如何在Swift中传递参数未知且返回值作为参数的函数

Question

I would like to pass a function as a parameter because I am dealing with web services, and I have noticed that the code is repetitive.

Snippet 1

Service.getAllVouchersUsingCallback() { (response, data, error) -> Void in
    guard let statusCode = response?.statusCode else {
        Util.showToastWithMessage(Messages.NO_INTERNET_CONNECTION)
        return
    }

    switch statusCode {
        case 200:
            self.loadVouchersWithData(data!)
        case 503:
            Util.showToastWithMessage(Messages.SERVICES_NOT_AVAILABLE)
        default:
            Util.showToastWithMessage(Messages.UNEXPECTED_RESPONSE)
    }
}

Snippet 2

Service.getAllCategoriesUsingCallback { (response, data, error) -> Void in

    guard let statusCode = response?.statusCode else {
        Util.showToastWithMessage(Messages.NO_INTERNET_CONNECTION)
        return
    }

    switch statusCode {
        case 200:
            self.loadAndGetCategories(data!, withInialText: "Category ")
        case 503:
            Util.showToastWithMessage(Messages.SERVICES_NOT_AVAILABLE)
        default:
            Util.showToastWithMessage(Messages.UNEXPECTED_RESPONSE)
    }
}

The part that is repetitive is what happens when the status code is nil , and the action I have to perform when the response is 200 . I guess the function signature should be like this:

func dealWithWebServiceResponse(response: NSURLResponse?, withData data: NSData?, whichActionIs action: whateverFunctionType)

So, I would like to know how I can pass whatever function, ie, whatever number of parameters or whatever number of return values because in this case I am passing just data, but probably in the future I will need another kind of function.

Thanks in advance.

Answer 1

This is a great problem to explore functions that return functions. So we have this block of code:

guard let statusCode = response?.statusCode else {
    Util.showToastWithMessage(Messages.NO_INTERNET_CONNECTION)
    return
}

switch statusCode {
case 200:
    // <<================ Right here, we want to do "something different"
case 503:
    Util.showToastWithMessage(Messages.SERVICES_NOT_AVAILABLE)
default:
    Util.showToastWithMessage(Messages.UNEXPECTED_RESPONSE)
}

So how do we do "something different?" We pass a function. That function needs to take "data" because that's the only thing we have. You might be thinking that the function takes "other things" (like "Category "), but it really doesn't. This code doesn't know anything about "Category ". Something else, earlier in the program, has to have dealt with that part. The only thing that varies here is the data. So let's pretend we have that function for a second:

let success: (NSData) -> Void = ...   
...
case 200:
   success(data!)
...

We just want to figure out what success is in this case. Well, in your first example, it's:

{ self.loadVouchersWithData($0) }

and in your second example it's:

{ self.loadAndGetCategories($0, withInialText: "Category ") }

Those are both functions that take an NSData and return nothing, just like we want.

So we want a way to take that first block of code and plug-in this thing that changes. We want a function that takes an "on success" function and returns a "handle all the stuff" function. Let's write that out the long way:

func successHandler(success: (NSData) -> Void) -> (NSHTTPURLResponse?, NSData?, NSError?) -> Void {
    return { (response: NSHTTPURLResponse?, data: NSData?, error: NSError?) in
        guard let statusCode = response?.statusCode else {
            Util.showToastWithMessage(Messages.NO_INTERNET_CONNECTION)
            return
        }

        switch statusCode {
        case 200:
            success(data!) // <==== Here's the part that changes!
        case 503:
            Util.showToastWithMessage(Messages.SERVICES_NOT_AVAILABLE)
        default:
            Util.showToastWithMessage(Messages.UNEXPECTED_RESPONSE)
        }
    }
}

Whoa, that first line is a doozy. Let's look at it:

func successHandler(success: (NSData) -> Void) -> (NSHTTPURLResponse?, NSData?, NSError?) -> Void {

This is a function, that takes a function that takes an NSData and returns nothing, and that whole function returns a function that takes a response,data,error tuple, and returns nothing. Meditate on that for a moment. You really want that to sink in because it's really, really powerful.

OK, hopefully that's starting to sink in a little bit, so I'm going to move on. The syntax is pretty enormous, so Swift gives us a nice trick to simplify it, called currying:

func successHandler(success: (NSData) -> Void)(response: NSHTTPURLResponse?, data: NSData?, error: NSError?) {
    guard let statusCode = response?.statusCode else {
        Util.showToastWithMessage(Messages.NO_INTERNET_CONNECTION)
        return
    }

    switch statusCode {
    case 200:
        success(data!) // <==== Here's the part that changes!
    case 503:
        Util.showToastWithMessage(Messages.SERVICES_NOT_AVAILABLE)
    default:
        Util.showToastWithMessage(Messages.UNEXPECTED_RESPONSE)
    }
}

The declaration is now:

func successHandler(success: (NSData) -> Void)(response: NSHTTPURLResponse?, data: NSData?, error: NSError?) {

(I know that probably doesn't seem much simpler, but it really is, and it definitely makes the rest of the function simpler.)

That is (almost) exactly identical to the previous declaration. Mediate on that line for a moment. Note the f(x: T)(y: U) double-parentheses syntax. Note where I could drop -> Void at the end.

Currying is like passing some parameters now, and being able to pass the rest later.

OK, so how do we use that?

Service.getAllVouchersUsingCallback(successHandler{ self.loadVouchersWithData($0) })
Service.getAllCategoriesUsingCallback(successHandler{ self.loadAndGetCategories($0, withInialText: "Category ") })

We call our thing that wants a (response,data,error) and pass it the result of calling successHandler with a function that takes a data.

And that should remove all the duplication you were talking about. This is a particularly complicated version of currying because there are a lot of levels of functions. But it also shows how powerful the technique is.

You may want to put this aside for a moment and go back to a simpler introduction such as Introduction to Function Currying in Swift . Then, when that makes sense, come back to this.