CODEWITHSUNDEEP
iosswiftreactive-cocoa
Dennis Farandy
Dennis Farandy

Reputation: 287

how to call signal producer that depend to different signal producer in reactivecocoa4

I have 2 signal producer like this

func textSignal() -> SignalProducer(String?,NoError)

and

func searchSignal(text:String) -> SignalProducer([User]?,NSError)

how to call searchSignal without nested function? since flatmap & attemptMap need the same error result like this case is NoError and NSError

Upvotes: 0

Views: 516

Answers (1)

Javier Soto
Javier Soto

Reputation: 4870

There are 2 type differences that one must fix to be able to compose both functions.

The original signal can carry nils, and the function you're trying to flatMap it with doesn't accept nils. The type system is telling you that you need to choose a policy as to what to do in those cases. Some options:

  • Filter nils:

textSignal.filter { $0 != nil }.map { $0! }

Not recommended because you'll ignore those values, so if the user searches for "foo", and then the text field produces a nil string, the app would still show the search results for "foo".

  • Make the search function allow nils: this would be easy to do, but you're really just shifting the problem over to the other function, which would have to handle the nil values.

  • Treat nil strings as empty strings

textSignal.map { $0 ?? "" }

This is probably the simplest and the one that produces the most natural results.

The second difference is the error type. The original signal doesn't produce errors, but the second one can. Using the promoteErrors function we can turn the first function from NoError to NSError like this:

textSignal.promoteErrors(NSError)

This is safe to do with NoError signals because we know at compile time that they won't actually produce errors, and therefore no casting needs to happen to change it to NSError.

Upvotes: 2

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