swift 4 Core Motion Magnetometer magnetometerUpdateInterval似乎没有做任何事情

Question

我不确定我错过了什么，更新间隔设置似乎没有做任何事情。无论我将其设置为3.0还是0.3，我的数字每秒都会更新几次。我没有看到CPU上升或下降太多，所以它看起来不像iOS剪辑......？最新的Xcode和Swift。

import UIKit
import CoreMotion

class ViewController: UIViewController {

@IBOutlet weak var xlabel: UILabel!
@IBOutlet weak var ylabel: UILabel!
@IBOutlet weak var zlabel: UILabel!

let manager = CMMotionManager()
let queue = OperationQueue()

override func viewDidLoad() {
    super.viewDidLoad()

    func setLabel(field: CMMagneticField) {
        DispatchQueue.main.async {
        self.xlabel.text = "\(floor(field.x))"
        self.ylabel.text = "\(floor(field.y))"
        self.zlabel.text = "\(floor(field.z))"}
    }

    if manager.isMagnetometerAvailable {
    manager.magnetometerUpdateInterval = 3.0
        manager.startDeviceMotionUpdates(using: CMAttitudeReferenceFrame.xArbitraryCorrectedZVertical,
            to: queue, withHandler: {(data: CMDeviceMotion?, error: Error?) in
            if let field = data?.magneticField.field {
                setLabel(field: field)}
    })} else {
        print("I guess there's nothing there")
    }
  }
}