UITableView cellForRowAtIndexPath causing slow scrolling

Question

My table is scrolling incredibly slow, and I think it's caused by the Core Data methods within my cellForRowAtIndexPath. Below is my cellForRowAtIndexPath method:

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {

static NSString *CellIdentifier = @"Cell";

UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) {
    cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier] autorelease];
}

NSManagedObject *info = [buildingArray objectAtIndex: [indexPath row]];
// all rooms have been scanned for building
if([self allRoomsScanned: [[info valueForKey:@"buildingid"] intValue]]) {
    cell.accessoryType = UITableViewCellAccessoryCheckmark;
    [cell.textLabel setTextColor: [UIColor lightGrayColor]];
}
else {
    cell.accessoryType = UITableViewCellAccessoryDisclosureIndicator;
    [cell.textLabel setTextColor: [UIColor blackColor]];
}
[cell.textLabel setFont:[UIFont fontWithName:@"Helvetica-Bold" size:16.0]];
[cell.textLabel setText:[info valueForKey:@"buildingname"]];

return cell;
}

And here is the allRoomsScanned method and allDevicesScanned method:

- (BOOL) allRoomsScanned: (int) buildingID {
NSMutableArray *scannedRoomArray = [[NSMutableArray alloc] init];
// Get all user_device
NSManagedObjectContext *context = [self managedObjectContext];
NSError *error;
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription 
                               entityForName:@"user_device" inManagedObjectContext:context];
[fetchRequest setEntity:entity];
NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:&error];
NSNumber *deviceid = [NSNumber numberWithInt: 0];
NSNumber *roomid = [NSNumber numberWithInt: 0];
//int lastRoomID = 0;

for (NSManagedObject *info in fetchedObjects) {
    // Get all device
    deviceid = [info valueForKey:@"deviceid"];
    fetchRequest = [[NSFetchRequest alloc] init];
    entity = [NSEntityDescription 
              entityForName:@"device" inManagedObjectContext:context];
    [fetchRequest setEntity:entity];

    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"(deviceid = %d)", [deviceid intValue]];
    [fetchRequest setPredicate:predicate]; 
    NSArray *fetchedDevices = [context executeFetchRequest:fetchRequest error:&error];
    for (NSManagedObject *infod in fetchedDevices) {
        // Get all room
        roomid = [infod valueForKey:@"roomid"];
        fetchRequest = [[NSFetchRequest alloc] init];
        entity = [NSEntityDescription 
                  entityForName:@"room" inManagedObjectContext:context];
        [fetchRequest setEntity:entity];

        NSPredicate *predicate = [NSPredicate predicateWithFormat:@"(roomid = %d) AND (buildingid = %d)", [roomid intValue], buildingID];
        [fetchRequest setPredicate:predicate]; 
        NSMutableArray *fetchedRoom = [[context executeFetchRequest:fetchRequest error:&error] mutableCopy];

        // add room to array if room belongs to selected building and room not already added
        if([fetchedRoom count] > 0) { //&& lastRoomID != [roomid intValue]) {
            for (NSManagedObject *info in fetchedRoom) {
                NSLog(@"room id: %@", [info valueForKey:@"roomid"]);
                // add room ids to array if not already there
                if (![scannedRoomArray containsObject:[info valueForKey:@"roomid"]] && [self allDevicesScanned:[[info valueForKey:@"roomid"] intValue]])
                    [scannedRoomArray addObject: [info valueForKey:@"roomid"]];
            }
            //lastRoomID = [roomid intValue];
        }
    }
}

fetchRequest = [[NSFetchRequest alloc] init];
entity = [NSEntityDescription 
          entityForName:@"room" inManagedObjectContext:context];
[fetchRequest setEntity:entity];

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"(buildingid = %d)", buildingID];
[fetchRequest setPredicate:predicate]; 
NSArray *fetchedRoomTotal = [context executeFetchRequest:fetchRequest error:&error];
//NSLog(@"Total Rooms for Building: %d", [fetchedRoomTotal count]);
//NSLog(@"Scanned Rooms for Building: %d", [scannedRoomArray count]);
//NSLog(@"Scanned rooms: %@", scannedRoomArray);
if([fetchedRoomTotal count] == [scannedRoomArray count] && [fetchedRoomTotal count] > 0) {
    return YES;
}
else {
    return NO;
}
}

- (BOOL) allDevicesScanned: (int) roomID {
NSMutableArray *scannedDeviceArray = [[NSMutableArray alloc] init];
// Get all user_device
NSManagedObjectContext *context = [self managedObjectContext];
NSError *error;
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription 
                               entityForName:@"user_device" inManagedObjectContext:context];
[fetchRequest setEntity:entity];
NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:&error];
NSNumber *deviceid = [NSNumber numberWithInt: 0];
//NSNumber *roomid = [NSNumber numberWithInt: 0];

for (NSManagedObject *info in fetchedObjects) {
    // Get all device
    deviceid = [info valueForKey:@"deviceid"];
    fetchRequest = [[NSFetchRequest alloc] init];
    entity = [NSEntityDescription 
              entityForName:@"device" inManagedObjectContext:context];
    [fetchRequest setEntity:entity];

    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"(deviceid = %d) AND (roomid = %d)", [deviceid intValue], roomID];
    [fetchRequest setPredicate:predicate]; 
    NSArray *fetchedDevices = [context executeFetchRequest:fetchRequest error:&error];
    for (NSManagedObject *infod in fetchedDevices) {

        // add device to array
        if([fetchedDevices count] > 0) {
            NSLog(@"room id: %d", roomID);
            // add device ids to array if not already there
            if (![scannedDeviceArray containsObject:deviceid])
                [scannedDeviceArray addObject: deviceid];
        }
    }
}

fetchRequest = [[NSFetchRequest alloc] init];
entity = [NSEntityDescription 
          entityForName:@"device" inManagedObjectContext:context];
[fetchRequest setEntity:entity];

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"(roomid = %d)", roomID];
[fetchRequest setPredicate:predicate]; 
NSArray *fetchedDeviceTotal = [context executeFetchRequest:fetchRequest error:&error];
//NSLog(@"Total Devices for Room: %d", [fetchedDeviceTotal count]);
//NSLog(@"Scanned Devices for Room: %d", [scannedDeviceArray count]);
//NSLog(@"Scanned Devices: %@", scannedDeviceArray);
if([fetchedDeviceTotal count] == [scannedDeviceArray count] && [fetchedDeviceTotal count] > 0) {
    return YES;
}
else {
    return NO;
}
}

Any idea on how to get rid of the latency when scrolling? I'm assuming I may be doing something inefficiently with either my core data calls or the way I'm calling the method in cellForRowAtIndexPath.

Thank for any help. It is greatly appreciated.

Answer 1

You really should not make Fetch Requests and processing like that on the main thread while scrolling a table view. As rokjarc said, you should definitely save the result of your (quite heavy) allRoomsScanned method. I'd suggest adding a new style, i.e. with an activity indicator, that the cell gets when you don't have a result for that input yet. As soon as the load is complete you refresh the table view cell.

Attention: You can't use your default NSManagedObjectContext in allRoomsScanned and allDevicesScanned. You need to initialize a new context in the background thread. Either initialize a new context at the beginning of the block and pass it as a method parameter or create a new one right in the methods.

NSManagedObject *info = [buildingArray objectAtIndex: [indexPath row]];

NSNumber *cachedResult = [self.scanResults objectForKey:info.objectID];
if (cachedResult == nil) {
    // style loading state

    int scanInfo = [[info valueForKey:@"buildingid"] intValue];
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        BOOL result = [self allRoomsScanned: scanInfo];
        [self.scanResults setObject:[NSNumber numberWithBool:result] forKey:info.objectID];
        [self.tableView reloadRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationAutomatic]
    });

} else if (cachedResult.boolValue == YES) {
    cell.accessoryType = UITableViewCellAccessoryCheckmark;
    [cell.textLabel setTextColor: [UIColor lightGrayColor]];
} else if (cachedResult.boolValue == NO) {
    cell.accessoryType = UITableViewCellAccessoryDisclosureIndicator;
    [cell.textLabel setTextColor: [UIColor blackColor]];
}

[cell.textLabel setFont:[UIFont fontWithName:@"Helvetica-Bold" size:16.0]];
[cell.textLabel setText:[info valueForKey:@"buildingname"]];

return cell;