Unity jump only once per trigger - unity3d

Question

I need help with my code where I can only jump once but then it's locked for me I think I'm onto something.
Here is my code:
// Start is called before the first frame update
void Start()
{
}
// Update is called once per frame
void Update()
{
Vector3 v = transform.position;
v.x += speed * Input.GetAxis("Horizontal") * Time.deltaTime;
v.z += speed * Input.GetAxis("Vertical") * Time.deltaTime;
transform.position = v;
if (Input.GetKeyDown(KeyCode.Space) && isGrounded==true)
{
GetComponent<Rigidbody2D>().velocity = Vector2.up * jumpVelocity;
isGrounded = false;
}
void OnCollisionEnter(Collision col)
{
if (col.gameObject.tag == ("Ground") && isGrounded == false)
{
isGrounded = true;
}
}
}
}

Answer 1

You nested the OnCollisionEnter method inside the Update.
This way it is a local function and not found by the Unity Physics / Message System -> it will never be called.
Another issue in your code: Whenever there is a Rigibody/Rigibody2D involved you do not want to set or access stuff via the Transform component. This will break the physics and collision detection!
And finally note that Physics and Physics2D are two completely separate engines! If you want to be moving and detect collisions in 3D space then you have to use a Rigidbody, not a Rigibody2D!
It should rather be
[Header("References")]
[SerializeField] private Rigidbody _rigidbody;
[Header("Debugging")]
[SerializeField] private bool isGrounded;
private void Awake()
{
if(!_rigidbody) _rigidbody = GetComponent<Rigidbody>();
}
// Update is called once per frame
private void Update()
{
var x = Input.GetAxis("Horizontal");
var y = Input.GetAxis("Vertical");
// Clamp the vector so it has a maximum magnitude of 1
// this prevents that diagonal movement is overall faster
var moveInput = Vector2.ClampMagnitude(new Vector2(x,y), 1) * speed;
// instead of using the Transform simply change the rigidbody velocity accordingly
var velocity = _rigidbody.velocity;
velocity.x = moveInput.x;
velocity.z = moveInput.y;
_rigidbody.velocity = velocity;
// it is cheaper to first check a simple bool flag
// if this already fails we can skip the more expensive "GetKeyDown"
if (isGrounded && Input.GetKeyDown(KeyCode.Space))
{
_rigidbody.velocity = Vector2.up * jumpVelocity;
isGrounded = false;
}
}
// This method should be on class level and not nested under Update
private void OnCollisionEnter(Collision col)
{
// if already "isGrounded" we can skip the more expensive "CompareTag"
if (!isGrounded && col.gameObject.CompareTag("Ground"))
{
isGrounded = true;
}
}