CODEWITHSUNDEEP
iosswift
cch
cch

Reputation: 3386

Downcast from 'String?' to 'String' only unwraps optionals

I am trying to read a .mid file from the DocumentDirectory. I tried this: 

var path = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true).first as! String
var midiFileURL = NSURL.fileURLWithPath(path)

The problem is:

Downcast from 'String?' to 'String' only unwraps optionals; did you mean to use '!'?

Once I read the file from the DocumentsDirectory I want to be able to play it back with this: 

guard let bankURL = NSBundle.mainBundle().URLForResource("gs_instruments", withExtension: "dls") else {
    fatalError("\"gs_instruments.dls\" file not found.")
}

do {
    try self.midiPlayer = AVMIDIPlayer(contentsOfURL: midiFileURL, soundBankURL: bankURL)
    print("created midi player with sound bank url \(bankURL)")
} catch let error as NSError {
    print("Error \(error.localizedDescription)")
}

self.midiPlayer.prepareToPlay()

self.midiPlayer.play({
    print("midi done")
})

How do I fix the error? Many thanks.

Upvotes: 3

Views: 1498

Answers (1)

Alexander
Alexander

Reputation: 63272

NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true).first is returning you an Optional<String> (a.k.a. String?). You're trying to cast this to String. The warning is telling you that you're doing this in a cumbersome way. You can just force unwrap it with !:

var path = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true).first!

Of course, because you're force unwrapping, you risk your application crashing in case the value returned is nil (in the case that there is no first... when the collection is empty).

I highly advise you read about Optionals in the Swift Language Guide - The Basics.

Upvotes: 9

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