How to add @noescape annotation to optional closure

问题

My function has this signature:

func foo(bar: String, baz: ((String) -> ())? = nil)

And now I want to make unecessary to escape self inside the given closure. But when I try this:

func foo(bar: String, @noescape baz: ((String) -> ())? = nil)

The compiler complains:

@noescape may only be applied to parameters of function type

Is it possible to use it in optional parameters?

回答1:

Requirements

If your requirements are the following:

the baz param is a closure
the baz param is marked with @noescape (because you want to omit self in the closure code)
the baz param can be omitted during the invocation of foo

Solution

Then you can use the following syntax

func foo(bar: String, @noescape baz: ((String) -> ()) = { _ in } ) {

}

As you can see the main difference from your code is that:

here baz is not an optional type (but it's an "optional parameter")
and its default value is an empty closure not a nil value.

Examples

As you requested you can now pass a closure to baz without the need of using self

class Boo {
    let world = "world"
    func boo() {
        foo("hello") { (something) -> () in
            print(world)
        }
    }
}

And you can also omit the baz param

class Boo {
    let world = "world"
    func boo() {
        foo("hello")
    }
}

Update: using a closure with return type different from Void

In a comment below users TadeasKriz asked about how to use this approach with a closure having the return value different the Void.

Here it is the solution

func foo(bar: String, @noescape baz: ((String) -> (Int)) = { _ in return 0 } ) {

}

Here the baz param does required a closure with 1 param of type String and a return value of type Int. As you can see I added a default value to the param, a closure that does return 0. Please note that the default closure will never be used so you can replace 0 with any Int value you want.

Now you can decide whether to use pass your closure to the baz param

class Boo {
    let world = "world"
    func boo() {
        foo("hello") { (something) -> Int in
            print(world)
            return 100
        }
    }
}

Or, again, you can totally omit the baz param.

class Boo {
    let world = "world"
    func boo() {
        foo("hello")
    }
}

来源：https://stackoverflow.com/questions/33779777/how-to-add-noescape-annotation-to-optional-closure

标签

ios

swift

closures