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

One of many objectives of the Swift crew with Swift’s concurrency options is to offer a mannequin that enables developer to put in writing secure code by default. Which means there’s plenty of time and vitality invested into ensuring that the Swift compiler helps builders detect, and stop entire lessons of bugs and concurrency points altogether.

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

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

On this put up, we’re going to take a more in-depth have a look at the Sendable protocol, and the @Sendable annotation for closures. By the top of this put up, it’s best to have a great understanding of the issues that Sendable (and @Sendable) goal to unravel, how they work, and the way you should use them in your code.

Understanding the issues solved by Sendable

One of many trickiest points of a concurrent program is to make sure knowledge consistency. Or in different phrases, thread security. After we move cases 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 unimaginable)

For instance, you could be grabbing knowledge from the community, after which passing the obtained knowledge round to a few capabilities in your foremost thread.

Because of the nature of the primary thread, you’ll be able to safely assume that all your code runs sequentially, and no two processes in your software might be engaged on the identical referencea on the similar time, doubtlessly creating a knowledge race.

To briefly outline a knowledge race, it’s when two or extra elements of your code try to entry the identical knowledge in reminiscence, and at the very least one in every of these accesses is a write motion. When this occurs, you’ll be able to by no means make certain concerning the order during which the reads and writes occur, and you may even run into crashes for unhealthy reminiscence accesses. All in all, knowledge races aren’t any enjoyable.

Whereas actors are a improbable approach 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 may not be cheap so that you can rewrite all your 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 may not look too unhealthy. We now have a category that acts as a easy cache for date formatters, and now we have a activity group that may run a bunch of code in parallel. Every activity will seize a random date format from the listing of potential format and asks the cache for a date formatter.

Ideally, we count on 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 could be to make our FormatterCache an actor and this may resolve our potential knowledge race. Whereas that may be a great resolution (and truly one of the best resolution when you ask me) the compiler tells us one thing else after 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 not be safely handed by concurrency boundaries in a closure that’s imagined to be safely handed by concurrency boundaries.

⚠️ If the instance above doesn’t produce a warning for you, you may wish to allow strict concurrency checking in your undertaking’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 when you’re undecided how to do that.

With the ability to be safely handed by concurrency boundaries primarily implies that a worth may 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 somewhat relying on the kind of objects you cope with. For instance, actor objects are Sendable by default as a result of they’ve knowledge security built-in.

Let’s check out different forms 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. Once you move a worth sort from one place to the subsequent, a replica is created which implies that every place that holds a replica 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 purpose regionally about our code with out having to contemplate whether or not different elements of our code have a reference to the identical occasion of our object.

Due to this conduct, 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 shouldn't 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 somewhat 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 replica of the FormatterCache however a reference, all copies of Film could be wanting on the similar cases of the FormatterCache, which implies that we could 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 every 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’ll be able to 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 usually are not. That’s as a result of lessons are so much much less secure by their nature; all people that receives an occasion of a category really receives a reference to that occasion. Which means 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 all people that holds a reference to that class occasion.

That doesn’t imply we are able to’t make our lessons Sendable, it simply implies that we have to add the conformance manually, and manually make sure 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. Which means 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 should be last. Inheritance would possibly break your Sendable conformance if a subclass provides incompatible overrides or options. Because of this, solely last lessons may 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 cases the place we would know a category or struct is secure to be handed throughout concurrency boundaries even when the compiler can’t show it.

In these circumstances, we are able to fall again on unchecked Sendable conformance.

Unchecked Sendable conformance

Once you’re working with codebases that predate Swift Concurrency, chances are high that you just’re slowly working your manner by your app with a purpose to introduce concurrency options. Which means a few of your objects might want to work in your async code, in addition to in your sync code. Which means utilizing actor to isolate mutable state in a reference sort may not 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 supreme (we could possibly be utilizing a barrier or perhaps even a plain outdated lock as an alternative), it really works. Nevertheless, we are able to’t add Sendable conformance to our class as a result of formatters isn’t Sendable.

To repair this, we are able to 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 all the necessities.

Having this function in our toolbox is extremely helpful if you’re slowly phasing Swift Concurrency into an present undertaking, however you’ll wish to suppose twice, or perhaps even thrice, if you’re reaching for @unchecked Sendable. It is best to solely use this function if you’re actually sure that your code is definitely secure for use in a concurrent surroundings.

Utilizing @Sendable on closures

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

Numerous 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. Which means any state that the closure captures should be Sendable as a result of the closure could be handed throughout concurrency boundaries.

In different phrases, this closure will run in a concurrent method so we wish to make it possible for we’re not by chance introducing a knowledge race. If all state captured by the closure is Sendable, then we all know for certain that the closure itself is Sendable. Or in different phrases, we all know that the closure can safely be handed round in a concurrent surroundings.

Tip: to be taught 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 with a purpose 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 may 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 make sure that all state captured in a closure is Sendable and that it’s secure to name that closure in a concurrent context.

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