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)
    }
  }
}

From Swift Documentarion

@escaping

Apply this attribute to a parameter’s type in a method or function declaration to indicate that the parameter’s value can be stored for later execution. This means that the value is allowed to outlive the lifetime of the call. Function type parameters with the escaping type attribute require explicit use of self. for properties or methods.

class ClassThree {

    var closure: (() -> ())?

    func doSomething(completion: @escaping () -> ()) {
        closure = finishBlock
    }
}

In the above example the completion block is saved to closure and will literally live beyond the function call. So complier will force to mark completion block as @escaping.