What is the meaning of keyword "throws" in Swift?

Question

What is meaning of "throws" keyword here:

This code take so long to be executed and i think "throws" keyword in the image above related:

let url = URL(string:"\(APIs.postsImages)\(postImg)")
let imgData = try? Data.init(contentsOf: url)
self.postImage.image = UIImage.init(data: imgData)

Answer 1

init() throws can return more information about the failure and let the caller decide if they care about it or not. Read a useful post about it here.

To indicate that a function, method, or initializer can throw an error, you need to write the throws keyword in the function’s declaration after its parameters. A function marked with throws is called a throwing function. If the function specifies a return type, you write the throws keyword before the return arrow.

func thisFunctionCanThrowErrors() throws -> String

or in real code it might look like this:

enum PossibleErrors: Error {
    case wrongUsername
    case wrongPassword
}

func loggingIn(name: String, pass: String) throws -> String {

    if name.isEmpty { 
        throw PossibleErrors.wrongUsername 
    }
    if pass.isEmpty { 
        throw PossibleErrors.wrongPassword 
    }
    return "Fine"
}

The throwing functions like this must be called using one of the following try operators:

• try
• try?
• try!

Using init() throws you can break the execution of a class initialiser without the need of populating all stored properties:

class TestClass {

    let textFile: String

    init() throws {
        do {
            textFile = try String(contentsOfFile: "/Users/swift/text.txt", 
                                        encoding: NSUTF8StringEncoding)
        catch let error as NSError {
            throw error
        }
    }
}

And in a structure you can even avoid the do/catch block:

struct TestStruct {

    var textFile: String

    init() throws {
        textFile = try String(contentsOfFile: "/Users/swift/text.txt", 
                                    encoding: NSUTF8StringEncoding)
    }
}