Async Channels

Performant channels for Swift concurrency.

Don't communicate by sharing memory; share memory by communicating

- Rob Pike

If you are familiar with golang and the go ecosystem, you can skip to the go comparisons section.

Channels are a typed conduit through which you can send and receive values - usually across threads or in this case, Swift async tasks. This library is modeled after go's channel behaviors.

Example

let msg = Channel<String>(capacity: 3)
let done = Channel<Bool>()

await msg <- "Swift"
await msg <- "❤️"
await msg <- "Channels"
msg.close()

Task {
    for await message in msg {
        print(message)
    }
    await done <- true
}
await <-done

Benchmarks

See the Benchmarks readme for results and comparing with go.

Usage

1. Add https://github.com/gh123man/Async-Channels as a Swift package dependency to your project.
2. import AsyncChannels and go!

Channel Operations

Un-Buffered Channels

// Create an un-buffered channel
let msg = Channel<String>()

Task {
    // Send a value. Send will suspend until the channel is read. 
    await msg <- "foo" 
}

Task {
    // Receive a value. Receive will suspend until a value is ready
    let foo = await <-msg
}

Buffered Channels

// Create a buffered channel that can hold 2 items
let msg = Channel<String>(capacity: 2)

// Writing to a buffered channel will not suspend until the channel is full
await msg <- "foo" 
await msg <- "bar" 

// Messages are received in the order they are sent
print(await <-msg) // foo
print(await <-msg) // bar

// The channel is now empty. 

Closing Channels

A Channel can be closed. In Swift, the channel receive (<-) operator returns T? because a channel read will return nil when the channel is closed. If you try to write to a closed channel, a fatalError will be triggered.

let a = Channel<String>()
let done = Channel<Bool>()

Task {
    // a will suspend because there is nothing to receive
    await <-a 
    await done <- true
}

// Close will send `nil` causing a to resume in the task above
a.close() 
// done is signaled 
await <-done

Sequence operations

Channel implements AsyncSequence so you may write:

let a = Channel<String>() 

for await message in a {
    print(message)
}

The loop will break when the channel is closed.

Select

select lets a single task wait on multiple channel operations. select will suspend until at least one of the cases is ready. If multiple cases are ready it will choose one randomly.

Operations

rx(c) receive a value, but do nothing with it.

rx(c) { v in ... } receive a value and do something with it.

tx(c, "foo") send a value

none { ... } if none of the channel operations were ready, none will execute instead.

Examples

let c = Channel<String>()
let d = Channel<String>()

Task {
    await c <- "foo"
    await d <- "bar"
}

// Will print foo or bar
await select {
    rx(d) { print($0!) }
    rx(c) { print($0!) }
}

let a = Channel<String>(capacity: 10)
let b = Channel<String>(capacity: 10)

// Fill up a
for _ in (0..<10) {
    await a <- "a"
}

for _ in (0..<20) {
    await select {
        // receive from a and print it
        rx(a) { print($0!) }
        // send "b" to b
        tx(b, "b")
        // if both a and b suspend, print "NONE"
        none {
            print("NONE")
        }
    }
}

Notes

If you are looking for a blocking variant of this library for traditional swift concurrency, check out my previous project Swigo which this library is based off of.

Special Thanks

I could not have gotten this far on my own without the help from the folks over at forums.swift.org. Especially a big shout-out and thank you to wadetregaskis for optimizing much of this code and finding the more challenging performance limitations.