ios - Swift优化嵌套异步调用

Question

在这种情况下，我需要进行3个嵌套的异步调用来接收所需的数据。
因此，第二个 call 需要第一个 call 的数据，而第三个 call 需要第二个 call 的数据。我没有很多这样的情况。只有一个和另一个只有两个嵌套调用，所以我在考虑一种没有任何外部库的纯快速解决方案，但我对所有内容都开放。

由于我使用的是Firebase，是否最好将此逻辑移至CloudFunctions？那么要在后端准备呢？

FirestoreService().fetchCollection(query: query) { (result: Result<[Request], Error>) in
    // do stuff
    FirestoreService().fetchCollection(query: query) { (result: Result<[Request], Error>) in
        // do stuff
            FirestoreService().fetchDocument(documentReference: documentReference) { (result: Result<Package, Error>) in
                // finish
            }
        }

    }
}

Answer 1

如果您不使用第3方库，则可能要考虑将那些操作包装在某个类中，并以命令式方式使用闭包。

这是示例:

class CustomFirestoreHandler {

    private var onFetchFirstQueryArrived: ((Result<[Request], Error>) -> ())? = nil
    private var onFetchSecondQueryArrived: ((Result<[Request], Error>) -> ())? = nil
    private var onFetchDocumentArrived: ((Result<Package, Error>) -> ())? = nil

    init() {

        onFetchFirstQueryArrived = { [weak self] (result: Result<[Request], Error>) in
            self?.executeSecondQuery()
        }

        onFetchSecondQueryArrived = { [weak self] (result: Result<[Request], Error>) in
            self?.executeFetchDocument()
        }
    }

    func executeQuery(completion: @escaping (Result<Package, Error>) -> ()) {

        self.onFetchDocumentArrived = completion

        FirestoreService().fetchCollection(query: query) { [weak self] (result: Result<[Request], Error>) in 

            // validate if some error occurred and do early return here, so that we don't need necessarily call second query.
            if (result.error == whatever) {
                 self?.onFetchDocumentArrived?(result)
                 return
            }

            self?.onFetchFirstQueryArrived?(result)
        }
    }

    private func executeSecondQuery() {
        FirestoreService().fetchCollection(query: query) { [weak self] (result: Result<[Request], Error>) in 

            // validate if some error occurred and do early return here, so that we don't need necessarily call fetch document.
            if (result.error == whatever) {
                 self?.onFetchDocumentArrived?(result)
                 return
            }

            self?.onFetchSecondQueryArrived?(result)
        }
    }

    private func executeFetchDocument() {
        FirestoreService().fetchDocument(documentReference: documentReference) { (result: Result<Package, Error>) in
            self?.onFetchDocumentArrived?(result)
        }
    }

}

这是上面 CustomFirestoreHandler的用法:

let firestoreHandler = CustomFirestoreHandler()
firestoreHandler.executeQuery { (result: Result<Package, Error>) in
    // Handle `result` here...
}

我知道它看起来很复杂，但这是我认为(CMIIW)目前防止厄运金字塔的唯一方法，因为swift没有异步等待样式(就像javascript一样)。