如何知道DispatchQueue的后台处理已经终止？

let dispatchGroup = DispatchGroup()

// 假设有多个后台任务
for i in 0..<10 {
    dispatchGroup.enter()
    DispatchQueue.global(qos: .background).async {
        // 执行后台任务
        print("Task \(i) is running")
        // 任务完成后调用 dispatchGroup.leave()
        self.dispatchGroup.leave()
    }
}

// 监听所有任务的完成
dispatchGroup.notify(queue: .main) {
    print("All background tasks have completed")
}

var tasksCompleted = 0
let totalTasks = 10

for i in 0..<totalTasks {
    DispatchQueue.global(qos: .background).async {
        // 执行后台任务
        print("Task \(i) is running")
        // 任务完成后更新计数器
        tasksCompleted += 1
        if tasksCompleted == totalTasks {
            print("All background tasks have completed")
        }
    }
}

let operationQueue = OperationQueue()
operationQueue.maxConcurrentOperationCount = 10

for i in 0..<10 {
    let operation = BlockOperation {
        // 执行后台任务
        print("Task \(i) is running")
    }
    operationQueue.addOperation(operation)
}

operationQueue.addBarrierBlock {
    print("All background tasks have completed")
}

let serialQueue = DispatchQueue(label: "com.example.serialQueue")
serialQueue.async {
    // 第一个任务
}
serialQueue.async {
    // 第二个任务
}