如何在 Firebase 的 iOS SDK 上执行分页查询？

Question

这是我的模型。

messagesRef = Firebase(url: "https://"+CONSTANTS.FirebaseDB+".firebaseio.com/messages/1:1000")
    messagesRef.queryLimitedToLast(5).observeEventType(FEventType.ChildAdded, withBlock: { (snapshot) in
        self.message_list.append(snapshot) // it works.
    });
});

我的代码有效 -- 它获取最后 5 条消息 (8-12)。

但是，如果我有一个函数查询接下来的 5 条消息 (2-6) 怎么办？有开始和偏移量。我该如何查询？

Answer 1

messagesRef = Firebase(url: "https://"+CONSTANTS.FirebaseDB+".firebaseio.com/messages/1:1000")messagesRef
.queryOrderedByKey()
.queryStartingAtValue(5)
.queryEndingAtValue(10)
.observeEventType(FEventType.ChildAdded, withBlock: { (snapshot) in self.message_list.append(snapshot) });

这有点摸不着头脑，但根据此处的文档，它似乎应该可行 https://www.firebase.com/docs/ios-api/Classes/Firebase.html#//api/name/queryStartingAtValue:

Answer 2

我花了太多时间想通了，这里是解决方案。这是 Objective-C 代码，您可以将其转换为 swift。调用以下函数进行分页。

- (void)loadMoreMessages {
    
    if (!lastMessageKey) {
        // Loading messages first time
        [[[msgsReference queryOrderedByKey] queryLimitedToLast:K_MESSAGES_PER_PAGE] observeSingleEventOfType:FIRDataEventTypeValue withBlock:^(FIRDataSnapshot * _Nonnull snapshot) {
            if (snapshot.exists) {
                
                for (FIRDataSnapshot *child in snapshot.children) {
                   
                    NSMutableDictionary *dict = [child.value mutableCopy];
                    [dict setObject:child.key forKey:@"id"];
                    [messages addObject:dict];
                }
                
                lastMessageKey = [[snapshot.children.allObjects firstObject] key];
                NSLog(@"%@", messages);
            }
        }];
    }
    else {
        // Paging started
        [[[[msgsReference queryOrderedByKey] queryLimitedToLast:K_MESSAGES_PER_PAGE + 1] queryEndingAtValue:lastMessageKey] observeSingleEventOfType:FIRDataEventTypeValue withBlock:^(FIRDataSnapshot * _Nonnull snapshot) {
           
            if (snapshot.exists) {
       
                NSInteger count = 0;
                NSMutableArray *newPage = [NSMutableArray new];
                for (FIRDataSnapshot *child in snapshot.children) {
                    
                    // Ignore last object because this is duplicate of last page
                    if (count == snapshot.childrenCount - 1) {
                        break;
                    }
                    
                    count += 1;
                    NSMutableDictionary *dict = [child.value mutableCopy];
                    [dict setObject:child.key forKey:@"id"];
                    [newPage addObject:dict];
                }
                
                lastMessageKey = [[snapshot.children.allObjects firstObject] key];

                // Insert new messages at top of old array
                NSIndexSet *indexes = [NSIndexSet indexSetWithIndexesInRange: NSMakeRange(0, [newPage count])];
                [messages insertObjects:newPage atIndexes:indexes];
                NSLog(@"%@", messages);
            }
        }];
    }
}

这里是您需要的对象的描述：

#define K_MESSAGES_PER_PAGE 50 // A macro defining the numbers in one request
msgsReference // Firebase database messages node reference I'm also attaching the screenshot of my db structure for make you more clear
lastMessageKey // Is a NSString object which store the first key of last page
messages // Is a NSMutableArray storing the result

祝你好运！！ (y)

Answer 3

Swift 3.x

func fetchEarlierMessages(chatGroupId: String, messageKey: String, completion: @escaping ([Message]?) -> ()) {
    previousMessageRef = root.child("messages").child(chatGroupId)
    messageValueHandle = previousMessageRef.queryOrderedByKey().queryLimited(toLast: 10).queryEnding(atValue: messageKey).observe(.value, with: { snapshot in
        var messages: [Message] = []
        for child in snapshot.children.allObjects as! [FIRDataSnapshot] {
            guard var item = child.value as? [String: AnyObject] else { return }
            item["message_id"] = child.key as AnyObject
            if let message = self.parseMessage(snapshot: child) {
                messages.append(message)
            }
        }
        messages.removeLast()
        completion(messages)
    })
}

这里的'parseMessage'函数是我自定义的将snapshot转换为message model的函数，你可以使用自己的。消息键是您在初始 firebase 调用中加载的最早消息的键。

Answer 4

Swift 2017

我想为这个使用 firebase 的分页任务提供一个尽可能优雅的解决方案。此方法在初始化和加载更多数据时都会调用：

var items: [ItemEntity] = []
var theEndOfResults = false
var lastLoadedItem: ItemEntity? {
    return items.last
}
    
func loadItems() {
    let isLoadingMore = lastLoadedItem != nil
        
    Decorator.showStatusBarLoader()
    self.databaseReference
        .child("items")
        .queryOrdered(byChild: "date")
        .queryEnding(atValue: isLoadingMore ? lastLoadedItem!.date.stringValue : Date.distantFuture.stringValue)
        .queryLimited(toLast: 5)
        .observeSingleEvent(of: .value) { snapshot in
            var items = self.array(from: snapshot)
                .map { ItemEntity(parentKey: [=10=].parentKey, dictionary: [=10=].dict) }

                self.theEndOfResults = (self.lastLoadedItem == items.last) // prevent loading when it's no more items
                if isLoadingMore { items.removeFirst() } // removing as the firebase sending a repeated one
                self.items.append(contentsOf: items)

                self.reloadData()
        }
    }

控制器重新加载数据的功能。

override func reloadData() {
    tableV.reloadData()
    refreshControl.endRefreshing()
    tableV.loadControl?.endLoading()
    Decorator.hideStatusBarLoader()
}

当用户到达 tableView 的末尾时调用。

@objc public func loadMore() {
    guard self.theEndOfResults == false else { tableV.loadControl?.endLoading(); return }
    self..loadItems()
}

从快照创建数组

func array(from snapshot: DataSnapshot) -> [ParseResult] {
    var items = [ParseResult]()
    if let snapshots = snapshot.children.allObjects as? [DataSnapshot] {
        for snap in snapshots {
            if let postDictionary = snap.value as? Dictionary<String, AnyObject> {
                items.append((snap.key, postDictionary))
            }
        }
    }
    print(" DATA COME:\r\(snapshot)")
    //or u can use: dump(items, indent: 2, maxDepth: 5, maxItems: 15)
    return items.reversed()
}

感谢 video 使用“酷”firebase 参考api澄清了一些时刻。
Good luck 正在开发中，如果发现任何不清楚的地方，请提出任何问题。

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