eraseToAnyPublisher()

Declaration

func eraseToAnyPublisher() -> AnyPublisher<Self.Output, Self.Failure>

Return Value

An AnyPublisher wrapping this publisher.

Discussion

Use eraseToAnyPublisher() to expose an instance of AnyPublisher to the downstream subscriber, rather than this publisher’s actual type. This form of type erasure preserves abstraction across API boundaries, such as different modules. When you expose your publishers as the AnyPublisher type, you can change the underlying implementation over time without affecting existing clients.

The following example shows two types that each have a publisher property. TypeWithSubject exposes this property as its actual type, PassthroughSubject, while TypeWithErasedSubject uses eraseToAnyPublisher() to expose it as an AnyPublisher. As seen in the output, a caller from another module can access TypeWithSubject.publisher as its native type. This means you can’t change your publisher to a different type without breaking the caller. By comparison, TypeWithErasedSubject.publisher appears to callers as an AnyPublisher, so you can change the underlying publisher type at will.

public class TypeWithSubject {
    public let publisher: some Publisher = PassthroughSubject<Int,Never>()
}
public class TypeWithErasedSubject {
    public let publisher: some Publisher = PassthroughSubject<Int,Never>()
        .eraseToAnyPublisher()
}

// In another module:
let nonErased = TypeWithSubject()
if let subject = nonErased.publisher as? PassthroughSubject<Int,Never> {
    print("Successfully cast nonErased.publisher.")
}
let erased = TypeWithErasedSubject()
if let subject = erased.publisher as? PassthroughSubject<Int,Never> {
    print("Successfully cast erased.publisher.")
}

// Prints "Successfully cast nonErased.publisher."