iOS中 Query 的幾個方法和如何使用 Wilddog 實現分頁加載

在開發項目中，咱們會常常用到數據查詢和分頁加載。咱們先了解一下 Query 的幾個方法，而後實現分頁加載。關於分頁加載，在傳統（有後端服務）的開發中，通常得須要後端同窗的協助，給前端同窗返回數據，再讓前端同窗實現功能，這樣會費事費人力。目前，給你們介紹一下 Wilddog 無後端開發分頁加載，你能夠自定製實現該功能。

1.SQL Queries 和 Wilddog Queries 之間的轉化

1.1 經過 ID 選 user (WHERE id = x)

將咱們的用戶存到了 /user 節點下面，因此，咱們能夠這樣獲取用戶信息：

Wilddog *ref = [[Wilddog alloc]initWithUrl:@"https://example-data-sql.wilddogio.com/user/1"];
[ref observeSingleEventOfType:WEventTypeValue withBlock:^(WDataSnapshot *snapshot) {
     NSLog(@"I fetched a user %@",snapshot.value);
}];

1.2 經過 email address 尋找 user (WHERE email = x)

Wilddog *ref = [[Wilddog alloc]initWithUrl:@"https://example-data-sql.wilddogio.com/user"];
WQuery *query = [ref queryEqualToValue:@"kato@wilddog.com"];
[query observeSingleEventOfType:WEventTypeValue withBlock:^(WDataSnapshot *snapshot) {
     NSLog(@"accounts matching email address %@",snapshot.value);
}];

1.3 獲取一段時間發送的信息(WHERE timestamp BETWEEN x AND y)

Wilddog *ref = [[Wilddog alloc]initWithUrl:@"https://example-data-sql.wilddogio.com/messages"];
Query *query = [ref queryStartingAtValue:startTime];
query = [query queryEndingAtValue:endTime];
[query observeSingleEventOfType:WEventTypeValue withBlock:^(WDataSnapshot *snapshot) {
     NSLog(@"messages in range %@",snapshot.value);
}];

2.介紹幾個方法

2.1 – queryStartingAtValue:返回一個WQuery引用，這個引用用來監測數據的變化，這些被監測的數據的值均大於或等於startValue

//篩選出高度大於等於3m的恐龍
Wilddog *ref = [[Wilddog alloc] initWithUrl:@"https://dinosaur-facts.wilddogio.com/dinosaurs"];
WQuery *query = [ref queryOrderedByChild:@"height"];
query =[query queryStartingAtValue:@3];
[query observeEventType:WEventTypeChildAdded withBlock:^(WDataSnapshot *snapshot) {
     NSLog(@"%@",snapshot.key);
}];

2.2 - queryEndingAtValue:返回一個WQuery引用，這個引用用來監測數據的變化，這些被監測的數據的值均小於或者等於endValue

//篩選出高度小於等於1m的恐龍
Wilddog *ref = [[Wilddog alloc] initWithUrl:@"https://dinosaur-facts.wilddogio.com/dinosaurs"];      
WQuery *query = [ref queryOrderedByChild:@"height"];
query =[query queryEndingAtValue:@1];
[query observeEventType:WEventTypeChildAdded withBlock:^(WDataSnapshot *snapshot) {
     NSLog(@"%@",snapshot.key);
}];

2.3 - queryLimitedToFirst:返回一個新WQuery引用，獲取從第一條開始的指定數量的數據

//篩選出數據庫前100只恐龍
Wilddog *ref = [[Wilddog alloc] initWithUrl:@"https://dinosaur-facts.wilddogio.com/dinosaurs"];
WQuery *query = [ref queryLimitedToFirst:100];
[query observeEventType:WEventTypeChildAdded withBlock:^(WDataSnapshot *snapshot) {
     NSLog(@"%@",snapshot.key);
}];

2.4 - observeEventType:withBlock:獲取初始數據和監聽某一節點處數據的變化

//獲取初始的那些恐龍和監聽實時增長的恐龍
Wilddog *ref = [[Wilddog alloc] initWithUrl:@"https://dinosaur-facts.wilddogio.com/dinosaurs"];
[ref observeEventType:WEventTypeChildAdded withBlock:^(WDataSnapshot *snapshot) {
     NSLog(@"%@",snapshot.key);
}];

3.實現分頁加載

如今用一個筆記本的實例來舉例實現下拉加載，結合 - queryOrderedByChild: 、– queryStartingAtValue: 、– queryLimitedToFirst: 等方法實現分頁加載。
在 .h 文件中

@interface NoteService ()
@property (nonatomic,strong) Wilddog *wilddog;
@property (nonatomic,strong) NSNumber *lastUpdatetimeOnPrevPage;
@end
@implementation NoteService
-(instancetype)init
{
     if (self = [super init]) {
         _wilddog = [[Wilddog alloc] initWithUrl: @"https://<your-noteapp-name>.wilddogio.com/note"];
         _lastUpdatetimeOnPrevPage = [NSNumber numberWithLong:0];
     }
     return self;
}

假如咱們按照最後編輯時間做爲排序，一次性把全部的筆記都拉下來，咱們能夠這樣寫（這兒不是分頁）:

-(void)getAllNotes:(void (^)(NSArray *))complete
 {
     WQuery *query = [self.wilddog queryOrderedByChild:@"lastUpdate"];
     [query observeSingleEventOfType:WEventTypeValue withBlock:^(WDataSnapshot *snapshot) {
        if (snapshot) {
            NSMutableArray *noteList = [NSMutableArray new];
            [snapshot.value enumerateKeysAndObjectsUsingBlock:^(id _Nonnull key, id _Nonnull obj, BOOL * _Nonnull stop) {
                NoteModel *model = [[NoteModel alloc] initWithDict:obj];
                [noteList addObject:model];
            }];
            if (complete) {
                complete(noteList);
            }
        }
     }];
 }

回到重點，咱們實現分頁加載：

-(void)getNotesWithPate:(int)page complete:(void (^)(NSArray *))complete
    {
         BOOL isFirstPath = page == 0; 
    
         NSString *orderKey = @"lastUpdate";
         WQuery *query = [self.wilddog queryOrderedByChild:orderKey];
         query = [query queryStartingAtValue:_lastUpdatetimeOnPrevPage];
    
         // kDefaultLimitPerPage 一次定義拉取多少條
         query = [query queryLimitedToFirst:kDefaultLimitPerPage + (isFirstPath ? 0 : 1)];      //queryStartingAtValue 是大於等於上次頁最後一個的值，因此要 kDefaultLimitPerPage ＋ 1個，而後去掉這個上一頁最後一個。
         [query observeSingleEventOfType:WEventTypeValue withBlock:^(WDataSnapshot *snapshot) {
              if (snapshot) {
                  NSMutableArray *noteList = [NSMutableArray new];
                  WDataSnapshot *child = nil;
                  NSEnumerator *enumerator = snapshot.children;
                  while (child = [enumerator nextObject]) {
                      NoteModel *model = [[NoteModel alloc] initWithDict:[child value]];
                      if ([self.lastUpdatetimeOnPrevPage longLongValue] == model.lastUpdate) {
                           continue; //去掉重複的一個
                      }
                      NSLog(@"model.lastUpdate %lld",model.lastUpdate);
                      [noteList addObject:model];
                  }
                  if (noteList.count > 0) {
                      self.lastUpdatetimeOnPrevPage = @([(NoteModel *)noteList.lastObject lastUpdate]);
                  }
                  if (complete) {
                        complete(noteList);
                  }
             }
          } withCancelBlock:^(NSError *error) {
    
        }];
    }

在上拉加載的時候，只要將參數 page 自加傳入便可。當每一次數據加載完成以後，咱們用 complete block 將參數回調。

數據的查詢和加載，更多須要的接口能夠參考 iOS API