为什么“打印到控制台”比“.text into label”要快得多

Question

我试图显示CMPedometer的节奏和步速数据。当我使用连接的手机运行应用程序时，它会立即通过print("...")函数将数据输出写入控制台，但需要多次转动，直到它显示UILabel中的数据。 如何尽可能快地获取数据，以便我可以使用它们？

最佳，扎克

import UIKit 
import CoreMotion 

class ViewController: UIViewController { 
    let pedometer = CMPedometer() 
    @IBOutlet weak var paceLabel: UILabel! 
    @IBOutlet weak var cadenceLabel: UILabel! 

    override func viewDidLoad() { 
     super.viewDidLoad() 

     guard CMPedometer.isCadenceAvailable() && CMPedometer.isPaceAvailable() else{ 
      print("Pace and cadence data are not available") 
      return 
     }   
     let oneWeekAgo = NSDate(timeIntervalSinceNow: -(7 * 24 * 60 * 60)) 
     pedometer.startUpdates(from: oneWeekAgo as Date) {data, error in 
      guard let pData = data , error == nil else{ 
       return 
      } 
      //The current pace of the user, measured in seconds per meter. (1 step = 83cm?) 
      if let pace = pData.currentPace{ 
       print("Pace = \(pace)") 
       self.paceLabel.text = "Pace = \(round(Double(pace))*10/10)" 
      } 
      //The rate at which steps are taken, measured in steps per second. 
      if let cadence = pData.currentCadence{ 
       self.cadenceLabel.text = "Cadence = \(cadence))" 
       print("Cadence = \(cadence)") 
      } 
     }// -----------------oneWeekAgo   
    }// -----------------ViewDidLoad 

    override func didReceiveMemoryWarning() { 
     super.didReceiveMemoryWarning() 
    } 
}//-------------------- UIViewController 

Answer 1

更新块被称为后台线程，你需要在主线程上更新UI。将UI更新调用包裹发送回主线程：

Dispatch.main.async { 
    //The current pace of the user, measured in seconds per meter. (1 step = 83cm?) 
    if let pace = pData.currentPace{ 
     print("Pace = \(pace)") 
     self.paceLabel.text = "Pace = \(round(Double(pace))*10/10)" 
    } 

    //The rate at which steps are taken, measured in steps per second. 
    if let cadence = pData.currentCadence{ 
     self.cadenceLabel.text = "Cadence = \(cadence))" 
      print("Cadence = \(cadence)") 
     } 
    } 
}