One of many objectives of the Swift group with Swift’s concurrency options is to offer a mannequin that enables developer to put in writing protected code by default. Because of this there’s quite a lot of time and vitality invested into ensuring that the Swift compiler helps builders detect, and stop entire courses of bugs and concurrency points altogether.

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

Whereas actors are nice while you need to synchronize entry to some mutable state, they don’t resolve each attainable subject 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, 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 features of a concurrent program is to make sure information consistency. Or in different phrases, thread security. Once we move situations of courses or structs, enum circumstances, 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 unimaginable)

For instance, you is likely to be grabbing information from the community, after which passing the obtained information round to a few features in your most important thread.

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

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

Whereas actors are a unbelievable technique to construct objects that appropriately isolate and synchronize entry to their mutable state, they’ll’t resolve all of our information races. And extra importantly, it won’t be cheap so that you can rewrite your whole code to utilize actors.

Think about 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 unhealthy. We now have a category that acts as a easy cache for date formatters, and we’ve a activity group that may run a bunch of code in parallel. Every activity will seize a random date format from the checklist 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 information races right here. One fast repair could be to make our FormatterCache an actor and this could resolve our potential information race. Whereas that may be a very good answer (and really the perfect answer in case 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 attempting to inform us that we’re doing one thing that’s probably harmful. We’re capturing a price that can not be safely handed by way of concurrency boundaries in a closure that’s alleged to be safely handed by way of concurrency boundaries.

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

Having the ability to be safely handed by way of concurrency boundaries basically implies that a price may be safely accessed and mutated from a number of duties concurrently with out inflicting information 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 differ slightly relying on the kind of objects you cope with. For instance, actor objects are Sendable by default as a result of they’ve information security built-in.

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

Sendable and worth sorts

In Swift, worth sorts present quite a lot of thread security out of the field. Whenever you move a price 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 courses 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 sorts 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 just isn't 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 replica of the FormatterCache however a reference, all copies of Film could be trying on the identical situations of the FormatterCache, which implies that we is likely to be information 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 also 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 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 courses

Whereas each structs and actors are implicitly Sendable, courses aren’t. That’s as a result of courses are loads 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 are able to’t make our courses Sendable, it simply implies that we have to add the conformance manually, and manually make sure that our courses are literally Sendable.

We will make our courses Sendable by including conformance to the Sendable protocol:

closing 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 need to both be Sendable courses, worth sorts, or actors. This requirement is similar to the necessities for Sendable structs.

Along with this requirement, your class should be closing. Inheritance may break your Sendable conformance if a subclass provides incompatible overrides or options. For that reason, solely closing courses 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 protected 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

Whenever you’re working with codebases that predate Swift Concurrency, chances are high that you just’re slowly working your manner by way of your app in an effort 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 {
    personal var formatters = [String: DateFormatter]()
    personal 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 perfect (we may very well be utilizing a barrier or perhaps even a plain outdated lock as a substitute), 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 characteristic in our toolbox is extremely helpful while you’re slowly phasing Swift Concurrency into an present mission, however you’ll need to assume twice, or perhaps even thrice, while you’re reaching for @unchecked Sendable. You must solely use this characteristic while you’re actually sure that your code is definitely protected 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 features and closures.

Plenty of closures in Swift Concurrency are annotated with the @Sendable annotation. For instance, right here’s what the declaration for TaskGroup‘s addTask appears to be like 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 is likely to be handed throughout concurrency boundaries.

In different phrases, this closure will run in a concurrent method so we need to make it possible for 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 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 in regards to the Sendable and @Sendable options of Swift Concurrency. You discovered why concurrent packages require further security round mutable state, and state that’s handed throughout concurrency boundaries in an effort to keep away from information races.

You discovered that structs are implicitly Sendable if all of their members are Sendable. You additionally discovered that courses may be made Sendable so long as they’re closing, 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 protected to name that closure in a concurrent context.

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