ios - Can swift closures be set to a default value when used as a parameter in a function?

Question

A pretty handy feature of Swift functions is that function parameters can have default values:

func someFunction(parameterWithDefault: Int = 42) {
    //if no arguments are passed to the function call,
    //value of parameterWithDefault is 42
}

If a parameter is a closure, is there a way to make it have a default value? See the example below:

func sendBody(
    body: NSData? = nil,
    success: (data: NSData) -> Void,
    failure: (data: NSData?) -> Void) {
}

Is there a way to not force the developer to pass a value for success or failure when calling sendBody?

Answer 1

Yes, functions are just values, so you can supply them as defaults

// just to show you can do it with inline closures or regular functions
func doNothing<T>(t: T) -> Void { }

func sendBody(
    body: NSData? = nil,
    success: (data: NSData) -> Void = { _ in return },
    failure: (data: NSData?) -> Void = doNothing
)
{  }

Alternatively, you could make them optional, that way you can detect if the caller passed one:

func sendBody(
    body: NSData? = nil,
    success: ((NSData) -> Void)? = nil,
    failure: ((NSData?) -> Void)? = nil
    )
{ success?(NSData()) }

sendBody(success: { _ in print("ah, yeah!") })

Also worth noting if you’re doing this: if the caller uses the trailing closure syntax, this will be the last closure in the argument list. So you want the last one to be the one the user is most likely to want to supply, which is probably the success closure:

func sendBody(
    body: NSData? = nil,
    success: ((NSData) -> Void)? = nil,
    failure: ((NSData?) -> Void)? = nil
    )
{
    if success != nil { print("passed a success closure") }
    if failure != nil { print("passed a failure closure") }
}

// this prints "passed a failure closure"
sendBody { data in
    print("which closure is this?")
}

Other than this, the order in the function declaration doesn’t matter to the caller – defaulted arguments can be supplied in any order.