我试图在执行一个blog请求时显示一个activity indicator。我所做的是创建一个ActivityTracker对象，它将跟踪发布者的生命周期。这个ActivityTracker是一个ObservableObject，将存储在视图模型中，视图模型也是一个ObservableObject。
看起来这种设置并没有刷新视图。下面是我的代码：

struct ContentView: View {
    @ObservedObject var viewModel = ContentViewModel()

    var body: some View {
        VStack(spacing: 16) {
            Text("Counter: \(viewModel.tracker.count)\nPerforming: \(viewModel.tracker.isPerformingActivity ? "true" : "false")")

            Button(action: {
                _ = request().trackActivity(self.viewModel.tracker).sink { }
            }) {
                Text("Request")
            }
        }
    }
}

class ContentViewModel: ObservableObject {
    @Published var tracker = Publishers.ActivityTracker()
}

private func request() -> AnyPublisher<Void, Never> {
    return Just(()).delay(for: 2.0, scheduler: RunLoop.main)
        .eraseToAnyPublisher()
}

extension Publishers {
    final class ActivityTracker: ObservableObject {
        // MARK: Properties

        @Published var count: Int = 0

        var isPerformingActivity: Bool {
            return count > 0
        }

        private var cancellables: [AnyCancellable] = []
        private let counterSubject = CurrentValueSubject<Int, Never>(0)
        private let lock: NSRecursiveLock = .init()

        init() {
            counterSubject.removeDuplicates()
                .receive(on: RunLoop.main)
                .print()
                .sink { [weak self] counter in
                    self?.count = counter
                }
                .store(in: &cancellables)
        }

        // MARK: Private methods

        fileprivate func trackActivity<Value, Error: Swift.Error>(
            ofPublisher publisher: AnyPublisher<Value, Error>
        ) {
            publisher
                .receive(on: RunLoop.main)
                .handleEvents(
                    receiveSubscription: { _ in self.increment() },
                    receiveOutput: nil,
                    receiveCompletion: { _ in self.decrement() },
                    receiveCancel: { self.decrement() },
                    receiveRequest: nil
                )
                .print()
                .sink(receiveCompletion: { _ in }, receiveValue: { _ in })
                .store(in: &cancellables)
        }

        private func increment() {
            lock.lock()
            defer { lock.unlock() }
            counterSubject.value += 1
        }

        private func decrement() {
            lock.lock()
            defer { lock.unlock() }
            counterSubject.value -= 1
        }
    }
}

extension AnyPublisher {
    func trackActivity(_ activityTracker: Publishers.ActivityTracker) -> AnyPublisher {
        activityTracker.trackActivity(ofPublisher: self)
        return self
    }
}

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我还尝试将我的ActivityTracker声明为@Published，但结果相同，我的文本没有更新。请注意，将Activity Tracker直接存储在视图中也可以，但这不是我想要的。
我错过什么了吗？