如何按用户对PFObject进行分组，然后对其进行排序？

Question

I am currently building a simple messaging service for iOS with Parse where users send alerts to each other.

On the main screen the alerts will be grouped by user and previously shown alerts will be shown as well.

Recent Alerts

Bob - 2
Jim - 3
Tom
Bill

Currently I am fetching the items by date and grouping them by userId like this:

{
"USER_ID1":[
{
"SENDER_NAME":"JOHN",
"SHOWN":0
},
{
"SENDER_NAME":"JOHN",
"SHOWN":1
}
],
"USER_ID2":[
{
"SENDER_NAME":"BOB",
"SHOWN":0
},
{
"SENDER_NAME":"BOB",
"SHOWN":0
}
]...
}

This is how the bit fetching the alerts works

PFQuery *query = [PFQuery queryWithClassName:@"Alerts"];
[query whereKey:@"recipiantIds" equalTo:[[PFUser currentUser] objectId]];
[query orderByAscending:@"createdAt"];
[query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {

    self.alerts = [NSMutableArray arrayWithArray:objects];
    if (error) {
        NSLog(@"Error: %@ %@", error, [error userInfo]);
        [[[UIAlertView alloc] initWithTitle:@"Could not fetch alerts" message:@"Please check your internet connection" delegate:nil cancelButtonTitle:@"OK" otherButtonTitles: nil] show];
    }
    else {
        for (PFObject *alert in objects) {
            [self addAlert:alert[@"sender_name"] andShown:alert[@"shown"] andId:alert.objectId andSenderId:alert[@"senderId"] andDate:alert.createdAt];
        }

        [self.tableView reloadData];
    }
}];

And the function for adding alerts to the Dictionary:

- (void)addAlert:(NSString *)senderName andShown:(NSNumber*)shown andId:(NSString*)alertId andSenderId:(NSString*)senderId andDate:(NSDate *)date {
    NSDictionary *items;
    if (shown != nil) {
        items = @{@"sender":senderName, @"shown":shown, @"alertid":alertId, @"senderId":senderId, @"datestamp":date};
    }
    else {
        items = @{@"sender":senderName, @"alertid":alertId, @"senderId":senderId, @"datestamp":date};
    }
    if ([self.myalerts objectForKey:senderId] != nil) {
        NSMutableArray *userAlerts = [self.myalerts objectForKey:senderId];
        [userAlerts addObject:items];
        [self.myalerts setObject:userAlerts forKey:senderId];
    }
    else {
        NSMutableArray *userAlerts = [NSMutableArray new];
        [userAlerts addObject:items];
        [self.myalerts setObject:userAlerts forKey:senderId];
    }
}

If there is any nice way to sort them like SnapChat does I'd love to know it.

Thanks guys!

Answer 1

Yes, this can be done. You'll want to sort your array of alerts with an NSSortDescriptor (key is whatever key in your objects you want to sort by):

NSSortDescriptor *descriptor = [[NSSortDescriptor alloc] initWithKey:@"SENDER_NAME" ascending:YES];
NSArray *sortDescriptors = [NSArray arrayWithObject:descriptor];
NSArray *sortedArray = [objects sortedArrayUsingDescriptors:sortDescriptors];

Update

Ok, so if you're looking to order it like snapchat does, you'll need a mixed approach here. I'm not going to spell out the whole algorithm, but it would probably be something like this:

1) create a new array to hold the newly sorted notifications

2) loop through your objects array and move all unread notifications to the array you created in step one

3) sort the array you created in step one by time (see below)

4) sort the rest of the objects in your objects array by time (see below)

5) add the objects you just sorted to the new array

Sorting by time (very similar to original answer):

NSSortDescriptor *dateDescriptor = [NSSortDescriptor
                                     sortDescriptorWithKey:@"date" 
                                                 ascending:NO];
NSArray *sortDescriptors = [NSArray arrayWithObject:dateDescriptor];
NSArray *sortedEventArray = [nodeEventArray
         sortedArrayUsingDescriptors:sortDescriptors];