iOS Memory issue (ARC) when downloading and saving large ammount of images from server

Question

The following code downloads 700+ images from a server with varying sizes, the issue here is that memory (even with ARC) is never released and eventually a memory warning appears followed by the application exiting. I've tried @autoreleasepool in this method and that didn't seem to work. Also I've tried stopping the for loop at different locations to see if memory is released after it finished, but it isn't.

This method is called inside a for loop and receives the image url and short name. It has been tried in a background thread and main thread with the same results (memory wise).

-(void)saveImage:(NSString*)image name:(NSString*)imageName{     
    int k = 0;
    for (int j = 0; j < [imageName length]; j++) {
        if ([imageName characterAtIndex:j] == '/') {
            k = j;
        }
    }if (k != 0) {
        imageName = [imageName substringFromIndex:k+1];
    }    

    NSString *fullPath = [documentsDirectory stringByAppendingPathComponent:[NSString stringWithFormat:@"%@", imageName]]; 

    if ([fileManager fileExistsAtPath:fullPath]) {
        [fileManager removeItemAtPath:fullPath error:nil];
    }

    NSURL *url = [NSURL URLWithString:image];
    NSData *data = [[NSData alloc]initWithContentsOfURL:url];
    NSLog(@"Saved: %d:%@", [fileManager createFileAtPath:fullPath contents:data attributes:nil], url); 
    data = nil;
}  


- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{

int cacheSizeMemory = 4*1024*1024; // 4MB
int cacheSizeDisk = 32*1024*1024; // 32MB
NSURLCache *sharedCache = [[NSURLCache alloc] initWithMemoryCapacity:cacheSizeMemory diskCapacity:cacheSizeDisk diskPath:@"nsurlcache"];
[NSURLCache setSharedURLCache:sharedCache];

self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
self.viewController = [[ViewController alloc] initWithNibName:@"ViewController" bundle:nil];
self.window.rootViewController = self.viewController;
[[UIApplication sharedApplication] setStatusBarHidden:YES];
[self.window makeKeyAndVisible];
return YES;
}

- (void)applicationDidReceiveMemoryWarning:(UIApplication *)application {
NSLog(@"mem warning, clearing cache");
[[NSURLCache sharedURLCache] removeAllCachedResponses];
}

Answer 1

Based on your screenshot, I think the issue is with the NSURL caching rather than the actual NSData objects. Can you try the following:

In your Application Delegate, setup an initial URL cache:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    // Do initial setup
    int cacheSizeMemory = 16*1024*1024; // 16MB
    int cacheSizeDisk = 32*1024*1024; // 32MB
    NSURLCache *sharedCache = [[NSURLCache alloc] initWithMemoryCapacity:cacheSizeMemory diskCapacity:cacheSizeDisk diskPath:@"nsurlcache"];
    [NSURLCache setSharedURLCache:sharedCache];

    // Finish the rest of your didFinishLaunchingWithOptions and head into the app proper
}

Add the following to your Application Delegate:

- (void)applicationDidReceiveMemoryWarning:(UIApplication *)application {
    [[NSURLCache sharedURLCache] removeAllCachedResponses];
}

A cache file will be created:"Library/Caches/your_app_id/nsurlcache"

The link to the Apple example is here: URL Cache

Code not tested but this (or something similar) should sort your problem + plus you can experiment with the cache sizes.

Can you post another screenshot of Allocations in action with this code? I would expect to see memory use stop growing and flatten out.

Answer 2

"dataWithContentsOfURL" returns an autoreleased NSData object and that doesn't normally get released until the end of the run loop or the end of the method, so it's no wonder you're filling up memory pretty quickly.

Change it to an explicit "initWithContentsOfURL" method and then force a release by doing "data = nil;" when you are absolutely done with the image data.