Swiftで磁気センサの値を取得する方法 ref: http://qiita.com/yesiwas/items/5f70bf56f572e39c4cb7

file0.swift

//CoreMotionマネージャを作成
let cmManager = CMMotionManager()

file1.swift

cmManager.magnetometerUpdateInterval = 0.1

file2.swift

//キューで実行するクロージャ
let handler:CMMagnetometerHandler = {(magnetoData:CMMagnetometerData?,error:NSError?) -> Void in
    self.showMagnetoData(magnetoData,error: error)
}
//キューを登録し、スタート
cmManager.startMagnetometerUpdatesToQueue(NSOperationQueue.mainQueue(), withHandler: handler)

file3.swift

func showMagnetoData(magnetoData:CMMagnetometerData?, error:NSError?){
        if let data = magnetoData {

            var x = data.magneticField.x
            var y = data.magneticField.y;
            var z = data.magneticField.z;

            ...

ViewController.swift

import UIKit
import CoreMotion

class ViewController: UIViewController {

    @IBOutlet weak var xLabel: UILabel!
    @IBOutlet weak var yLabel: UILabel!
    @IBOutlet weak var zLabel: UILabel!

    let cmManager = CMMotionManager()

    override func viewDidLoad() {
        super.viewDidLoad()

        cmManager.magnetometerUpdateInterval = 0.1


        let handler:CMMagnetometerHandler = {(magnetoData:CMMagnetometerData?,error:NSError?) -> Void in
            self.showMagnetoData(magnetoData,error: error)
        }
        cmManager.startMagnetometerUpdatesToQueue(NSOperationQueue.mainQueue(), withHandler: handler)

    }

    func showMagnetoData(magnetoData:CMMagnetometerData?, error:NSError?){
        if let data = magnetoData {

            var x = data.magneticField.x
            var y = data.magneticField.y;
            var z = data.magneticField.z;

            x = round(x*100)/100
            y = round(y*100)/100
            z = round(z*100)/100

            xLabel.text = String(x)
            yLabel.text = String(y)
            zLabel.text = String(z)

        }
    }
}