityEngine;
using UnityEngine.Events;

public class PressurePlate : MonoBehaviour
{
    [SerializeField] private UnityEvent onPressed;
    [SerializeField] private UnityEvent onReleased;

    void OnTriggerEnter(Collider other)
    {
        if (other.CompareTag("Player")) onPressed.Invoke();
    }

    void OnTriggerExit(Collider other)
    {
        if (other.CompareTag("Player")) onReleased.Invoke();
    }
}

In the Inspector, click + under "On Pressed", drag in a door, and choose Door > Open(). Add another entry for a light: GameObject > SetActive (bool) with the box ticked.

An event that passes a value

using UnityEngine;
using UnityEngine.Events;

public class Health : MonoBehaviour
{
    [SerializeField] private float maxHealth = 100f;
    public UnityEvent<float> onHealthChanged; // passes health as a 0-1 fraction
    public UnityEvent onDied;

    private float health;

    void Start()
    {
        health = maxHealth;
        onHealthChanged.Invoke(1f);
    }

    public void TakeDamage(float amount)
    {
        if (health <= 0f) return;
        health = Mathf.Max(health - amount, 0f);
        onHealthChanged.Invoke(health / maxHealth);
        if (health == 0f) onDied.Invoke();
    }
}

Now drag a UI Slider into "On Health Changed" and choose Dynamic float > value. The bar updates by itself.

Listening from code

using UnityEngine;

public class DeathCounter : MonoBehaviour
{
    [SerializeField] private Health health;
    private int deaths;

    void OnEnable()  { health.onDied.AddListener(HandleDied); }
    void OnDisable() { health.onDied.RemoveListener(HandleDied); }

    private void HandleDied()
    {
        deaths++;
        Debug.Log("Deaths: " + deaths);
    }
}

UnityEvent vs. C# event

	UnityEvent	C# `event Action`
Connect in Inspector	Yes	No
Speed	Slower (fine for gameplay moments)	Faster
Best for	Designer-facing hooks: doors, buttons, triggers, "on death"	Code-to-code events, things fired very often
Safety	Anyone with access can call `Invoke()`	Only the owning class can invoke

What happens at runtime

The Inspector connections are saved with the scene/prefab as data (object reference + method name).
The first time you call Invoke, Unity looks up those methods by name and caches them.
Invoke then calls each listener in order, one after another, on the same frame.
If a listener's object was destroyed, Unity skips it for Inspector connections. Code listeners on destroyed objects can throw errors, which is why you remove them in OnDisable.

Common Mistakes

My method isn't in the dropdown. It must be public, return void (or be a property setter), and have 0 or 1 simple parameter.
Renaming a method breaks the connection. The Inspector shows "Missing". Reconnect it. This is the main downside of Inspector wiring.
RemoveListener with a lambda does nothing. AddListener(() => X()) and RemoveListener(() => X()) are two different lambdas. Store the delegate in a variable, or use a named method.
Invoking every frame. Fine occasionally, but for per-frame values, a C# event or direct reference is lighter.

UnityEvent

Methods

What the Inspector can call

Code Samples

A reusable pressure plate

An event that passes a value

Listening from code

UnityEvent vs. C# event

What happens at runtime

Common Mistakes

See also