Loading images in the background for a table view

Question

I have fetchData method which fills my imagesArray:

func fetchData(){

    let imageQuery = PFUser.query()
    imageQuery?.whereKey("username", containedIn: namesArray)

    imageQuery!.findObjectsInBackgroundWithBlock { (objects: [PFObject]?, error: NSError?) -> Void in

        if error == nil {

            for object in objects! {
                print("\(object["username"] as! String)")

                self.imagesArray.append(object["image"] as! PFFile)
                if self.imagesArray.count == self.namesArray.count {
                    self.tableView.reloadData()
                    print("Names Order: \(self.namesArray)")
                }
            }
        } else {
            print("error: \(error?.localizedDescription)")
        }
    }
}

And I have tableView:cellForRowAtIndexPath method which uses this array to fill the UIImageView instance:

override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {

    let cell = tableView.dequeueReusableCellWithIdentifier("Cell") as! ChatsCell
    cell.nameLabel.text = namesArray[indexPath.row]

    if imagesArray.count == namesArray.count && self.imagesLoaded == false{
        imagesArray[indexPath.row].getDataInBackgroundWithBlock { (imageData: NSData?, error: NSError?) -> Void in
            if error == nil {
                let image = UIImage(data: imageData!)
                cell.imageView?.image = image
                self.tableView.reloadData()
                self.imagesLoaded = true
            }
        }
    }

    return cell
}

Actually I use if condition for waiting my array imagesArray to be filled by query of another method and I use variable imagesLoaded to prevent infinite call of reloadData method. How can I write this method without if statement?

Answer 1

Is there any reason that imagesArray and namesArray have to be separated? If not, I probably will design a struct and join above two arrays to one since name and image can be fetched at the same time in func fetchData().

For example, let's assume we have a struct like:

struct Person {
    var username: String
    var image: PFFile
}

Then we have a global mutable array:

var items = [Person]()

Inside the for-loop in func fetchData(), we can create a new person and append it each time once we have a name and an image.

let person = Person(username: "myName", image: "myImage")
self.items.append(person)

Therefore, in the func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {}, we can get a person and associated username and image directly according to indexPath.row

let person = self.items[indexPath.row]
let username = person.username
let image = person.image

The benefits like this is that,

• Cell count is restricted by self.items.count; therefore, no worries to take care of indexPath.row beyond an array.

• Also username and image are always paired. It also helps to avoid concerning the order as imagesArray and namesArray are separated. No doubt, the condition imagesArray.count == namesArray.count can be removed as well.

• As to boolean self.imagesLoaded which is able to be removed. Owing to before remote data has been fetched, self.items.count is 0 and none cell will be rendered at that moment.