arfoundation-samples/Assets/Scenes/Meshing/Scripts/ProjectileLauncher.cs

using UnityEngine;

/// <summary>
/// Launches projectiles from a touch point with the specified <see cref="initialSpeed"/>.
/// </summary>
[RequireComponent(typeof(Camera))]
public class ProjectileLauncher : MonoBehaviour
{
    [SerializeField]
    Rigidbody m_ProjectilePrefab;

    public Rigidbody projectilePrefab
    {
        get => m_ProjectilePrefab;
        set => m_ProjectilePrefab = value;
    }

    [SerializeField]
    float m_InitialSpeed = 25;

    public float initialSpeed
    {
        get => m_InitialSpeed;
        set => m_InitialSpeed = value;
    }

    void Update()
    {
        if (m_ProjectilePrefab == null)
            return;

        if (Input.touchCount == 0)        
            return;

        var touch = Input.touches[0];
        if (touch.phase == TouchPhase.Began)
        {
            var ray = GetComponent<Camera>().ScreenPointToRay(touch.position);
            var projectile = Instantiate(m_ProjectilePrefab, ray.origin, Quaternion.identity);
            var rigidbody = projectile.GetComponent<Rigidbody>();
            rigidbody.velocity = ray.direction * m_InitialSpeed;
        }
    }
}