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How to make a custom sequence

Swift version: 5.1

Paul Hudson    @twostraws   

When you use a for-in loop, Swift maps that code to a while loop that generates an iterator for your data. Swift then calls next() on that iterator repeatedly until it gets back nil to signal that the loop has ended.

This functionality is all handled through two protocols: Sequence describes something that can be looped over, and IteratorProtocol describes something that iterates over a sequence. If you combine them both into a single type, all you need to do is implement a next() method returning whatever value comes next in your sequence.

Let’s walk through a simple sequence that counts doubles up from 1, so it will count 1, 2, 4, 8, 16, 32, 64, 128, 256, and so on until we run out of integers that can be stored in Int.

In code this means creating a new DoubleGenerator struct that conforms to both Sequence and IteratorProtocol. Because this type conforms to both protocols, we only need to implement one method: next(). This should double the current number then return it, however we need to do that doubling inside a defer block so that we only double after we’ve returned.

Here’s the code:

struct DoubleGenerator: Sequence, IteratorProtocol {
    var current = 1

    mutating func next() -> Int? {
        defer {
            current *= 2
        }

        return current
    }
}

As that’s an infinite sequence you should be careful using it. For example, this code increments a counter each time its loop goes around and exits the loop when the counter reaches 10.

var i = 0

let numbers = DoubleGenerator()
for number in numbers {
    i += 1
    if i == 10 { break }

    print(number)
}

That will print doubles of 1 through to 256.

If you want your iterator and sequences to be different types, you’ll need to add a makeIterator() method to your sequence.

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