Detect when your iPhone or iPad is near a magnet or ferrous material

magnetometer-sample.swift

import CoreMotion
import Foundation

let queue: OperationQueue = OperationQueue()
queue.qualityOfService = .utility 

let motionManager: CMMotionManager = CMMotionManager()
motionManager.magnetometerUpdateInterval = 0.5

motionManager.startMagnetometerUpdates(to: queue, withHandler: { (data: CMMagnetometerData?, error: Error?) -> Void in 
    guard let data = data else
    {
        if let error = error
        {
            print("err @ \(#function): \(error.localizedString)")
        }

        return
    }

    // Values units: Microteslas
    print("Magnetic Field. x: \(data.magneticField.x), y: \(data.magneticField.y), z: \(data.magneticField.z)")
    
    let average: Double = (data.magneticField.x + data.magneticField.y + data.magneticField.z) / 3.0
    print("Average: \(average)")
})

// Don't forget to stop magnetometer updates 
// when you don't need it anymore...

// motionManager.stopMagnetometerUpdates()