uitableview - 为什么事件指示器显示两次

Question

我正在实现一个事件指示器，以在获取/加载图像时显示。但是，事件指示器有时会在同一帧中显示两次。

我多次检查了代码，甚至尝试了其他方法，例如与行号匹配的计数器。知道为什么这会出现两次吗？ (见下图)

事件指示器代码(在 cellForRowAtIndexPath 内):

 // start indicator when loading images
 var indicatorPhoto: MaterialActivityIndicatorView! = MaterialActivityIndicatorView(style: .Small)
 indicatorPhoto.center = cell.mainRestaurantImageView.center
 cell.mainRestaurantImageView.addSubview(indicatorPhoto)
 indicatorPhoto!.startAnimating()

 cell.mainRestaurantImageView.loadInBackground {
      (success: UIImage!, error: NSError!) -> Void in
      if ((success) != nil) {
           // stop indicator when loading images
           if indicatorPhoto?.isAnimating == true {
                 indicatorPhoto!.stopAnimating()
                 indicatorPhoto!.removeFromSuperview()
           }
      } else {
           println("Unsuccessful Fetch Image")
           if indicatorPhoto?.isAnimating == true {
                indicatorPhoto!.stopAnimating()
                indicatorPhoto!.removeFromSuperview()
           }
      }
 }

更新:

这是 cellForRowAtIndexPath 代码的其余部分

func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
        let cell = tableView.dequeueReusableCellWithIdentifier("RestaurantCell", forIndexPath: indexPath) as FeedCell

        cell.nameLabel.text = restaurantNames[indexPath.row]
        cell.photoNameLabel.text = photoNames[indexPath.row]
        cell.cityLabel.text = " " + addressCity[indexPath.row]
        cell.distanceLabel?.text = arrayRoundedDistances[indexPath.row] + "mi"

        // check if there are images
        if foodPhotoObjects.isEmpty {  } else {
            var restaurantArrayData = self.foodPhotoObjects[indexPath.row] as PFObject
            cell.mainRestaurantImageView.image = UIImage(named: "") // set placeholder
            cell.mainRestaurantImageView.file = restaurantArrayData["SmPhotoUploaded"] as PFFile

            // start indicator when loading images
            var indicatorPhoto: MaterialActivityIndicatorView! = MaterialActivityIndicatorView(style: .Small)
            indicatorPhoto.center = cell.mainRestaurantImageView.center
            cell.mainRestaurantImageView.addSubview(indicatorPhoto)
            indicatorPhoto!.startAnimating()

            cell.mainRestaurantImageView.loadInBackground {
                (success: UIImage!, error: NSError!) -> Void in
                if ((success) != nil) {
                    // stop indicator when loading images
                    if indicatorPhoto?.isAnimating == true {
                        indicatorPhoto!.stopAnimating()
                        indicatorPhoto!.removeFromSuperview()
                    }
                } else {
                    println("Unsuccessful Fetch Image")
                    if indicatorPhoto?.isAnimating == true {
                        indicatorPhoto!.stopAnimating()
                        indicatorPhoto!.removeFromSuperview()
                    }
                }
            }

            cell.mainRestaurantImageView.contentMode = .ScaleAspectFill
            cell.mainRestaurantImageView.clipsToBounds = true

        }

        return cell
    }

更新2

// create indicator when loading images
        var indicatorPhoto : MaterialActivityIndicatorView? = cell.mainRestaurantImageView.viewWithTag(123) as? MaterialActivityIndicatorView;
        if indicatorPhoto == nil{
            indicatorPhoto = MaterialActivityIndicatorView(style: .Small)
            indicatorPhoto!.center = cell.mainRestaurantImageView.center
            indicatorPhoto!.tag = 123
            cell.mainRestaurantImageView.addSubview(indicatorPhoto!)
            indicatorPhoto!.startAnimating()
        }

Answer 1

此显示多次，因为您多次添加它。事实上，每次调用 foodPhotoObjects.isEmpty 的 else 情况时。

这是因为，你的方法的第一行:

let cell = tableView.dequeueReusableCellWithIdentifier("RestaurantCell", forIndexPath: indexPath) as FeedCell

使单元格从 TableView 中出列。出列工作如下:

1. 它根据标识符维护一个队列。
2. 如果队列中没有单元格，则会创建一个新单元格。
3. 如果已有一个单元格，它会将该单元格返回给您。它将被重复使用。

因此，您所做的是，每次都将 MaterialActivityIndi​​catorView 添加到单元格中，无论之前是否添加过。

解决方案:

1. 从 xib 将自定义 View 添加到您的单元格，并将其类设置为MaterialActivityIndi​​catorView。并在此处获取引用隐藏/显示和动画。
2. 检查cell.mainRestaurantImageView的 subview ，看看是否已经有一个 MaterialActivityIndi​​catorView，获取其引用并做动画之类的。如果没有 MaterialActivityIndi​​catorView subview ，请创建一个 subview 并将其作为 subview 添加到 ImageView 中。为此，您将使用标签属性。

第二种方法可以这样做:

//first find the activity indication with tag 123, if its found, cast it to its proper class
var indicatorPhoto : MaterialActivityIndicatorView? = cell.mainRestaurantImageView.viewWithTag(123) as? MaterialActivityIndicatorView;
if indicatorPhoto == nil{
    //seems it wasn't found as subview initialize here and add to mainRestaurantImageView with tag 123
}
//do rest of the stuff.