1-distance_between_nodes.swift

import SceneKit

// Part 1: Distance
extension SCNVector3 {
  func distance(to vector: SCNVector3) -> Float {
    return simd_distance(simd_float3(self), simd_float3(vector))
  }
}

2-direction_between_nodes.swift

import SceneKit

// Part 2: Angle
extension SCNVector3 {
    
    /// Calculate the magnitude of this vector
    var magnitude:SCNFloat {
        get {
            return sqrt(dotProduct(self))
        }
    }
    
    /**
     Calculate the dot product of two vectors
     
     - parameter vectorB: Other vector in the calculation
     */
    func dotProduct(_ vectorB:SCNVector3) -> SCNFloat {
        return (x * vectorB.x) + (y * vectorB.y) + (z * vectorB.z)
    }
    
    /**
     Calculate the angle between two vectors
     
     - parameter vectorB: Other vector in the calculation
     */
    func angleBetweenVectors(_ vectorB:SCNVector3) -> SCNFloat {
        
        //cos(angle) = (A.B)/(|A||B|)
        let cosineAngle = (dotProduct(vectorB) / (magnitude * vectorB.magnitude))
        return SCNFloat(acos(cosineAngle))
    }
}

3-rotating_weapons.swift

import SceneKit
// Part 3: Rotating weapons

/// Changes the node's orientation so that its local forward vector points toward the specified location.
myWeapon.look(at: targetLocationVector)
// https://developer.apple.com/documentation/scenekit/scnnode/2867394-look

/// Changes the node's orientation so that the specified forward vector points toward the specified location.
myWeapon.look(at: targetLocationVector, up: upLocationVector, localFront: localFrontVector)
// https://developer.apple.com/documentation/scenekit/scnnode/2867396-look