unexpectedly found nil while unwrapping an Optional value(optional binding) - xcode

Question

import UIKit
import Parse
class HomePageViewController: UIViewController, UITableViewDelegate {
#IBOutlet weak var homPageTableView: UITableView!
var imageFiles = [PFFile]()
var imageText = [String]()
override func viewDidLoad() {
super.viewDidLoad()
// DO any additional setup after loading the view
var query = PFQuery(className: "Posts")
query.orderByAscending("createdAt")
query.findObjectsInBackgroundWithBlock {
(posts : [AnyObject]?, error : NSError?) -> Void in
if error == nil {
//success fetxhing objects
println(posts?.count)
for post in posts! {
self.imageFiles.append(post["imageFile"] as! PFFile) ---------error here
self.imageText.append(post["imageText"] as! String)
}
println(self.imageFiles.count)
}else{ println(error)
}
}
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
}
}
As a title, It is keep saying "unexpectedly found nil while unwrapping an Optional value" at the line I draw.
There are a lot of questions about this, it is hard to read code for me, And even if I undertood, mine didn't go right.
where should I use optional binding?
And can u explain it with really easy example what is optional binding?
Thank you

Answer 1

Your post dictionary does not contain an imageFile key. When you access a dictionary using post["imageFile"] the result is the value (if the key exists) and nil (if the key does not exist). You can distinguish between these cases using
if let imageFile = post["imageFile"],
let imageText = post["imageText"] {
self.imageFiles.append(imageFile as! PFFile)
self.imageText.append(imageText as! String)
} else {
print("imageFile and/or imageText missing from \(post)")
}

Answer 2

You need to unwrap your posts before looping through them because posts: [AnyObject]? is optional.
if error == nil {
if let postData = posts{
//success fetxhing objects
println(postData?.count)
for post in postData! {
self.imageFiles.append(post["imageFile"] as! PFFile)
self.imageText.append(post["imageText"] as! String)
}
println(self.imageFiles.count)
}
}else{
println(error)
}