Thread-safe counter using DispatchSemaphore

SynchronizedSemaphore.swift

import XCTest

struct SynchronizedSemaphore<Value> {
    private let mutex = DispatchSemaphore(value: 1)
    private var _value: Value
 
    init(_ value: Value) {
        self._value = value
    }
 
    var value: Value {
        mutex.lock { _value }
    }
 
    mutating func value(execute task: (inout Value) -> Void) {
        mutex.lock { task(&_value) }
    }
}
 
private extension DispatchSemaphore {
    
    func lock<T>(execute task: () throws -> T) rethrows -> T {
        wait()
        defer { signal() }
        return try task()
    }
}


func test() {
    var temp = SynchronizedSemaphore<Int>(0)
 
    temp.value { $0 = 0 } // Reset

    DispatchQueue.concurrentPerform(iterations: 1_000) { _ in
        temp.value { $0 += 1 }
    }
    
    print(temp.value)
    XCTAssertEqual(temp.value, 1_000)
}

test()