嵌套查询块将数据加载到UITableView中

Question

我有一个以UIViewController作为子视图的tableView。表视图有三个部分，我使用了问题PFQueryTableViewController with 3 Sections的答案来完成这一任务，但是数据并没有被加载到表中。我正在viewWillAppear的后台执行我的查询。我担心这是由于嵌套的查询块造成的。我怎么才能解决这个问题？这是我的代码：

-(void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];


    PFQuery *gameQuery = [PFQuery queryWithClassName:@"Game"];
    [gameQuery whereKey:@"players" equalTo:[PFUser currentUser]];
    [gameQuery orderByDescending:@"createdAt"];

    [gameQuery findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error){

        self.myTurn = [NSMutableArray array];
        self.theirTurn = [NSMutableArray array];
        self.gameOver = [NSMutableArray array];
        self.allGames = [NSArray array];

        for(PFObject *object in objects)
        {
            if([object objectForKey:@"isOver"] == [NSNumber numberWithBool:YES])
            {
                [self.gameOver addObject:object];
            }

            else
            {

                PFRelation *relation = [object relationForKey:@"whoseTurn"];
                PFQuery *relQuery = [relation query];
                [relQuery findObjectsInBackgroundWithBlock:^(NSArray *userObjects, NSError *error1){


                    NSMutableArray *arr = [NSMutableArray array];
                    for(PFUser *user in userObjects)
                    {
                        [arr addObject:user.objectId];
                    }

                    if([arr containsObject:[PFUser currentUser].objectId])
                    {
                       [self.myTurn addObject:object];
                    }

                    else
                        [self.theirTurn addObject:object];

                }];
            }

        }

        self.allGames = [NSArray arrayWithObjects:self.myTurn, self.theirTurn, self.gameOver, nil];
        [self.tableView reloadData];

    }];

}

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    return [self.allGames count];
}


- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {

    return [[self.allGames objectAtIndex:section] count];
}

- (UITableViewCell *)tableView:(UITableView *)tableView
         cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *MyIdentifier = @"MyIdentifier";

    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:MyIdentifier];

    if (cell == nil)
    {
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault
                                       reuseIdentifier:MyIdentifier];
    }

    PFObject *object = self.allGames[indexPath.section][indexPath.row];
    cell.textLabel.text = [object objectForKey:@"numPlayers"];

    return cell;
}

Answer 1

下面是你可以做的事情：

-(void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];

    PFQuery *gameQuery = [PFQuery queryWithClassName:@"Game"];
    [gameQuery whereKey:@"players" equalTo:[PFUser currentUser]];
    [gameQuery orderByDescending:@"createdAt"];

    [gameQuery findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error){

        @synchronized(self.allGames) {
            self.allGames = [NSArray arrayWithObjects: [NSMutableArray array], [NSMutableArray array], [NSMutableArray array], nil];
            // These two instance variables contain values used in asynchronous blocks, to know how much responses are still expected
            self.expectedNbGames = [objects count];
            self.fetchedNbGames = 0;
        }

        for(PFObject *object in objects)
        {
            if([object objectForKey:@"isOver"] == [NSNumber numberWithBool:YES])
            {
                @synchronized(self.allGames) {
                    [((NSMutableArray *)self.allGames[2]) addObject:object];
                    self.fetchedNbGames++;
                }
            }

            else
            {
                PFRelation *relation = [object relationForKey:@"whoseTurn"];
                PFQuery *relQuery = [relation query];
                [relQuery findObjectsInBackgroundWithBlock:^(NSArray *userObjects, NSError *error1){

                    NSMutableArray *arr = [NSMutableArray array];
                    for(PFUser *user in userObjects)
                    {
                        [arr addObject:user.objectId];
                    }

                    @synchronized(self.allGames) {
                        if([arr containsObject:[PFUser currentUser].objectId])
                        {
                            [((NSMutableArray *)self.allGames[0]) addObject:object];
                        }

                        else
                            [((NSMutableArray *)self.allGames[1]) addObject:object];
                        }

                        self.fetchedNbGames++;
                        if (self.fetchedNbGames == self.expectedNbGames)
                        {
                            // We have now received the last expected response, it's time to sort everything again in descending order based on the "createdAt" value
                            NSSortDescriptor *dateDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"createdAt" ascending:NO];
                            NSArray *sortDescriptors = [NSArray arrayWithObject:dateDescriptor];
                            // self.allGames[0] is already sorted anyway because it's been left untouched from the initial PFQuery
                            self.allGames[1] = [self.allGames[1] sortedArrayUsingDescriptors:sortDescriptors];
                            self.allGames[2] = [self.allGames[2] sortedArrayUsingDescriptors:sortDescriptors];
                            // And... reload one last time!
                            dispatch_async(dispatch_get_main_queue(), ^{
                                [self.tableView reloadData];
                            });
                        }
                    }

                    dispatch_async(dispatch_get_main_queue(), ^{
                        [self.tableView reloadData];
                    });
                }];
            }

        }

        [self.tableView reloadData];
    }];
}

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    @synchronized(self.allGames) {
        return [self.allGames count];
    }
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
    @synchronized(self.allGames) {
        return [[self.allGames objectAtIndex:section] count];
    }
}

- (UITableViewCell *)tableView:(UITableView *)tableView
         cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *MyIdentifier = @"MyIdentifier";

    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:MyIdentifier];

    if (cell == nil)
    {
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault
                                       reuseIdentifier:MyIdentifier];
    }

    @synchronized(self.allGames) {
        PFObject *object = self.allGames[indexPath.section][indexPath.row];
        cell.textLabel.text = [object objectForKey:@"numPlayers"];
    }

    return cell;
}

@synchronized()构造用于确保线程安全，因为您将从不同的线程(主线程，但也是来自解析答复的异步块)访问(读写) sell.allGames数组。

请注意，我们要从异步块中多次分派reloadData，因为您的代码目前无法知道所有块何时完成运行。我们需要在主线程上分派这个调用，因为它将触发UI更新，而这些更新总是必须在主线程上完成。

看看这是否有效，也许您可以使用属性来稍微改进它(避免每次使用@synchronized()时编写self.allGames)。

UPDATE：我改进了我的答案，提供了一种在收到所有回复后重新排序的方法。这个过程涉及创建两个新的实例变量(名为fetchedNbGames和expectedNbGames的整数)，它们必须在收到来自Parse的每个响应后进行更新和相互比较。一旦接收到所有响应，结果数组就可以重新排序(由于请求被异步发送和处理，结果数组可能被无序排序)，表视图再次刷新。

这是未经测试的，可能有很大的改进空间，但这取决于您调整它以满足您的需要，您得到了基本的想法。

还请参阅this other SO thread，以更直观地解释与线程和异步调用相关的问题。