How to pass an Error up the stack trace in Swift

Question

In java, if one method throws an error, the method that calls it can pass it on to the next method.

public void foo() throws Exception {
     throw new Excep         


        

Answer 1

A Swift function can call a throwing function and pass an error up to the caller, but

• The function itself must be marked with throws, and
• the throwing function has to be called with try.

Example:

func foo() throws {
    print("in foo")
    throw NSError(domain: "mydomain", code: 123, userInfo: nil)
}

func bar() throws -> String {
    print("in bar")
    try foo()
    return "bar"
}

do {
    let result = try bar()
    print("result:", result)
} catch {
    print(error.localizedDescription)
}

Output:

in bar
in foo
The operation couldn’t be completed. (mydomain error 123.)

If try foo() fails then bar() returns immediately, propagating the error thrown by foo() to its caller. In other words, try foo() inside a throwing function is equivalent to

do {
    try foo()
} catch let error {
    throw error
}