ios - 更新 UICollectionView 中的 UILabel

Question

这是我的 collectionView 代码:

  func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCellWithReuseIdentifier("Cell", forIndexPath: indexPath)
    let cellWidth = self.view.frame.size.width / 3 - 15
    cell.layer.cornerRadius = cellWidth / 15

    // Fetch final image - if it exists
    if let finalImage = collectedRows[indexPath.row]["picture"] as? PFFile {
      finalImage.getDataInBackgroundWithBlock {
        (imageData: NSData?, error: NSError?) -> Void in
        if error == nil {
          if let imageData = imageData {

            let imageView = UIImageView(frame: CGRectMake(0, 0, cell.frame.size.width, cell.frame.size.height))
            imageView.image = UIImage(data: imageData)
            imageView.contentMode = UIViewContentMode.ScaleToFill
            cell.addSubview(imageView)

            if #available(iOS 8.0, *) {
                if !UIAccessibilityIsReduceTransparencyEnabled() {
                  cell.backgroundColor = UIColor.clearColor()

                  let blurEffect = UIBlurEffect(style: UIBlurEffectStyle.Light)
                  let blurEffectView = UIVisualEffectView(effect: blurEffect)
                  //always fill the view
                  blurEffectView.frame = cell.bounds
                  blurEffectView.autoresizingMask = [.FlexibleWidth, .FlexibleHeight]

                  cell.addSubview(blurEffectView)
            } else {
                  let overLayer = UIView(frame: CGRectMake(0, 0, cell.frame.size.width, cell.frame.size.height))
                  overLayer.backgroundColor = UIColor.blackColor()
                  overLayer.alpha = 0.75
                  cell.addSubview(overLayer)
                }
            } else {
                // Fallback on earlier versions
              let overLayer = UIView(frame: CGRectMake(0, 0, cell.frame.size.width, cell.frame.size.height))
              overLayer.backgroundColor = UIColor.blackColor()
              overLayer.alpha = 0.75
              cell.addSubview(overLayer)
            }

            //Create time Left label

            if let sentDate = self.collectedRows[indexPath.row]["sentAt"] as? NSDate {
              print("sentDate: \(sentDate)")
              let calendar = NSCalendar.currentCalendar()

              if #available(iOS 8.0, *) {
                let expiryDate = calendar.dateByAddingUnit(.Day, value: 1, toDate: sentDate, options: NSCalendarOptions())
                let currentDate = NSDate()
                let differenceTime = calendar.components([.Day, .Hour, .Minute, .Second], fromDate: currentDate, toDate: expiryDate!, options: NSCalendarOptions())
                print("sendDate2: \(differenceTime)")
                let hoursLeft = String(differenceTime.hour)
                let minutesLeft = String(differenceTime.minute)
                let secondsLeft = String(differenceTime.second)

                if (Int(hoursLeft)! <= 0 && Int(minutesLeft)! <= 0 && Int(secondsLeft)! <= 0) {
                  //remove object from parse
                  self.collectedRows[indexPath.row].deleteInBackground()
                  self.collectedRows[indexPath.row].saveInBackground()
                } else {
                  let timerLabel = UILabel(frame: CGRectMake(0, 0, cell.frame.size.width, cell.frame.size.height))
                  timerLabel.font = UIFont(name: "SourceSansPro-Semibold", size: 16)
                  timerLabel.textAlignment = .Center
                  timerLabel.textColor = UIColor.whiteColor()
                  timerLabel.text = "\(hoursLeft):\(minutesLeft):\(secondsLeft)"
                  cell.addSubview(timerLabel)
                  cell.bringSubviewToFront(timerLabel)
                }
              } else {
                // Fallback on earlier versions
                let timerLabel = UILabel(frame: CGRectMake(0, 0, cell.frame.size.width, cell.frame.size.height))
                timerLabel.font = UIFont(name: "SourceSansPro-Semibold", size: 16)
                timerLabel.textAlignment = .Center
                timerLabel.textColor = UIColor.whiteColor()
                timerLabel.numberOfLines = 2
                timerLabel.text = "Less than \n24 Hours Left"
                cell.addSubview(timerLabel)
                cell.bringSubviewToFront(timerLabel)

              }
            }

            if #available(iOS 8.0, *) {
            let deleteButton = UIButton(frame: CGRectMake(cell.frame.size.width/2, cell.frame.size.height * 0.75, cell.frame.size.width/2, cell.frame.size.height/4))
            deleteButton.backgroundColor = UIColor.clearColor()
            deleteButton.setTitleColor(UIColor.whiteColor(), forState: .Normal)
            deleteButton.setTitle("...", forState: .Normal)
            deleteButton.titleLabel!.font = UIFont(name: "SourceSansPro-Bold", size: 32)
            cell.addSubview(deleteButton)
            deleteButton.addTarget(self, action: "removeFromProfile:", forControlEvents: UIControlEvents.TouchUpInside)
            } else {
              // DO NOT even show button
            }
            cell.layer.cornerRadius = cell.frame.size.width / 15
            cell.layer.masksToBounds = true
          }
        }
      }
    }
    return cell
  }

我正在创建一个 UICollectionViewCell，其中有一张模糊的照片，顶部有一个 UILabel，它是 24 小时倒计时标签。一切都很好。但是，我希望 UILabel 每秒更新一次，以便它不断倒计时。我知道我可以创建一个 NSTimer 来每秒更新整个 UICollectionView (使用 collection.reloadData() )。这显然会导致内存问题并使应用程序崩溃。有谁知道如何仅更新标签？或者有谁知道如何从此方法中提取标签创建，然后将标签应用到 IndexPath 处的适当单元格，然后使用 NSTimer.scheduledWithInterval 调用新方法(标签创建)？标记的部分:

//Create time Left label

这是我每秒都需要更新的唯一部分。

Answer 1

我建议在您的类中创建一个弱属性来保存对 timerLabel 的引用，并在您的 cellForItemAtIndexPath 函数中设置该属性。

您的计时器函数应该能够使用该属性来更新标签，而无需重新加载 Collection View (只要它仍然是 View 层次结构的一部分)。