Question

我使用 Parse.com构建 iOS 应用

我想首先从缓存检索数据＆amp; 然后从网络

为此，我使用了 kPFCachePolicyCacheThenNetwork 缓存政策，选择我的要求。

PFQuery *employesquery = [PFQuery queryWithClassName:@"employesTable"];
    [employesquery whereKey:@"UserID" equalTo:[PFUser currentUser]];
    [employesquery includeKey:@"empID"];
    [employesquery includeKey:@"empID.user"];

    MBProgressHUD *hud = [MBProgressHUD showHUDAddedTo:self.navigationController.view  animated:YES];


//APPLIES CACHE... ********
        employesquery.cachePolicy = kPFCachePolicyCacheThenNetwork;

    [employesquery findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
        [hud hide:YES];
        NSMutableArray *empData = [[NSMutableArray alloc] init];
        for (PFObject *object in objects)
        {
            //Getting Data For Item
            PFObject *employeeObject = [object objectForKey:@"empID"];
            [empData addObject:employeeObject];
        }
        [self fetchEmployeeData:empData];

    }];

但使用此每个数据检索两次，可重复的数据。

如何避免此可重复数据，

数据从网络获取后，先前显示的数据（使用缓存）将被清除/隐藏。

我尝试了[PFQuery clearAllCachedResults];

清除了所有缓存，以便缓存中没有数据用于下一次迭代。

Answer 1

使用kPFCachePolicyCacheThenNetwork作为缓存策略时，您的回调始终会被触发两次。这是因为策略是使用缓存然后转到网络，而不是专门使用缓存。

解决方案是跟踪您的块被调用的次数，这样您就可以确定是从网络获取缓存结果还是新结果。 Parse有more information about this here。

Answer 2

您必须清除用于在块findObjectsInBackgroundWithBlock中解析的存储对象的数据数组：^（NSArray * objects，NSError * error）。

- (void)refreshData
{
    PFUser *currentUser = [PFUser currentUser];

    NSPredicate *predicate = [NSPredicate predicateWithFormat:[NSString stringWithFormat:@"%@ = '%@' OR %@ = '%@'",
                                                           kMessageUserSendIdKey, currentUser.objectId,
                                                           kMessageUserReceiveIdKey, currentUser.objectId]];

PFQuery *query = [PFQuery queryWithClassName:kMessageClassKey predicate:predicate];
[query addDescendingOrder:kMessageTimeCreatedKey];
[query includeKey:kMessageUserSendKey];
[query includeKey:kMessageUserReceiveKey];
[query setCachePolicy:kPFCachePolicyCacheThenNetwork];

[query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
    if (!error) {
        // TODO: Clear the data array
        self.messagesDataArray = [NSMutableArray new];

        if (objects.count > 0) {
            [self.messagesDataArray addObject:objects[0]];
            for (int i = 1; i < objects.count; i++) {
                PFObject *messageChat = [objects objectAtIndex:i];
                if (![self isExistWithMessageChat:messageChat]){
                    [self.messagesDataArray addObject:messageChat];
                }
            }
            [self.tableView reloadData];
        }
     }
     else {
        NSLog(@"Error: %@ %@", error, [error userInfo]);
     }
   }];
}

iOS + Parse.com：kPFCachePolicyCacheThenNetwork，检索数据两次

2 个答案: