Cache Facebook Profile Picture iOS

问题

I'm saving images to disk for my app users, who either upload a picture or connect with facebook and we display their profile picture.

This works great, but if a user changes their facebook profile picture, the code below never updates the image.

How can I check the facebook graph api to check if the image has been uploaded without redownloading the image? I am using AFNetworking so checking the 302 header is an option if its possible-just not sure how to implement.

//load image if exists on disk

    NSFileManager *fileManager = [NSFileManager defaultManager];
    NSString *documentsPath = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) firstObject];
    NSString *filePath = [documentsPath stringByAppendingPathComponent:uniqueCustomerPath];
    BOOL fileExists = [fileManager fileExistsAtPath:filePath];
    if(fileExists){
        UIImage* tempImage = [UIImage imageWithContentsOfFile:filePath];
        completionHandler(tempImage);
    }

//download image if not
    else{                
            NSString *imageUrl=[NSString stringWithFormat:@"https://graph.facebook.com/%@/picture?type=large", (long)self.customer.facebook_id];
            UIImage *tempImage = [UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:imageUrl]]];
            NSData *myData = UIImageJPEGRepresentation(tempImage, 1);                
            [myData writeToFile:filePath atomically:YES];
            completionHandler(tempImage);
        }

来源：https://stackoverflow.com/questions/29811388/cache-facebook-profile-picture-ios