How to Spawn Objects on Mouse Path? Unity 3D

Question

This is what I want to achieve:

When moving the mouse (the mouse button is down) I want to save the whole mouse movement, I mean from the start (button click) point till the end (release button) point. And after releasing button I want to spawn some objects on the whole saved mouse path (like the picture), and the space between each two spawned objects should be the same.

• The world is in 3D, but the objects should spawn only on the X and Y axis, the Z axis will be fixe.

How can I do that in the code? I've already started it:

void Update()
{
    if (Input.GetMouseButtonDown(0))
    {
        screenPoint = Camera.main.WorldToScreenPoint(transform.position);
    }

    if (Input.GetMouseButton(0) && !Input.GetMouseButtonDown(0))
    {
        Vector3 curScreenPoint = new Vector3(Input.mousePosition.x, Input.mousePosition.y, transform.position.z);

        Vector3 curPosition = new Vector3(-Camera.main.ScreenToWorldPoint(curScreenPoint).x, -Camera.main.ScreenToWorldPoint(curScreenPoint).y, transform.position.z);

        if ((Input.GetAxis("Mouse X") != 0) || (Input.GetAxis("Mouse Y") != 0));
        {
            GameObject TheBall = Instantiate(Ball, curPosition, Quaternion.identity);
            TheBall.transform.SetParent(transform, false);
        }         
    }

Answer 1

Like I said you first may get the average space between each point. After that you would always create new point from the position of the one before. You could do this like that:

public GameObject prefab;
List<Vector2> points = new List<Vector2>();

void Update() {
    // You add the points in update to the List
    if(Input.GetMouseButtonUp(0)) {
        float totalDistance = 0;
        for(int i = 0; i < points.Count-1; i++) {
            totalDistance += Vector2.Distance(points[i], points[i+1]);
        }
        float averageDistance = totalDistance / (points.Count-1);

        // Spawning first Point
        Vector2 lastPoint = points[0];
        Instantiate(
            prefab, 
            new Vector3(lastPoint.x, z, lastPoint.y), 
            Quaternion.identity
        );

        // Spawning other Points
        for(int i = 0; i < points.Count - 1; i++) {
            Vector2 spawnPos = lastPoint + ((points[i+1] - lastPoint).normalized * averageDistance)
            lastPoint = spawnPos; //Save the new point because you want create the next one from this one
            int z = 0; // The z you need
            Instantiate(
                prefab, 
                new Vector3(spawnPos.x, z, spawnPos.y), 
                Quaternion.identity
            );
        }
    }
}

Edit: Created the code for you. Your start code also had some issues but this here is working.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class SpawnCircles : MonoBehaviour {
    public GameObject prefab;
    public float Y;

    private Camera cam;
    private List<Vector2> points = new List<Vector2>();

    void Update() {
        if(Input.GetMouseButton(0)) {
            cam = Camera.main;
            Vector2 mousePos = new Vector2();
            mousePos.x = Input.mousePosition.x;
            mousePos.y = cam.pixelHeight - Input.mousePosition.y;
            Vector3 point = cam.ScreenToWorldPoint(new Vector3(mousePos.x, mousePos.y, 10));
            points.Add(new Vector2(point.x, point.z * -1));
        }
        if(Input.GetMouseButtonUp(0)) {
            float totalDistance = 0;
            for(int i = 0; i < points.Count - 1; i++) {
                totalDistance += Vector2.Distance(points[i], points[i + 1]);
            }
            float averageDistance = totalDistance / (points.Count - 1);

            // Spawning first Point
            Vector2 lastPoint = points[0];
            Instantiate(
                prefab,
                new Vector3(lastPoint.x, Y, lastPoint.y),
                Quaternion.identity
            );
            // Spawning other Points
            for(int i = 0; i < points.Count - 1; i++) {
                Vector2 spawnPos = lastPoint + ((points[i + 1] - lastPoint).normalized * averageDistance);

                lastPoint = spawnPos; //Save the new point because you want create the next one from this one
                Instantiate(
                    prefab,
                    new Vector3(spawnPos.x, Y, spawnPos.y),
                    Quaternion.identity
                );
            }
            // Resetting List
            points = new List<Vector2>();
        }
    }
}