This is Understanding Combine, written by Matt Neuburg. It is a work in progress. Corrections and suggestions are greatly appreciated (you can comment here). So are donations; please consider keeping me going by funding this work at http://www.paypal.me/mattneub. Or buy my books: the current editions are iOS 13 Programming Fundamentals with Swift and Programming iOS 13. Thank you!


Prefix and Drop

.prefix(_:) is actually a form of .output(in:). You supply an Int representing a count. This operator turns that into a Range starting at zero and behaves as if you had called .output(in:). Thus, this operator passes along the first n values received from upstream (where n is the count you supplied), then cancels the upstream and sends a .finished completion.

.prefix(while:) (Publishers.PrefixWhile) takes a function that receives a value from upstream and returns a Bool. If the function returns true, it passes the value along downstream. If the function returns false, it cancels the upstream and emits a .finished completion.

.tryPrefix(while:) (Publishers.TryPrefixWhile) is like .prefix(while) except that the function can throw. If it does throw, this operator cancels the upstream and the error is passed downstream as a failure.

.dropFirst() (Publishers.Drop) is the inverse of .prefix(1): it swallows the first value received from upstream and then just passes along every subsequent value received from upstream.

.drop(while:) (Publishers.DropWhile) takes a function that receives a value from upstream and returns a Bool. If the function returns true, it swallows the received value. If the function returns false, it passes the received value along downstream and never calls the function again; it just keeps passing received values downstream.

.tryDrop(while:) (Publishers.TryDropWhile) is like .drop(while:) except that the function can throw. I think you know what happens if it does throw.

You’re probably wondering: what about suffix(_:)? How can we receive only the last bunch of n values emitted by the upstream publisher? There is no built-in suffix operator, but you could build one, sort of, using collect. Let’s say we want only the last two values emitted by the upstream publisher:

    [1,2,3,4].publisher
        .collect()
        .flatMap {$0.suffix(2).publisher} // 3, then 4, then `.finished`

I say “sort of” because there’s no way to implement suffix so as to pass along the last values as they arrive, so this is not really comparable to what prefix does.


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