What are Sendable and @Sendable closures in Swift? – Donny Wals

One of many objectives of the Swift group with Swift’s concurrency options is to supply a mannequin that enables developer to write down protected code by default. Because of this there’s plenty of time and vitality invested into ensuring that the Swift compiler helps builders detect, and forestall entire lessons of bugs and concurrency points altogether.

One of many options that helps you forestall knowledge races (a standard concurrency difficulty) comes within the type of actors which I’ve written about earlier than.

Whereas actors are nice if you need to synchronize entry to some mutable state, they don’t resolve each doable difficulty you may need in concurrent code.

On this put up, we’re going to take a better have a look at the Sendable protocol, and the @Sendable annotation for closures. By the tip of this put up, you need to have an excellent understanding of the issues that Sendable (and @Sendable) intention to resolve, how they work, and the way you need to use them in your code.

Understanding the issues solved by Sendable

One of many trickiest facets of a concurrent program is to make sure knowledge consistency. Or in different phrases, thread security. After we move situations of lessons or structs, enum circumstances, and even closures round in an software that doesn’t do a lot concurrent work, we don’t want to fret about thread security so much. In apps that don’t actually carry out concurrent work, it’s unlikely that two duties try to entry and / or mutate a chunk of state at the very same time. (However not unattainable)

For instance, you may be grabbing knowledge from the community, after which passing the obtained knowledge round to a few features in your primary thread.

Because of the nature of the primary thread, you possibly can safely assume that your entire code runs sequentially, and no two processes in your software will probably be engaged on the identical referencea on the identical time, doubtlessly creating an information race.

To briefly outline an information race, it’s when two or extra elements of your code try to entry the identical knowledge in reminiscence, and a minimum of one among these accesses is a write motion. When this occurs, you possibly can by no means make certain concerning the order wherein the reads and writes occur, and you may even run into crashes for dangerous reminiscence accesses. All in all, knowledge races aren’t any enjoyable.

Whereas actors are a improbable strategy to construct objects that accurately isolate and synchronize entry to their mutable state, they will’t resolve all of our knowledge races. And extra importantly, it won’t be affordable so that you can rewrite your entire code to utilize actors.

Contemplate one thing like the next code:

class FormatterCache {
    var formatters = [String: DateFormatter]()

    func formatter(for format: String) -> DateFormatter {
        if let formatter = formatters[format] {
            return formatter
        }

        let formatter = DateFormatter()
        formatter.dateFormat = format
        formatters[format] = formatter

        return formatter
    }
}

func performWork() async {
    let cache = FormatterCache()
    let possibleFormatters = ["YYYYMMDD", "YYYY", "YYYY-MM-DD"]

    await withTaskGroup(of: Void.self) { group in
        for _ in 0..<10 {
            group.addTask {
                let format = possibleFormatters.randomElement()!
                let formatter = cache.formatter(for: format)
            }
        }
    }
}

On first look, this code won’t look too dangerous. We have now a category that acts as a easy cache for date formatters, and we’ve got a activity group that can run a bunch of code in parallel. Every activity will seize a random date format from the record of doable format and asks the cache for a date formatter.

Ideally, we anticipate the formatter cache to solely create one date formatter for every date format, and return a cached formatter after a formatter has been created.

Nevertheless, as a result of our duties run in parallel there’s an opportunity for knowledge races right here. One fast repair can be to make our FormatterCache an actor and this may resolve our potential knowledge race. Whereas that will be an excellent resolution (and really one of the best resolution should you ask me) the compiler tells us one thing else once we attempt to compile the code above:

Seize of ‘cache’ with non-sendable sort ‘FormatterCache’ in a @Sendable closure

This warning is making an attempt to inform us that we’re doing one thing that’s doubtlessly harmful. We’re capturing a worth that can’t be safely handed via concurrency boundaries in a closure that’s purported to be safely handed via concurrency boundaries.

⚠️ If the instance above doesn’t produce a warning for you, you will need to allow strict concurrency checking in your challenge’s construct settings for stricter Sendable checks (amongst different concurrency checks). You possibly can allow strict concurrecy settings in your goal’s construct settings. Check out this web page should you’re undecided how to do that.

With the ability to be safely handed via concurrency boundaries basically implies that a worth could be safely accessed and mutated from a number of duties concurrently with out inflicting knowledge races. Swift makes use of the Sendable protocol and the @Sendable annotation to speak this thread-safety requirement to the compiler, and the compiler can then verify whether or not an object is certainly Sendable by assembly the Sendable necessities.

What these necessities are precisely will differ a bit relying on the kind of objects you take care of. For instance, actor objects are Sendable by default as a result of they’ve knowledge security built-in.

Let’s check out different varieties of objects to see what their Sendable necessities are precisely.

Sendable and worth varieties

In Swift, worth varieties present plenty of thread security out of the field. While you move a worth sort from one place to the following, a duplicate is created which implies that every place that holds a duplicate of your worth sort can freely mutate its copy with out affecting different elements of the code.

This an enormous good thing about structs over lessons as a result of they permit use to cause domestically about our code with out having to think about whether or not different elements of our code have a reference to the identical occasion of our object.

Due to this habits, worth varieties like structs and enums are Sendable by default so long as all of their members are additionally Sendable.

Let’s have a look at an instance:

// This struct will not be sendable
struct Film {
    let formatterCache = FormatterCache()
    let releaseDate = Date()
    var formattedReleaseDate: String {
        let formatter = formatterCache.formatter(for: "YYYY")
        return formatter.string(from: releaseDate)
    }
}

// This struct is sendable
struct Film {
    var formattedReleaseDate = "2022"
}

I do know that this instance is a bit bizarre; they don’t have the very same performance however that’s not the purpose.

The purpose is that the primary struct does not likely maintain mutable state; all of its properties are both constants, or they’re computed properties. Nevertheless, FormatterCache is a category that is not Sendable. Since our Film struct doesn’t maintain a duplicate of the FormatterCache however a reference, all copies of Film can be trying on the identical situations of the FormatterCache, which implies that we may be taking a look at knowledge races if a number of Film copies would try to, for instance, work together with the formatterCache.

The second struct solely holds Sendable state. String is Sendable and because it’s the one property outlined on Film, film can be Sendable.

The rule right here is that each one worth varieties are Sendable so long as their members are additionally Sendable.

Typically talking, the compiler will infer your structs to be Sendable when wanted. Nevertheless, you possibly can manually add Sendable conformance if you would like:

struct Film: Sendable {
    let formatterCache = FormatterCache()
    let releaseDate = Date()
    var formattedReleaseDate: String {
        let formatter = formatterCache.formatter(for: "YYYY")
        return formatter.string(from: releaseDate)
    }
}

Sendable and lessons

Whereas each structs and actors are implicitly Sendable, lessons are usually not. That’s as a result of lessons are so much much less protected by their nature; everyone that receives an occasion of a category really receives a reference to that occasion. Because of this a number of locations in your code maintain a reference to the very same reminiscence location and all mutations you make on a category occasion are shared amongst everyone that holds a reference to that class occasion.

That doesn’t imply we will’t make our lessons Sendable, it simply implies that we have to add the conformance manually, and manually be certain that our lessons are literally Sendable.

We are able to make our lessons Sendable by including conformance to the Sendable protocol:

last class Film: Sendable {
    let formattedReleaseDate = "2022"
}

The necessities for a category to be Sendable are much like these for a struct.

For instance, a category can solely be Sendable if all of its members are Sendable. Because of this they have to both be Sendable lessons, worth varieties, or actors. This requirement is similar to the necessities for Sendable structs.

Along with this requirement, your class have to be last. Inheritance may break your Sendable conformance if a subclass provides incompatible overrides or options. For that reason, solely last lessons could be made Sendable.

Lastly, your Sendable class shouldn’t maintain any mutable state. Mutable state would imply that a number of duties can try to mutate your state, main to an information race.

Nevertheless, there are situations the place we’d know a category or struct is protected to be handed throughout concurrency boundaries even when the compiler can’t show it.

In these circumstances, we will fall again on unchecked Sendable conformance.

Unchecked Sendable conformance

While you’re working with codebases that predate Swift Concurrency, likelihood is that you simply’re slowly working your method via your app as a way to introduce concurrency options. Because of this a few of your objects might want to work in your async code, in addition to in your sync code. Because of this utilizing actor to isolate mutable state in a reference sort won’t work so that you’re caught with a category that may’t conform to Sendable. For instance, you may need one thing like the next code:

class FormatterCache {
    non-public var formatters = [String: DateFormatter]()
    non-public let queue = DispatchQueue(label: "com.dw.FormatterCache.(UUID().uuidString)")

    func formatter(for format: String) -> DateFormatter {
        return queue.sync {
            if let formatter = formatters[format] {
                return formatter
            }

            let formatter = DateFormatter()
            formatter.dateFormat = format
            formatters[format] = formatter

            return formatter
        }
    }
}

This formatter cache makes use of a serial queue to make sure synchronized entry to its formatters dictionary. Whereas the implementation isn’t best (we may very well be utilizing a barrier or possibly even a plain previous lock as a substitute), it really works. Nevertheless, we will’t add Sendable conformance to our class as a result of formatters isn’t Sendable.

To repair this, we will add @unchecked Sendable conformance to our FormatterCache:

class FormatterCache: @unchecked Sendable {
    // implementation unchanged
}

By including this @unchecked Sendable we’re instructing the compiler to imagine that our FormatterCache is Sendable even when it doesn’t meet the entire necessities.

Having this characteristic in our toolbox is extremely helpful if you’re slowly phasing Swift Concurrency into an present challenge, however you’ll need to suppose twice, or possibly even thrice, if you’re reaching for @unchecked Sendable. It is best to solely use this characteristic if you’re actually sure that your code is definitely protected for use in a concurrent setting.

Utilizing @Sendable on closures

There’s one final place the place Sendable comes into play and that’s on features and closures.

A number of closures in Swift Concurrency are annotated with the @Sendable annotation. For instance, right here’s what the declaration for TaskGroup‘s addTask appears like:

public mutating func addTask(precedence: TaskPriority? = nil, operation: @escaping @Sendable () async -> ChildTaskResult)

The operation closure that’s handed to addTask is marked with @Sendable. Because of this any state that the closure captures should be Sendable as a result of the closure may be handed throughout concurrency boundaries.

In different phrases, this closure will run in a concurrent method so we need to guarantee that we’re not by accident introducing an information race. If all state captured by the closure is Sendable, then we all know for positive that the closure itself is Sendable. Or in different phrases, we all know that the closure can safely be handed round in a concurrent setting.

Tip: to study extra about closures in Swift, check out my put up that explains closures in nice element.

Abstract

On this put up, you’ve discovered concerning the Sendable and @Sendable options of Swift Concurrency. You discovered why concurrent applications require further security round mutable state, and state that’s handed throughout concurrency boundaries as a way to keep away from knowledge races.

You discovered that structs are implicitly Sendable if all of their members are Sendable. You additionally discovered that lessons could be made Sendable so long as they’re last, and so long as all of their members are additionally Sendable.

Lastly, you discovered that the @Sendable annotation for closures helps the compiler be certain that all state captured in a closure is Sendable and that it’s protected to name that closure in a concurrent context.

I hope you’ve loved this put up. When you’ve got any questions, suggestions, or solutions to assist me enhance the reference then be at liberty to achieve out to me on Twitter.