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

Question

I have build an iOS app using Parse.com

I want to retrieve data from cache initially & then from network

For this i have used kPFCachePolicyCacheThenNetwork cache policy wchich is opt for my requirement.

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];

    }];

But using this Each data retrieve two times, repeatable data.

How to avoid this repeatable data,

Once the data is getting from networks the previously shown data (using cache) gets cleared/ hidden.

I have tried with [PFQuery clearAllCachedResults];

It was cleared all the cache so that there no data in cache for the next iteration.

Answer 1

When using kPFCachePolicyCacheThenNetwork as your caching policy your callback is always triggered twice. This is because the policy is to use the cache and then go to the network, not to use the cache exclusively.

The solution is to keep track of how many times your block has been called, so you can establish whether you're getting the cached result or the fresh result from the network. Parse have more information about this here .

Answer 2

You must clear the data array which is used for store objects queried from parse in the block findObjectsInBackgroundWithBlock:^(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]);
     }
   }];
}