Chaining promises in Swift to initialize a custom object - swift

Question

I implemented PromiseKit in Swift to avoid callback hell with completion blocks. I need to know the best way to chain promises together to init custom objects that have other associated objects. For example a Comment object that has a User object attached to it.
First I fetch the comments from the DB, which all have a uid property in the DB structure. I ultimately want to end up with an array of comments, where each one has the correct user attached to it, so I can load both the comment and user data. This all seemed much easier with completion blocks, but I'm a total Promise noob so idk.
Here is the code in the controller that handles fetch
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)")
}
Here is comment fetch function:
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)
}
}
}
}
Here is fetch users function
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)
}
}
}
}
Here is comment object:
struct Comment: Decodable {
let uid: String
let commentText: String
let creationDate: Date
var user: User?
}
This is how simple it is with completion blocks, starting to think Promises aren't worth it?
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

Ok I think I see what you are trying to do. The issue is that you need to capture the comments along with the users so you can return then together and later combine them. It should look something like this:
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)")
}
The transforms are [Comment] -> [User] -> ([Comment], [User]) -> [Comments with users attached]