A property wrapper that creates an AsyncStream yielding values when the property is set.

Streaming.swift

import os

@propertyWrapper
public struct Streaming<Wrapped: Sendable>: Sendable {
    public var wrappedValue: Wrapped {
        get { value }
        set {
            value = newValue
            let continuations = protectedContinuations.withLock { $0.values }
            for continuation in continuations {
                continuation.yield(newValue)
            }
        }
    }

    public var projectedValue: Stream {
        get {
            let id = nextID
            let (stream, continuation) = Stream.makeStream(bufferingPolicy: bufferingPolicy)
            protectedContinuations.withLock { $0[id] = continuation }
            continuation.onTermination = { [protectedContinuations] _ in
                protectedContinuations.withLock { $0[id] = nil }
            }
            return stream
        }
    }

    public init(wrappedValue: Wrapped, bufferingPolicy: Stream.Continuation.BufferingPolicy = .bufferingNewest(1)) {
        self.value = wrappedValue
        self.bufferingPolicy = bufferingPolicy
    }

    public typealias Stream = AsyncStream<Wrapped>

    private var value: Wrapped
    private let bufferingPolicy: Stream.Continuation.BufferingPolicy
    private let protectedContinuations: OSAllocatedUnfairLock<[Int: Stream.Continuation]> = .init(initialState: [:])
}

fileprivate var nextID: Int { _nextID.withLock { id in defer { id += 1 }; return id } }
fileprivate let _nextID: OSAllocatedUnfairLock<Int> = .init(initialState: 42)

Usage:

class BluetoothManager {
    @Streaming
    var bluetoothState: CBManagerState = .unknown

    func waitForBluetoothToPowerOn() async {
        for await state in $bluetoothState where state == .poweredOn {
            break
        }
    }
}