How to set rotation of object using angle between two points?

Question

I have a LineRenderer that draws a line between two points, Vector3 startPos and Vector3 endPos. How can I get the angle between these points, and apply it to an objects transform rotation, so that it points in that direction?

Here is my code that draws a line between 2 points by dragging your mouse, if you want to test just Replace the Interact methods with Input.MouseButtonDown(1), hold and up.

Vector3 startPos;
Vector3 endPos;
float angle;

LineRenderer lr;

private void Awake()
{
    lr = GetComponent<LineRenderer>();
    lr.useWorldSpace = true;
}

public override void InteractOnDown()
{
    base.InteractOnDown();

    lr.enabled = true;
    lr.positionCount = 2;
    startPos = PointerHandler.RaycastScreenPoint(Input.mousePosition, UnitManager.instance.movementMask, GameManager.instance.cam).point;
    lr.SetPosition(0, startPos);
}

public override void InteractOnHold()
{
    base.InteractOnHold();

    endPos = PointerHandler.RaycastScreenPoint(Input.mousePosition, UnitManager.instance.movementMask, GameManager.instance.cam).point;
    lr.SetPosition(1, endPos);
}

public override void InteractOnUp()
{
    base.InteractOnUp();

    lr.positionCount = 0;
    lr.enabled = false;
}

Answer 1

You should always question whether an angle is actually necessary, or if a vector would suffice.

Here, all we need is the vector from the start to the end:

Vector3 direction = endPos - startPos;

Then you can use that vector to align the forward vector of your transform:

transform.forward = direction;

Or you can compute a new orientation with transform.rotation= Quaternion.LookRotation(direction, localUp) if you want more control over the object's twist around this direction axis.