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

One of many objectives of the Swift staff with Swift’s concurrency options is to supply a mannequin that permits developer to put in writing secure code by default. Which means that there’s loads of time and power 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 forestall knowledge races (a standard concurrency situation) comes within the type of actors which I’ve written about earlier than.

Whereas actors are nice whenever you wish to synchronize entry to some mutable state, they don’t remedy each attainable situation you might need in concurrent code.

On this put up, we’re going to take a better take a look at the Sendable protocol, and the @Sendable annotation for closures. By the tip of this put up, it’s best to have a very good 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 elements of a concurrent program is to make sure knowledge consistency. Or in different phrases, thread security. After we go situations of lessons or structs, enum instances, and even closures round in an utility that doesn’t do a lot concurrent work, we don’t want to fret about thread security loads. In apps that don’t actually carry out concurrent work, it’s unlikely that two duties try and entry and / or mutate a bit of state at the very same time. (However not inconceivable)

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

As a result of nature of the principle thread, you may safely assume that your whole code runs sequentially, and no two processes in your utility can 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 components of your code try and entry the identical knowledge in reminiscence, and not less than certainly one of these accesses is a write motion. When this occurs, you may by no means be sure concerning the order during which the reads and writes occur, and you may even run into crashes for dangerous reminiscence accesses. All in all, knowledge races are not any enjoyable.

Whereas actors are a incredible method to construct objects that accurately isolate and synchronize entry to their mutable state, they’ll’t remedy all of our knowledge races. And extra importantly, it won’t be cheap so that you can rewrite your whole 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. Now we have a category that acts as a easy cache for date formatters, and now we have a job group that may run a bunch of code in parallel. Every job will seize a random date format from the listing of attainable 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.

Nonetheless, 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 is able to remedy our potential knowledge race. Whereas that will be a very good answer (and truly the most effective answer should you ask me) the compiler tells us one thing else after we attempt to compile the code above:

Seize of ‘cache’ with non-sendable kind ‘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 by way of concurrency boundaries in a closure that’s speculated to be safely handed by way of concurrency boundaries.

?? If the instance above doesn’t produce a warning for you, you may wish to allow strict concurrency checking in your venture’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 unsure how to do that.

Having the ability to be safely handed by way of 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 examine whether or not an object is certainly Sendable by assembly the Sendable necessities.

What these necessities are precisely will fluctuate slightly 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 kinds of objects to see what their Sendable necessities are precisely.

Sendable and worth sorts

In Swift, worth sorts present loads of thread security out of the field. While you go a worth kind from one place to the following, a duplicate is created which implies that every place that holds a duplicate of your worth kind can freely mutate its copy with out affecting different components of the code.

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

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

Let’s take a look at an instance:

// This struct is just not 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 slightly bizarre; they don’t have the very same performance however that’s not the purpose.

The purpose is that the primary struct does probably not maintain mutable state; all of its properties are both constants, or they’re computed properties. Nonetheless, 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 wanting on the identical situations of the FormatterCache, which implies that we may be knowledge races if a number of Film copies would try and, 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 sorts are Sendable so long as their members are additionally Sendable.

Typically talking, the compiler will infer your structs to be Sendable when wanted. Nonetheless, you may 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 loads much less secure by their nature; all people that receives an occasion of a category truly receives a reference to that occasion. Which means that 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:

remaining 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 that they need to both be Sendable lessons, worth sorts, or actors. This requirement is equivalent to the necessities for Sendable structs.

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

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

Nonetheless, there are situations 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 instances, we are able to fall again on unchecked Sendable conformance.

Unchecked Sendable conformance

While you’re working with codebases that predate Swift Concurrency, likelihood is that you just’re slowly working your approach by way of your app with a purpose to introduce concurrency options. Which means that a few of your objects might want to work in your async code, in addition to in your sync code. Which means that utilizing actor to isolate mutable state in a reference kind won’t work so that you’re caught with a category that may’t conform to Sendable. For instance, you might 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 very best (we may very well be utilizing a barrier or possibly even a plain previous lock as an alternative), it really works. Nonetheless, 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 whenever you’re slowly phasing Swift Concurrency into an present venture, however you’ll wish to assume twice, or possibly even thrice, whenever you’re reaching for @unchecked Sendable. It is best to solely use this function whenever you’re actually sure that your code is definitely secure 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 capabilities 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 seems 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 that 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 wish to be sure 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 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 realized concerning the Sendable and @Sendable options of Swift Concurrency. You realized 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 realized that structs are implicitly Sendable if all of their members are Sendable. You additionally realized that lessons may be made Sendable so long as they’re remaining, and so long as all of their members are additionally Sendable.

Lastly, you realized 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. In case you have any questions, suggestions, or ideas to assist me enhance the reference then be at liberty to achieve out to me on Twitter.