Swift 4 异步调用 for 循环使用 DispatchGroup、DispatchQueue 和 DispatchSemaphore 按顺序执行

Question

我想在 swift 中按顺序运行 for 循环，DispatchGroup 会将它们一起触发，所以我想使用 DispatchQueue 和 DispatchSemaphore 来实现我的目标。 我的程序运行失败，如何强制他们等待并一一运行？

let dispatchGroup = DispatchGroup()
let dispatchQueue = DispatchQueue(label: "taskQueue")
let dispatchSemaphore = DispatchSemaphore(value: 1)

for c in self.categories {

    dispatchSemaphore.wait()

    dispatchQueue.async(group: dispatchGroup) {

        if let id = c.categoryId {

            dispatchGroup.enter()

            self.downloadProductsByCategory(categoryId: id) { success, data in

                if success, let products = data {

                    self.products.append(products)
                }

                dispatchSemaphore.signal()
                dispatchGroup.leave()
            }
        }
    }
}

dispatchGroup.notify(queue: dispatchQueue) {

    self.refreshOrderTable { _ in

        self.productCollectionView.reloadData()

        NVActivityIndicatorPresenter.sharedInstance.stopAnimating()
    }
}

---

感谢Palle ，这是我的最终代码：

let dispatchGroup = DispatchGroup()
let dispatchQueue = DispatchQueue(label: "taskQueue")
let dispatchSemaphore = DispatchSemaphore(value: 0)

dispatchQueue.async {

    for c in self.categories {

        if let id = c.categoryId {

            dispatchGroup.enter()

            self.downloadProductsByCategory(categoryId: id) { success, data in

                if success, let products = data {

                    self.products.append(products)
                }

                dispatchSemaphore.signal()
                dispatchGroup.leave()
            }

            dispatchSemaphore.wait()
        }
    }
}

dispatchGroup.notify(queue: dispatchQueue) {

    DispatchQueue.main.async {

        self.refreshOrderTable { _ in

            self.productCollectionView.reloadData()
        }
    }
}

Answer 1

您可以将整个循环放在一个块中，而不是只将下载函数放在一个块中：

dispatchQueue.async {
    for c in self.categories {
        if let id = c.categoryId {
            self.downloadProductsByCategory(categoryId: id) { success, data in
                if success, let products = data {
                    self.products.append(products)
                }

                dispatchSemaphore.signal()
            }
            dispatchSemaphore.wait()
        }
    }
}

---

您可以通过使用 compactMap 来解开您的产品 ID 来简化您的代码：

for id in self.categories.compactMap({$0.categoryId}) { ... }