Sync.m

#import "Sync.h"
#import "NSData+MD5Digest.h"

@implementation Sync

+ (Sync *) defaultWorker {
    //GCD singleton pattern: http://www.galloway.me.uk/tutorials/singleton-classes/
    static Sync *defaultWorker = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        defaultWorker = [[self alloc] init];
    });
    return defaultWorker;
}

- (void) downloadUpdates {
    __weak id this = self;
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSArray *documentPaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
        NSString *documentsDirectoryPath = [documentPaths objectAtIndex:0];
        
        NSString *checksum = nil;
        BOOL initialSync = YES;
        if([this isUpdatePending]){
            NSString *filePath = [documentsDirectoryPath stringByAppendingPathComponent:@"updated.db"];
            NSData *contents = [NSData dataWithContentsOfFile:filePath];
            checksum = [contents MD5HexDigest];
            initialSync = NO;
        }else if([this hasPerformedInitialSync]){
            NSString *filePath = [documentsDirectoryPath stringByAppendingPathComponent:@"database.db"];
            NSData *contents = [NSData dataWithContentsOfFile:filePath];
            checksum = [contents MD5HexDigest];
            initialSync = NO;
        }
        
        
        NSMutableURLRequest *request = [[NSMutableURLRequest alloc] init];
        [request setHTTPMethod:@"GET"];
        NSString *url = @"http://localhost:5000/api/updates/";
        if(checksum){
            url = [NSString stringWithFormat:@"%@?checksum=%@", url, checksum];
        }
        [request setURL:[NSURL URLWithString:url]];
        
        [[[NSURLSession sharedSession] dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *err) {
            NSInteger statusCode = [(NSHTTPURLResponse *)response statusCode];
            
            if (statusCode == 304) {
                //no database update available
                return;
            }else if(statusCode == 200){
                //updated database
                if(initialSync){
                    [data writeToFile:@"database.db" atomically:YES];
                }else{
                    [data writeToFile:@"updated.db" atomically:YES];
                }
            }
            
        }] resume];
    });
}

- (void) applyOutstandingUpdates {
    __weak id this = self;
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        if(![this isUpdatePending]){
            return; //no update pending
        }
        
        NSArray *documentPaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
        NSString *documentsDirectoryPath = [documentPaths objectAtIndex:0];
        
        NSString *updatePath = [documentsDirectoryPath stringByAppendingPathComponent:@"updated.db"];
        NSString *dbPath = [documentsDirectoryPath stringByAppendingPathComponent:@"database.db"];
        
        NSFileManager *fileManager = [NSFileManager defaultManager];
        NSError *err;
        [fileManager moveItemAtPath:updatePath toPath:dbPath error:&err];
    });

}

- (BOOL) isUpdatePending {
    NSArray *documentPaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectoryPath = [documentPaths objectAtIndex:0];
    
    NSString *filePath = [documentsDirectoryPath stringByAppendingPathComponent:@"updated.db"];
    
    NSFileManager *fileManager = [NSFileManager defaultManager];
    return [fileManager fileExistsAtPath:filePath];
}

- (BOOL) hasPerformedInitialSync {
    NSArray *documentPaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectoryPath = [documentPaths objectAtIndex:0];
    
    NSString *filePath = [documentsDirectoryPath stringByAppendingPathComponent:@"database.db"];
    
    NSFileManager *fileManager = [NSFileManager defaultManager];
    return [fileManager fileExistsAtPath:filePath];
}

@end