ios - Swift:与 DispatchGroup 的死锁

Question

我遇到了 CoreData 并发问题。 当目标线程被 DispatchGroup 阻塞时，我无法执行 context.perform。

这是一个简单的例子来说明这个问题:

func upload(objects: [NSManagedObject]) {
    let group = DispatchGroup()
    for object in objects {
        group.enter()
        upload(object) {
            group.leave()
        }
    }
    group.wait()    // current thread is blocked here

    someAdditionalWorkToDoInSameThread()
}

func upload(object: NSManagedObject, completion: ()->()) {
    let context = object.managedObjectContext
    performAlamofireRequest(object) {
        context.perform {
            // can't reach here because the thread is blocked
            update(object)
            completion()
        }
    }
}

请帮我正确地重新实现它。谢谢。

Answer 1

使用 notify在调度组而不是等待，应该可以解决您的问题。

调用 wait() 会阻塞当前线程以完成先前提交的工作。
notify(queue:execute:) 将通知您作为参数传递的队列组任务已完成。

func upload(objects: [NSManagedObject], completion: ()->()) {
    let group = DispatchGroup()
    for object in objects {
        group.enter()
        upload(object) {
            group.leave()
        }
    }
    group.notify(queue: DispatchQueue.main) {
         completion()
    }
}