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Shoot from gun in top down shooter Unity3D

Currently I made bullet shoot from players position. Problem is that it shoots from player's center and not from the gun. I changed position of the bullet to make it shoot out from the gun. But the problem is that, when I start to rotate the player, bullet copies the rotation but not the position of the gun. It just stays on the same place. Here's my code:
public Transform firePoint;
public GameObject bulletPrefab;
public float bulletForce = 20f;
public void Shoot()
{
Vector3 newPosition = new Vector3(firePoint.position.x + 1, firePoint.position.y - 0.1f, firePoint.position.z);
GameObject bullet = Instantiate(bulletPrefab, newPosition, firePoint.rotation);
Rigidbody2D rb = bullet.GetComponent<Rigidbody2D>();
rb.AddForce(firePoint.right * bulletForce, ForceMode2D.Impulse);
yield return new WaitForSeconds(0.5f);
}

Your issue is that you have hard-coded offset (x = 1, y = -0.1f) change. When you rotate your player 90 degrees, the offset for gun end becomes different.
There are two solutions for this:
Solution #1
Place your fire point Transform at the tip of the gun, make it child of the gun. That way the Transform will always follow the end of the gun.
Now, for instantiating, you can use firepoint.position without any modifications.
There are some drawbacks to this, mainly having strict objects hierarchy and it becomes harder to dynamically change guns as you will have to find and reassign fire point for each of them.
Solution #2
Have Vector3 firePointOffset;.
Once instantiating, calcualte the position of the fire point by doing
var firePointPosition = playerTransform.position + Vector3.Scale(playerTransform.forward, firePointOffset)`;
transform.forward returns Vector3 that points, well, forward for that specific transform. Vector3.Scale allows multiplying two vectors x * x, y * y, z * z;

Instantiate a prefab pointing in the direction of another object

The Issue
I'm making a 2D Unity game where the main weapon of your character is a fireball gun. The idea is that a fireball will shoot out of the player's hand at the same angle the player's hand is pointing. I have 3 issues:
When I shoot the fireball, since the fireball is a RididBody, it pushes the player. This is because I've made the centre of the player's arm (the same place where the fireball shoots from) the point at which the arm rotates around the player (what is meant to be the shoulder);
To instantiate the fireball prefab on the arm, the only way I know how to do it is by using a piece of code which requires the arm to be a RigidBody. This means that the arm is affected by gravity and falls off the player on start unless I freeze the arm's y-axis movement, which means that when the player jumps, while the arm does not fall, it floats at the same y-position as where it started while moving along the x-axis; and
When the fireball is shot, the angle from which it is propelled after being shot is not the same angle as the angle of the player's arm.
Instantiating the Fireball
if (Input.GetKeyDown(KeyCode.Space))
{
pew.Play();
var fireballTransform = Instantiate(fireballPrefab); //creates a new shot sprite
fireballTransform.position = new Vector3(transform.position.x + horizMultiplier, transform.position.y, transform.position.z);
fireballTransform.rotation = orientation;
fireballTransform.transform.Rotate(0, 0, transform.rotation.z);
}
if (Input.GetKeyDown(KeyCode.D)) // moves right
{
orientation = 0;
horizMultiplier = 0.08F;
}
if (Input.GetKeyDown(KeyCode.A)) // moves left
{
orientation = 180;
horizMultiplier = -0.08F;
}
This piece of code is located within the script applied to the player's arm. The movement of the arm works fine and the problem seems to be either within this piece of code or the code for my fireball (which I will put next). A few definitions:
pew is a sound effect played when the fireball is shot;
horizMultiplier is the distance from the arm's centre which I would like the fireball to instantiate (also dependant of if the player) is facing left or right); and
orientation is which direction the player is facing (left or right). The fireball is then instantiated facing that same direction.
Fireball Script
public Vector2 speed = new Vector2(); // x and y forces respectively
private Rigidbody2D rb; // shorthand
private float rotation;
void Start()
{
rb = GetComponent<Rigidbody2D>(); // shorthand
rotation = rb.rotation;
if (rotation == 0)
{
rb.AddForce(Vector3.right * speed.x); // propels right
}
if (rotation == 180)
{
rb.AddForce(Vector3.left * speed.x); // propels left
}
}
I believe this code is explanatory enough with comments (if not please comment and I'll address any question). I believe an issue could also be in this piece of code because of the lines: rb.AddForce(Vector3.right * speed.x); and rb.AddForce(Vector3.left * speed.x); as these add directional forces to the object. I don't know is this is objective direction (right or left no matter what direction the object the force is being applied to is facing) or if it's right or left in terms of the object-- say if an object was rotated 90 degrees clockwise and that object had a force applied so that it moves right making the object move downwards.
What I'm expecting to happen is the player's arm will turn so that when a fireball is fired it is fired in the direction the arm is facing. The arms turning mechanics are fine, it's just trying to properly instantiate the fireball. Can anyone help with any of the issues I've laid out?

How I would go about this:
Add an empty transform as child of the arm and move it to where the fireball should spawn. Also make sure the rotation of it is such that its forward vector (the local z-axis) points to where the fireball should go. To see this, you need to set "Local" left of the play button.
Spawn the fireball at the transform's position with its rotation.
Ignore collision between the arm's/player's collider and the fireball's collider with https://docs.unity3d.com/ScriptReference/Physics.IgnoreCollision.html. If necessary, you can enable the collision between the two colliders again after 1s or so.
Set the fireball rigidbody's velocity to speed * rigidbody.forward.
If you need help with the code, please post it so I can see what's going on.
Edit:
The arm definitely doesn't require a Rigidbody.
You can just use Instantiate(prefab, position, rotation) as shorthand.
Is this for a 2D game?
Also, I'm going to sleep now but I'll gladly try to help tomorrow.

Your Issues
1) You should be masking the fireball's layer not to collide with your player's layer.
You can find more info about this here: https://docs.unity3d.com/Manual/LayerBasedCollision.html
(note: make sure you're on the Physics2D panel, and not the Physics one, as that's for 3D)
2) There is a setting called gravityScale in RigidBody2D. You should set this to 0 if you don't want gravity to be affecting your object. More info: https://docs.unity3d.com/Manual/class-Rigidbody2D.html
Fireball Instantiate
if (Input.GetKeyDown(KeyCode.Space))
{
pew.Play();
var position = hand.transform.position + hand.transform.forward * horizMultiplier;
var rotation = hand.transform.rotation;
var fireball = Instantiate<Fireball>(fireballPrefab, position, rotation);
fireball.StartMoving();
}
Fireball Script
private Rigidbody2D rb; // shorthand
private float rotation;
public float Speed;
public void StartMoving()
{
rb = GetComponent<Rigidbody2D>(); // shorthand
rb.velocity = transform.forward * Speed;
}
void OnTriggerEnter(....) { .... }

Moving Prefabs, Unity Spawner, Move Position

I have one problem. I want my prefabs to spawn every time my player picks them up. I did research on Google and YouTube and I tried to use the random function and instantiate. I don't know how to use them. I wrote this code I saw on YouTube and my prefab Sphere moves like 1cm to z position. I want to every time when I pick up object or my player go to spawn more of this on the z position. How do I do this?
My smaller script:
public GameObject Sphere;
public float zrange;
// Use this for initialization
void Start () {
RandomPosition();
}
void RandomPosition()
{
zrange = Random.Range(0f, 2f);
this.transform.position = new Vector3(0, 0, zrange);
}

You achieve that by not messing with the x and y values (your code sets them both to 0).
Vector3 p = transform.position;
p.z = zrange;
transform.position = p;
This assumes that your code to instantiate the object is already correctly placing the object. If not, more information is needed.

Set player's boundary within sphere

I want to restrict player movement in the sphere, the schematic diagram show as below. If player movement is out of range, then restrict player to sphere max radius range.
How can I write C# code to implement it, like this?
These are my current steps:
Create 3D sphere
Create C# code append to sphere object
My code so far:
public Transform player;
void update(){
Vector3 pos = player.position;
}

I don't know how you calculate your player`s position but before assigning the new position to the player you should check and see if the move is eligible by
checking the new position distance form the center of the sphere
//so calculate your player`s position
//before moving it then assign it to a variable named NewPosition
//then we check and see if we can make this move then we make it
//this way you don't have to make your player suddenly stop or move it
//back to the bounds manually
if( Vector3.Distance(sphereGameObject.transform.position, NewPosition)< radius)
{
//player is in bounds and clear to move
SetThePlayerNewPosition();
}

What #Milad suggested is right but also include the fact you won't be able to "slide" on the sphere border if your movement vector even slightly goes outside the sphere :
(sorry for the crappy graphic skills...)
What you can do if you want to be able to "slide" on the sphere interior surface is get the angle formed between the player position and the X vector and then apply this angle with the :
public Transform player;
public float sphereRadius;
void LateUpdate()
{
Vector3 pos = player.position;
float angle = Mathf.Atan2(pos.y, pos.x);
float distance = Mathf.Clamp(pos.magnitude, 0.0f, sphereRadius);
pos.x = Mathf.Cos(angle) * distance;
pos.y = Mathf.Sin(angle) * distance;
player.position = pos;
}
Just make sure using this won't counter effect your player movement script (that's why I put it in LateUpdate() in my example).

Getting distance between point of an object and another object

First, I'd like to apologize because this is a very basic and repetitive question, but I am completely new to game development.
I understand that I can find a distance between two objects using:
float dist = Vector3.Distance(other.position, transform.position);
However, how can I find the distance between a point of one object to other object?
For instance let's say my object is this sphere
Now, how can I return an array that says that there are no objects to the left (null), in the front there is an object at 1, and to the right there is an object at 0.5?
Thank you for your patience and understanding

I'm not exactly sure what you wan't to achieve...
If you wan't to get potential objects at 0.5, 1, 1.5, etc. on lets say the Z Axis you probably would want to do this with raycasting.
If you wish to check for any objects returning the direction dependant to the Z Axis (0.5, 0.856, 1.45, etc) in contrast, you probably would either
use a scaled sphere collider and add the colliding object with the OnCollisionEnter Callback to the array/List
iterate through every object of the scene and check it's relative pos
Use Raycast Probes procedurally (using a density float and Raycasting every density offset on the Z Axis and checking if the ray hit anything ...)
...
Totally dependant on your case of use, and whether you use 2D or 3D.
Seneral
EDIT: Here's what you would want in 2D to get a wall withing range maxRange on the layer optionalWallLayer in direction y 1 (up in unity 2D)
float maxRange = 10.0f;
RaycastHit hit;
Vector3 dir = transform.TransformDirection(new Vector3 (0, 1, 0));
if (Physics.Raycast(transform.position, dir, maxRange, out hit, LayerMask optionalWallLayer))
{
Debug.Log ("Wall infront of this object in Range" + hit.distance);
// hit contains every information you need about the hit
// Look into the docs for more information on these
}
This would likely go into the Update Function of your Sphere's MonoBehaviour.
The option with the Sphere collider is useful if you are not sure if you are going to hit your obstacles. If these are small, you would likely want to add the said Sphere Collider as a component to your sphere, scale it up to your maximum distance, and add a MonoBehaviour script to the sphere, which would contain something like this:
public List<Transform> allObjectsInRange; // GameObjects to enclose into calculations, basically all which ever entered the sphere collider.
public List<float> relatedDistances;
void Update () {
// basic loop to iterate through each object in range to update it's distance in the Lists IF YOU NEED...
for (int cnt = 0; cnt < allObjectsInRange.Count; cnt++) {
relatedDistances[cnt] = Vector2.Distance (transform.position, allObjectsInRange[cnt].position);
}
}
// Add new entered Colliders (Walls, entities, .. all Objects with a collider on the same layer) to the watched ones
void OnCollisionEnter (Collision col) {
allObjectsInRange.Add (col.collider.transform);
relatedDistances.Add (Vector2.Distance (transform.position, col.collider.transform));
}
// And remove them if they are no longer in reasonable range
void OnCollisionExit (Collision col) {
if (allObjectsInRange.Contains (col.collider.transform)) {
relatedDistances.RemoveAt (allObjectsInRange.IndexOf (col.collider.transform));
allObjectsInRange.Remove (col.collider.transform);
}
}
That should do it, either of the options, based on your exact case, again:) Note both are pseudo-codes... Hope it helps!