In Swift can you trap "fatal error unexpectedly found nil while unwrapping an optional value"? - swift

Question

I'm parsing an NSDictionary. I don't want to go thru item by item to make sure everything is really there and nothing is unexpectedly nil. I figured I'd just do-try-catch. But I'm getting a compiler warning. Here's my code: saying:
do {
try adminMsg = NSDictionary(objects: [rawMsg["msg"]!["Title"]!!,
rawMsg["msg"]!["Text"]!!,
rawMsg["msg"]!["Time"]!!],
forKeys: ["Title", "Text", "Time"])
} catch {
adminMsg = nil
}
But I get this warning:
"no calls to throwing function occur within 'try' expression"
Does this mean I have no choice but to crash if an item is missing from the dictionary? I can't trap it and let my code gracefully tell the sender they sent me an invalid NSDictionary (unless I check it all out item-by-item in code)?

Answer 1

I can't trap it and let my code gracefully tell the sender they sent me an invalid NSDictionary
You cannot trap a fatal error, no. What you can do is catch the missing elements by using optional binding.
guard let title = message["Title"],
let text = message["Text"],
let time = message["Time"]
else {
// Missing data; inform caller
}
This is how you gracefully tell the client that there's something wrong with the data. You could return nil, an empty dictionary, or throw an error: whichever suits you best.
To make this properly Swifty, you should first define your data:
typealias RawMessage = Dictionary<String, [String : AnyObject]>
/** Important keys in a `RawMessage` */
enum RawMessageKey : String
{
case message = "msg"
case title = "Title"
case text = "Text"
case time = "Time"
}
enum RawMessageError : ErrorType
{
/** The `RawMessage` has no "message" key */
case NoMessage
/** The `RawMessage` is missing an expected inner key */
case MissingKey
}
Then your extraction function. This uses optional binding, not force unwrapping, to check that the key "msg" is present. Force unwrapping failure cannot be "caught"; that's not what it's for.
If the key is not present, you signal that to the caller by throwing your own error. Then use further optional binding to get the rest of the items. If any are missing, again throw an error.
/** Pull important values from `RawMessage` and repackage as `NSDictionary` */
func extractAdminMessage(rawMsg: RawMessage) throws -> NSDictionary
{
guard let message = rawMsg[String(RawMessageKey.message)] else {
throw RawMessageError.NoMessage
}
let keys = [String(RawMessageKey.title),
String(RawMessageKey.text),
String(RawMessageKey.time)]
guard let title = message[String(RawMessageKey.title)],
let text = message[String(RawMessageKey.text)],
let time = message[String(RawMessageKey.time)]
else {
throw RawMessageError.MissingKey
}
return NSDictionary(objects: [title, text, time],
forKeys: keys)
}
If you prefer, using flatMap could be an alternative to the stacked unbind:
let keys = [String(RawMessageKey.title),
String(RawMessageKey.text),
String(RawMessageKey.time)]
let objects = keys.flatMap { message[$0] }
guard objects.count == keys.count else {
throw RawMessageError.MissingKey
}
Here, any missing value will be dropped by flatMap; then if there aren't the same number of keys as objects, signal your caller.

Answer 2

I came up with this:
let rawMsg : NSDictionary =
["msg":["Title":"sometitle", "Text":"sometext", "Time":"sometime"]]
let adminMsg = NSMutableDictionary()
if let msg = rawMsg["msg"] as? NSDictionary {
for key in ["Title", "Text", "Time"] {
adminMsg[key] = msg[key]
}
}
That works even if a key is missing from the "msg" dictionary. You won't crash at any point during that, not matter how malformed rawMsg may be.
(It would be better if adminMsg were a Swift dictionary rather than a Cocoa NSDictionary, but at least this seems to cover the original problem domain.)

Answer 3

A few suggestions
Stop using NSDictionary and use Swift dictionary instead
Don't use exceptions to manage control flow
The force unwrap ! produce a fatal error, you cannot catch it
What you should do instead
if let
msg = rawMsg["msg"],
title = msg["Title"],
text = msg["Text"],
time = msg["Time"] {
let adminMsg:[String:Any] = ["Title": title, "Text": text, "Time": time]
}