Printed on: September 13, 2022
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.