如何在Swift中的函数中为通用参数返回默认值？

Question

I'm using generics to dynamically assign the type of the value that the response should be, at the call site of the function.

• The function below is the only accessible method my FirebaseClient class has.
• It is called in many different places.
• To make sure the correct object is in the response, I insert what object I'm expecting at the call site of the function. Here is the catch. If I'm posting a value to database, I dont expect any response, and only care about error, but am forced to put an arbitrary type for param/argument, even though i know it will be nil for such cases.

How can I set a default value for generic parameter, so that no one could hypothetically come along and insert an object as response when they won't get one?

Hopefully the code speaks for itself and makes more sense than I did.

Other info:

Inside the method below, if its a GET request, I initialize T with the json response. As you see the generic has to conform to QueryType protocol, which just contains an initializer, so each object that is T, has an initializer to handle response in its own way.

func fireRequest<T: QueryType>(_ request: FirebaseRequest, completion: @escaping (_ data: [T]? = [], _ error: FirebaseError?) -> Void ) { 

// setting default value to data param throws error. 

}

Successful use case:

client.fireRequest(FirebaseRequest.observe(path: path), completion: { (data: [User], error: FirebaseError? ) in

// Since I infer 'T' to be of type User, i can expect a collection of users, or single user as my response. This is what I want. 


})

Error Use case:

client.fireRequest(FirebaseRequest.setValue(path: path, value: locationData), completion: { (_, error: FirebaseError? ) in

// error: generic parameter 'T' could not be inferred. 

// In this case, I am posting some data to database, so I dont expect a response, but putting underscore throws error. 

})

Answer 1

I am not exactly sure what you are asking but you can use T() to generate the defualt value for the type.

As for the underscore it is because (_, FirebaseError?)->Void is not the same type as ([T], FirebaseError?)->Void . So your closure should be

{(_:[AnyObject]?, error: FirebaseError?) in code }

If you want this to use generics you would need to define some function like

func errorCaseCompletetion<T>( _ : [T]?, error: FirebaseError?){
     //code
 }

And then you just pass the function name as the completetion block.