UITableView cellForRowAtIndexPath causing slow scrolling - iphone

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;