swift - 在 Swift 中链接 promise 以初始化自定义对象

Question

我在 Swift 中实现了 PromiseKit 以避免带有完成 block 的回调 hell 。我需要知道将 promise 链接在一起以初始化具有其他关联对象的自定义对象的最佳方法。例如，一个 Comment 对象附加了一个 User 对象。

首先，我从数据库中获取comments，它们在数据库结构中都有一个uid 属性。我最终想得到一组 comments，其中每个都有正确的用户附加到它，所以我可以同时加载 comment 和 user 数据。使用完成 block ，这一切似乎要容易得多，但我是一个完全 Promise 的菜鸟，所以 idk。

这是 Controller 中处理获取的代码

CommentsService.shared.fetchComments(withPostKey: postKey)
            .then { comments -> Promise<[User]> in
                let uids = comments.map({ $0.uid })
                return UserService.shared.fetchUsers(withUids: uids)
        }.done({ users in
            // how to init Comment object with users now?
        })
            .catch { error in
                print("DEBUG: Failed with error \(error)")
        }

这是评论获取功能:

func fetchComments(withPostKey postKey: String) -> Promise<[Comment]> {
        return Promise { resolver in
            REF_COMMENTS.child(postKey).observeSingleEvent(of: .value) { snapshot in
                guard let dictionary = snapshot.value as? [String: AnyObject] else { return }
                let data = Array(dictionary.values)
                do {
                    let comments = try FirebaseDecoder().decode([Comment].self, from: data)
                    resolver.fulfill(comments)
                } catch let error {
                    resolver.reject(error)
                }
            }
        }
    }

这里是获取用户函数

func fetchUsers(withUids uids: [String]) -> Promise<[User]> {
        var users = [User]()

        return Promise { resolver in
            uids.forEach { uid in
                self.fetchUser(withUid: uid).done { user in
                    users.append(user)
                    guard users.count == uids.count else { return }
                    resolver.fulfill(users)
                }.catch { error in
                    resolver.reject(error)
                }
            }
        }
    }

这里是评论对象:

struct Comment: Decodable {
    let uid: String
    let commentText: String
    let creationDate: Date
    var user: User?
}

这就是完成 block 的简单之处，开始认为 Promises 不值得吗？

func fetchComments(withPostKey postKey: String, completion: @escaping([Comment]) -> Void) {
        var comments = [Comment]()
        REF_COMMENTS.child(postKey).observe(.childAdded) { (snapshot) in

            guard let dictionary = snapshot.value as? [String: AnyObject] else { return }
            guard let uid = dictionary["uid"] as? String else { return }

            UserService.shared.fetchUser(withUid: uid, completion: { (user) in
                let comment = Comment(user: user, dictionary: dictionary)
                comments.append(comment)
                completion(comments)
            })
        }
    }

Answer 1

好的，我想我明白你想做什么了。问题是您需要与用户一起捕获评论，这样您就可以一起返回，然后再将它们组合起来。它应该看起来像这样:

CommentsService.shared.fetchComments(withPostKey: postKey)
  .then { comments -> Promise<[Comment], [User]> in
    let uids = comments.map({ $0.uid })
    return UserService.shared.fetchUsers(withUids: uids)
      .then { users in
        return Promise<[Comment], [User]>(comments, users)
      }
    }.done({ combined in
      let (comments, users) = combined
      //Do combiney stuff here
    })
      .catch { error in
        print("DEBUG: Failed with error \(error)")
}

转换是 [评论] -> [用户] -> ([评论], [用户]) -> [附加用户的评论]