Swift Language Closures Closure basics
Example
Closures (also known as blocks or lambdas) are pieces of code which can be stored and passed around within your program.
let sayHi = { print("Hello") }
// The type of sayHi is "() -> ()", aka "() -> Void"
sayHi() // prints "Hello"
Like other functions, closures can accept arguments and return results or throw errors:
let addInts = { (x: Int, y: Int) -> Int in
return x + y
}
// The type of addInts is "(Int, Int) -> Int"
let result = addInts(1, 2) // result is 3
let divideInts = { (x: Int, y: Int) throws -> Int in
if y == 0 {
throw MyErrors.DivisionByZero
}
return x / y
}
// The type of divideInts is "(Int, Int) throws -> Int"
Closures can capture values from their scope:
// This function returns another function which returns an integer
func makeProducer(x: Int) -> (() -> Int) {
let closure = { x } // x is captured by the closure
return closure
}
// These two function calls use the exact same code,
// but each closure has captured different values.
let three = makeProducer(3)
let four = makeProducer(4)
three() // returns 3
four() // returns 4
Closures can be passed directly into functions:
let squares = (1...10).map({ $0 * $0 }) // returns [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
let squares = (1...10).map { $0 * $0 }
NSURLSession.sharedSession().dataTaskWithURL(myURL,
completionHandler: { (data: NSData?, response: NSURLResponse?, error: NSError?) in
if let data = data {
print("Request succeeded, data: \(data)")
} else {
print("Request failed: \(error)")
}
}).resume()
