Swift：如何刷新tableview沒有滯後（UIRefreshControl）

Question

使用PHP和JSON填充我的表視圖與來自MYSQL數據庫的對象Swift 3.我有一個拉下來刷新功能，但當我拉下來刷新它在中途停留一秒鐘，然後繼續（如輪子不會旋轉一秒鐘）。

如何更新我的tableview更平滑，因爲我猜測，因爲我將更多的內容添加到數據庫中，未來滯後更大。我目前在我的數據庫中有12個對象，所以想象100個對象。

在viewDidLoad中

// Pull to Refresh 
    let refreshControl = UIRefreshControl() 
    refreshControl.addTarget(self, action: #selector(handleRefresh), for: .valueChanged) 
    if #available(iOS 10.0, *) { 
     myTableView.refreshControl = refreshControl 
     print("iOS 10") 
    } else { 
     myTableView.addSubview(refreshControl) 
     print("iOS 9 or iOS 8") 
    } 

然後拉刷新功能

// Pull to Refresh 
func handleRefresh(refreshControl: UIRefreshControl) { 

    // Fetching Data for TableView 
    retrieveDataFromServer() 

    // Stop Refreshing 
    refreshControl.endRefreshing() 
} 

// Retrieving Data from Server 
func retrieveDataFromServer() { 

    // Loading Data from File Manager 
    loadData() 

    let getDataURL = "http://example.com/receiving.php" 
    let url: NSURL = NSURL(string: getDataURL)! 

    do { 
     let data: Data = try Data(contentsOf: url as URL) 
     let jsonArray = try JSONSerialization.jsonObject(with: data, options: .mutableContainers) as! NSMutableArray 

     // Clear the arrays 
     self.followedArray = [Blog]() 
     self.mainArray = [Blog]() 

     // Looping through jsonArray 
     for jsonObject in jsonArray { 

      if let blog = Blog(jsonObject:jsonObject as! [String : Any]) { 

       // Check if Identifiers Match 
       if followedIdentifiers.contains(blog.blogID) { 
        self.followedArray.append(blog) 
       } else { 
        self.mainArray.append(blog) 
       } 
      } 
     } 
    } catch { 
     print("Error: (Retrieving Data)") 
    } 
    myTableView.reloadData() 
} 

Answer 1

Spinner.isHidden = false 
    Spinner.startAnimating() 
    DispatchQueue.global(qos: .background).async { 
     loadData() 

    let getDataURL = "http://example.com/receiving.php" 
    let url: NSURL = NSURL(string: getDataURL)! 

    do { 
     let data: Data = try Data(contentsOf: url as URL) 
     let jsonArray = try JSONSerialization.jsonObject(with: data, options: .mutableContainers) as! NSMutableArray 

     // Clear the arrays 
     self.followedArray = [Blog]() 
     self.mainArray = [Blog]() 

     // Looping through jsonArray 
     for jsonObject in jsonArray { 

      if let blog = Blog(jsonObject:jsonObject as! [String : Any]) { 

       // Check if Identifiers Match 
       if followedIdentifiers.contains(blog.blogID) { 
        self.followedArray.append(blog) 
       } else { 
        self.mainArray.append(blog) 
       } 
      } 
     } 
    } catch { 
     print("Error: (Retrieving Data)") 
    } 

     DispatchQueue.main.async 
     { 

      myTableView.reloadData() 
      self.Spinner.startAnimating() 
      self.Spinner.isHidden = true 
     } 
    } 

Answer 2

參考蘋果樣本代碼在以下位置：

http://developer.apple.com/library/ios/#samplecode/LazyTableImages/Introduction/Intro.html

夫婦建議的：

不要在cellForRowAtIndexPath：method中顯示數據，因爲此時單元格尚未顯示。嘗試在UITableView的委託中使用tableView：willDisplayCell：forRowAtIndexPath：方法。

即使您需要顯示更多，也可重複使用單元/頁眉/頁腳的單個實例。

讓我知道是否需要任何具體的東西。

Answer 3

我的猜測是你的retrieveDataFromServer（）阻塞了主線程，因此造成了滯後。嘗試在一個異步塊

// Pull to Refresh 
func handleRefresh(refreshControl: UIRefreshControl) { 

    // Fetching Data for TableView 
    retrieveDataFromServer { [weak refreshControl] in 
     // This block will run once retrieveDataFromServer() is completed 

     // Reload data 
     myTableView.reloadData() 

     // Stop Refreshing 
     refreshControl?.endRefreshing() 
    } 
} 

// Retrieving Data from Server 
func retrieveDataFromServer(completion: (() -> Void)?) { 

    // Loading Data from File Manager 
    loadData() 

    let getDataURL = "http://example.com/receiving.php" 
    let url: NSURL = NSURL(string: getDataURL)! 

    do { 
     let data: Data = try Data(contentsOf: url as URL) 
     let jsonArray = try JSONSerialization.jsonObject(with: data, options: .mutableContainers) as! NSMutableArray 

     // Clear the arrays 
     self.followedArray = [Blog]() 
     self.mainArray = [Blog]() 

     // Looping through jsonArray 
     for jsonObject in jsonArray { 

      if let blog = Blog(jsonObject:jsonObject as! [String : Any]) { 

       // Check if Identifiers Match 
       if followedIdentifiers.contains(blog.blogID) { 
        self.followedArray.append(blog) 
       } else { 
        self.mainArray.append(blog) 
       } 
      } 
     } 
    } catch { 
     print("Error: (Retrieving Data)") 
    } 

    // Calls completion block when finished 
    completion?() 
} 

Answer 4

包裝它，我設想在主線程同步執行您遇到的滯後是由於網絡請求：

let data: Data = try Data(contentsOf: url as URL) 

網絡請求是緩慢的，應該差不多了當然要完成主線程。這裏的解決方案是將網絡調用移動到後臺線程，以便主線程（UI）不會被阻塞（延遲）。

那麼你怎麼做到這一點？那麼這是一個有很多不同答案的大問題。

我強烈建議你花一些時間在Swift中學習多線程編程（也稱爲併發）。通過這個Ray Wenderlich tutorial應該給你一個很好的基礎。

然後，瞭解用於在iOS應用中執行異步網絡請求的URLSession可能是個好主意。雷文德里赫又一次很棒starter tutorial。

終於...這是一個快速和骯髒的解決方案。這是「哈克」，你可能不應該使用它，但它可能會解決您的滯後問題：

func retrieveDataFromServer() { 

    // Loading Data from File Manager 
    loadData() 

    let getDataURL = "http://example.com/receiving.php" 
    let url: NSURL = NSURL(string: getDataURL)! 

    // Move to a background queue to fetch and process data from network. 
    DispatchQueue.global().async { 

     // Don't touch anything related to the UI here. 
     do { 
      let data: Data = try Data(contentsOf: url as URL) 
      let jsonArray = try JSONSerialization.jsonObject(with: data, options: .mutableContainers) as! NSMutableArray 

      // Create new temp arrays to process json 
      var tempFollowedArray = [Blog]() 
      var tempMainArray = [Blog]() 

      // Looping through jsonArray 
      for jsonObject in jsonArray { 

       if let blog = Blog(jsonObject:jsonObject as! [String : Any]) { 

        // Check if Identifiers Match 
        if self.followedIdentifiers.contains(blog.blogID) { 
         tempFollowedArray.append(blog) 
        } else { 
         tempMainArray.append(blog) 
        } 
       } 
      } 

      // Jump back to main (UI) thread to update results 
      DispatchQueue.main.async { 
       print("success") 
       self.followedArray = tempFollowedArray 
       self.mainArray = tempMainArray 

       self.myTableView.reloadData() 
      } 
     } catch { 
      DispatchQueue.main.async { 
       print("Error: (Retrieving Data)") 
       // This reload is probably not necessary, but it was 
       // in your original code so I included it. 
       self.myTableView.reloadData() 
      } 
     } 
    } 
}