In Swift 1 and 2, closure parameters were escaping by default. If you knew your closure wouldn’t escape the function body, you could mark the parameter with the @noescape attribute.

In Swift 3, it’s the other way around: closure parameters are non-escaping by default. If you intend for it to escape the function, you have to mark it with the @escaping attribute.

class ClassOne {
  // @noescape is applied here as default
  func methodOne(completion: () -> Void) {
    // 
  }
}

class ClassTwo {
  let obj = ClassOne()
  var greeting = "Hello, World!"

  func methodTwo() {
    obj.methodOne() {
      // self.greeting is required
      print(greeting)
    }
  }
}

In Swift 1 and 2, closure parameters were escaping by default. If you knew your closure wouldn’t escape the function body, you could mark the parameter with the @noescape attribute.

In Swift 3, it’s the other way around: closure parameters are non-escaping by default. If you intend for it to escape the function, you have to mark it with the @escaping attribute.

class ClassOne {
  // @noescape is applied here as default
  func methodOne(completion: () -> Void) {
    // 
  }
}

class ClassTwo {
  let obj = ClassOne()
  var greeting = "Hello, World!"

  func methodTwo() {
    obj.methodOne() {
      // self.greeting is required
      print(greeting)
    }
  }
}