如何在Firebase的iOS SDK上执行分页查询？

Question

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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.
    });
});

我的代码工作--它收到最后5条消息(8-12条)。

但是，如果我有一个函数查询接下来的5条消息(2-6条)，该怎么办？有一个开始和偏移。我怎样才能质疑这个问题？

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) });

这是在黑暗中拍摄的，但它似乎应该基于这里的文档https://www.firebase.com/docs/ios-api/Classes/Firebase.html#//api/name/queryStartingAtValue工作。

Answer 2

花了太多的时间，我想出了办法，这就是解决办法。这是目标-C代码，你可以把它转换成swift。为分页目的调用下面的函数。

- (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);
            }
        }];
    }
}

下面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

好运！！(y)

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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)
    })
}

在这里，'parseMessage‘函数是我的自定义函数，可以将快照转换为消息模型，您可以使用自己的。消息密钥是您在初始防火墙调用中加载的最早消息的密钥。