Question

I am currently building an application that generates images and stores them in an NSMutableArray, which then used in a UINavigation (Cell.imageView.image). I need to be able to handle up to 2000 images without causing lag in my application.

Currently how I set this generating is by calling the generating method when cellForRowAtIndexPath is accessed. Which seems to cause a 4-5 second lag before the next navigation is called.

fortunately after those 4-5 seconds the generating is done and there is no issues.

In the world of iProducts waiting 4-5 seconds isn’t really an option. I am wondering what my options are for generating these images in the background. I tried using threads [self performSelectorInBackground:@selector(presetSnapshots) withObject:nil];
but that only gave me issues about vectors for some reason.

Heres the generating code:

-(void)presetSnapshots{
    //NSAutoreleasePool* autoReleasePool = [[NSAutoreleasePool alloc] init];
    for (int i = 0; i < [selectedPresets count]; ++i){
        GraphInfo* graphInfo = [selectedPresets objectAtIndex:i];
        graphInfo.snapshot = [avc takePictureOfGraphInfo:graphInfo PreserveCurrentGraph:false];
        [graphInfo.snapshot retain];
    }
    //[autoReleasePool drain];
    presetSnapshotFinished = YES;

}

inside - (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath { is

   if (presetSnapshotFinished == NO){
        [self presetSnapshots];
        //[self performSelectorInBackground:@selector(presetSnapshots) withObject:nil];

    }

    cell.imageView.image = [[selectedPresets objectAtIndex:indexPath.row] snapshot];

Edit:

I also rather not use coreData for this. The images are 23×23 and coming out to about 7kb. So its about 6MB being use at any giving time in memory.

Answer 1

You can use Grand Central Dispatch (GCD) to fire up [self presetSnapshots]

dispatch_queue_t working_queue = dispatch_queue_create("com.yourcompany.image_processing", NULL);
dispatch_queue_t high = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH,NULL);
dispatch_set_target_queue(working_queue,high);
dispatch_queue_t main_queue = dispatch_get_main_queue();

dispatch_async(working_queue,^{
    if (presetSnapshotFinished == NO){
       [self presetSnapshots];
    }
    dispatch_async(main_queue,^{
        cell.imageView.image = [[selectedPresets objectAtIndex:indexPath.row] snapshot];
    });
});