Difference between CurrentValueSubject and @Published

Question

So I'm digging into combine and this question came up.

Is there any real difference between using CurrentValueSubject (and setting its value using currentValueSubject.value) or using a @Published var and accessing its publisher with a $? I mean I know one returns a Subject instead of a Publisher, but the only real difference I could find is that CurrentValueSubject is way more useful because you can declare it on a protocol.

I really don't understand how @Published can be useful if we can just use PassthroughSubject, am I missing something here? Mind you, this is using UIKit, it may have other uses for SwiftUI.

Thank you.

Answer 1

@Published is just a quick way to use CurrentValueSubject a little neater. When I debug one of my apps and look at the type returned by $paramName , it's actually just a CurrentValueSubject:

po self.$books
? Publisher
  ? subject : <CurrentValueSubject<Array<Book>, Never>: 0x6000034b8910>

I guess one benefit of using CurrentValueSubject instead of @Published may be to allow you to use the error type?

Note: Despite being a CurrentValueSubject right now I'd never rely on that assumption.

Answer 2

CurrentValueSubject is a value, a publisher and a subscriber all in one.

Sadly it doesn’t fire objectWillChange.send() when used inside an ObservableObject.

You can specify an error type.

@Published is a property wrapper, thus:

• It is not yet supported in top-level code.
• It is not supported in a protocol declaration.
• It can only be used within a class.

@Published automatically fires objectWillChange.send() when used inside an ObservableObject.

Xcode will emit a warning if your try to publish to @Published wrapped property from a background queue. Probably because objectWillChange.send() must be called from the main thread.

The error type of its publisher is Never

My biggest beef against @Published is that it can’t behave as a subscriber and setting up Combine pipelines requires additional plumbing compared to a Current Value Subject.

We can declare a @Published property inside a protocol. Not very pretty...

protocol TestProtocol {
    var isEnabled: Bool { get }
    var isEnabledPublished: Published<Bool> { get }
    var isEnabledPublisher: Published<Bool>.Publisher { get }
}

class Test: ObservableObject, TestProtocol {
    @Published var isEnabled: Bool = false
    var isEnabledPublished: Published<Bool> { _isEnabled }
    var isEnabledPublisher: Published<Bool>.Publisher { $isEnabled }
}

Answer 3

I found myself coming back to this post so felt I'd add some extra insight in to the difference between @Published and CurrentValueSubject.

One main difference can be found in the documentation for @Published:

When the property changes, publishing occurs in the property’s willSet block, meaning subscribers receive the new value before it’s actually set on the property.

Additionally, conversation on the Swift Forums note that @Published is intended for use with SwiftUI.

With regards to @Published publishing in the willSet block of it's property, consider the following example:

class PublishedModel {
    @Published var number: Int = 0
}

let pModel = PublishedModel()

pModel.$number.sink { number in
    print("Closure: \(number)")
    print("Object:  \(pModel.number) [read via closure]")
}

pModel.number = 1
print("Object:  \(pModel.number) [read after assignment]")

This produces the following output:

Closure: 0
Object:  0 [read via closure]
Closure: 1
Object:  0 [read via closure]
Object:  1 [read after assignment]

Contrast this with another example where we keep everything the same, except replacing @Published with CurrentValueSubject:

class CurrentValueSubjectModel {
    var number: CurrentValueSubject<Int, Never> = .init(0)
}

let cvsModel = CurrentValueSubjectModel()

cvsModel.number.sink { number in
    print("Closure: \(number)")
    print("Object:  \(cvsModel.number.value) [read via closure]")
}

cvsModel.number.send(1)

print("Object:  \(cvsModel.number.value) [read after assignment]")

Output:

Closure: 0
Object:  0 [read via closure]
Closure: 1
Object:  1 [read via closure] // <— Here is the difference
Object:  1 [read after assignment]

After updating number to 1, reading the object's CurrentValueSubject's value property within the closure prints the new value instead of the old value as with @Published.

In summary, use @Published within your ObservableObjects for your SwiftUI views. If you're looking to create some sort of model object with an instance property that holds a current value and also publishes it's changes after they are set, use CurrentValueSubject.

Answer 4

One advantage on @Published is that it can act as a private-mutable, public-immutable CurrrentValueSubject.

Compare:

@Published private(set) var text = "someText"

with:

let text = CurrentValueSubject<String, Never>("someText")

When designing APIs you often want to allow clients to read the current value and subscribe to updates but prevent them from setting values directly.

Answer 5

There is one limitation when using @Published.

You can only use @Published on properties of a Class whereas CurrentValueSubject can be used for struct as well