element-x-ios/ElementX/Sources/Other/CurrentValuePublisher.swift

//
// Copyright 2025 Element Creations Ltd.
// Copyright 2023-2025 New Vector Ltd.
//
// SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
// Please see LICENSE files in the repository root for full details.
//

import Combine

/// A wrapper of CurrentValueSubject.
/// The purpose of this type is to remove the possibility to send new values on the underlying subject.
///
/// `CurrentValueSubject` is documented as thread-safe but is not formally `Sendable`, hence `@unchecked`.
nonisolated struct CurrentValuePublisher<Output, Failure: Error>: Publisher, @unchecked Sendable {
    private let upstream: AnyPublisher<Output, Failure>
    private let valueProvider: () -> Output
    
    init(_ subject: CurrentValueSubject<Output, Failure>) {
        upstream = subject.eraseToAnyPublisher()
        valueProvider = { subject.value }
    }
    
    init(_ value: Output) {
        self.init(CurrentValueSubject(value))
    }
    
    private init(upstream: AnyPublisher<Output, Failure>, valueProvider: @escaping () -> Output) {
        self.upstream = upstream
        self.valueProvider = valueProvider
    }
    
    func receive<S: Subscriber>(subscriber: S) where Failure == S.Failure, Output == S.Input {
        upstream.receive(subscriber: subscriber)
    }
    
    var value: Output {
        valueProvider()
    }
}

nonisolated extension CurrentValuePublisher where Failure == Never {
    /// Transforms the published values and ``value``.
    ///
    /// The transform runs lazily, once per subscriber and per ``value`` read. It must stay lazy:
    /// this struct is discarded when the mapped publisher is chained inline, so anything eager
    /// (a live subscription feeding a second subject) would be cancelled and stop the updates.
    func map<T>(_ transform: @escaping (Output) -> T) -> CurrentValuePublisher<T, Never> {
        .init(upstream: upstream.map(transform).eraseToAnyPublisher()) {
            transform(value)
        }
    }
}

nonisolated extension CurrentValueSubject {
    func asCurrentValuePublisher() -> CurrentValuePublisher<Output, Failure> {
        .init(self)
    }
}