How do I use Swift Combine to take the last two values?

To use Swift Combine to take the last two values emitted by a Publisher, you can utilize the collect(_:) operator in conjunction with a few other techniques. Here's a detailed explanation:

Understanding the Challenge

Combine publishers emit values over time. If you want to specifically capture the last two values, you need to manage the stream of data and select those specific items. The collect() operator by itself collects all values into an array, but we need to make it so that it only saves the last two emitted ones.

Using collect(_:) and dropFirst()

The most efficient way to achieve this is by using collect(2) which will emit an array of size 2 and dropFirst to avoid the first element to be added to the final result. This ensures that the array always keeps the last two values.

Here is the code example:

import Combine

let subject = PassthroughSubject<Int, Never>()

let lastTwoValues = subject
.collect(2) // Collects values in groups of 2
.dropFirst() // Discard the first array of two values which are the first two emitted elements
.sink { array in
print("Last two values: \\(array)")
}

// Emitting example values
subject.send(1)
subject.send(2)
subject.send(3)
subject.send(4)
subject.send(5)


Explanation

1. Create a PassthroughSubject: A PassthroughSubject is used as a sample publisher that emits integer values. You can replace this with your specific publisher.

2. Use collect(2): The collect(2) operator transforms the stream so that it publishes an array of two values once they are available.

3. Use dropFirst(): We drop the first array of two elements using the dropFirst operator as we are only interested in getting the last two values.

4. sink to Receive Values: The sink operator is used to subscribe to the publisher and print the last two emitted values.

5. Send sample values: You can use this to test the publisher

Output

The output of this code when all the values are sent will be:

Last two values: [4, 5]

Key Points

- collect(2) is crucial in this approach because it allows you to receive values in groups of two. - dropFirst() ensures that only the final array of two is emitted - Make sure the publisher you are using is finite to ensure the final values are taken into consideration.

This method provides a clean and efficient way to capture the last two emitted values from a Combine publisher. You can adapt it to work with other types of publishers and values by simply changing the PassthroughSubject to your specific source.

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