Strange behaviour of optionals in Swift 3 - json

Question

I have experienced a strange behaviour when parsing JSON data using Swift 3.
do {
let json = try JSONSerialization.jsonObject(with: data!, options: []) as! NSDictionary
let items:[AnyObject] = (json["items"] as? [AnyObject])!
for item in items {
let id:String = item["id"] as! String
print("ID: \(id)")
let info = item["volumeInfo"] as AnyObject
print(info)
let title = info["title"]
print(title)
}
} catch {
print("error thrown")
}
This produces the following output. Notice that info is an optional but if I try to unwrap it it states it is not an optional! The script crashes on let title = info["title"] As a result I can't access the title key. This behaviour has changed since Swift 2.
ID: lbvUD6LUyV8C
Optional({
publishedDate = 2002;
publisher = "Sams Publishing";
title = "Java Deployment with JNLP and WebStart";
})

Answer 1

You can do something like:
do {
let json = try JSONSerialization.jsonObject(with: data!) as! [String: Any]
let items = json["items"] as! [[String: Any]]
for item in items {
let id = item["id"] as! String
let info = item["volumeInfo"] as! [String: Any]
let title = info["title"] as! String
print(id)
print(info)
print(title)
}
} catch {
print("error thrown: \(error)")
}
I might suggest excising the code of the ! forced unwrapping (if the JSON was not in the form you expected, do you really want this to crash?), but hopefully this illustrates the basic idea.

Answer 2

The runtime type of info is Optional<Something>, but the compile time type (as you explicitly cast it) is AnyObject. How is the compiler supposed to know that the AnyObject will happen to be an Optional<Something> unless you tell it (in the form of a cast)?
Try:
let info = item["volumeInfo"] as AnyObject?
Now the compiler knows it's an Optional<AnyObject>, so you can unwrap it.

Answer 3

In Swift 3 the compiler must know the types of all subscripted objects if it's an array or dictionary. AnyObject – which has been changed to Any in Swift 3 – is not sufficient.
Since you know that the value for key volumeInfo is a dictionary cast it accordingly preferably using optional bindings
let info = item["volumeInfo"] as? [String:Any] {
print(info)
let title = info["title"] as! String
print(title)
}

Answer 4

This should do:
guard let json = try? JSONSerialization.jsonObject(with: data!, options: []) as! [String: AnyObject],
let items = json["items"] as! Array<AnyObject>? else {
return
}
for item in items {
let id = item["id"] as! String
if let info = item["volumeInfo"] as? [String: AnyObject] {
let title = info["title"] as! String
} else {
// do something
}
}