ActivityTracker.swift

//
//  ActivityTracker.swift
//
//  Created by Daniel Tartaglia on 05 Dec 2021.
//  Copyright © 2021 Daniel Tartaglia. MIT License.
//

extension Publisher {
    func trackActivity(_ activityTracker: ActivityTracker) -> AnyPublisher<Output, Failure> {
        activityTracker.trackActivity(of: self)
    }
}

final class ActivityTracker {
    let isActive: AnyPublisher<Bool, Never>
    private let subject = CurrentValueSubject<Int, Never>(0)
    private let lock = NSRecursiveLock()
    
    init() {
        isActive = subject
            .map { $0 > 0 }
            .removeDuplicates()
            .eraseToAnyPublisher()
    }
    
    func trackActivity<Pub>(of pub: Pub) -> AnyPublisher<Pub.Output, Pub.Failure> where Pub: Publisher {
        Publishers.Using(
            { () -> AnyCancellable in
                self.increment()
                return AnyCancellable({ self.decrement() })
            },
            publisherFactory: { _ in pub }
        )
            .eraseToAnyPublisher()
    }
    
    private func increment() {
        lock.lock()
        subject.send(subject.value + 1)
        lock.unlock()
    }
    
    private func decrement() {
        lock.lock()
        subject.send(subject.value - 1)
        lock.unlock()
    }
}