iOS code skipping over dispatch_async - ios

Question

I need to download data from a URL (this prints the data in JSON format) and store it in a "configuration" file for the app in the app's AppDelegate.m file. When I run the app, it simply skips over the dispatch_async code for some reason. Why is this happening and how do I fix this ?
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
//Download the config.json file
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
NSString *configFileUrl = #"http://webserviceurl";
//NSString *downloadToFile = #"Configuration.json";
NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:configFileUrl]];
[self performSelectorOnMainThread:#selector(writeDataToConfigurationJsonFile:) withObject:data waitUntilDone:YES];
});
//More code below
And this is where I am writing the data to a file in the Documents directory of the app:
-(void)writeDataToConfigurationJsonFile:(NSData*)jsonData{
NSString *content = [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding];
//get the documents directory:
NSArray *paths = NSSearchPathForDirectoriesInDomains
(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
//make a file name to write the data to using the documents directory:
NSString *fileName = [NSString stringWithFormat:#"%#/Configuration.json", documentsDirectory];
//save content to the documents directory
[content writeToFile:fileName
atomically:YES
encoding:NSUTF8StringEncoding
error:nil];
}

Answer 1

performSelectorOnMainThread is a run loop method, you need to use:
dispatch_async(dispatch_get_main_queue(), ^{/*code*/});

Answer 2

You can nest a call to dispatch_sync() within dispatch_async() to ensure after the data is downloaded, the data is written synchronously on the main thread.
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
//Download the config.json file
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
NSString *configFileUrl = #"http://webserviceurl";
//NSString *downloadToFile = #"Configuration.json";
NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:configFileUrl]];
dispatch_sync(dispatch_get_main_queue(), ^{
[self writeDataToConfigurationJsonFile:data];
});
});
}

Answer 3

When you're asynchronously dispatching a thread, create a new serial/concurrent queue.
And for the synchronous dispatch, go back to the main queue (try without using 'waitUntilDone:YES'):
dispatch_async(dispatch_queue_create("com.yourOrgName", DISPATCH_QUEUE_SERIAL), ^{
NSString *configFileUrl = #"http://webserviceurl";
//NSString *downloadToFile = #"Configuration.json";
NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:configFileUrl]];
dispatch_sync(dispatch_get_main_queue(), ^ {
[self performSelector:#selector(writeDataToConfigurationJsonFile:) withObject:data afterDelay:0.0f];
});
});

Answer 4

For the purpose of this application, the best way was to use a synchronous request rather than an asynchronous request. Here's the final chunk of code -
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:#"some url"]];
NSError *error = nil;
NSURLResponse *response = nil;
NSData *receivedData = [NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&error];
NSString *string = [[NSString alloc] initWithData:receivedData encoding:NSUTF8StringEncoding];
NSLog(#"response");
NSArray *paths = NSSearchPathForDirectoriesInDomains
(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
//make a file name to write the data to using the documents directory:
NSString *fileName = [NSString stringWithFormat:#"%#/Configuration.json", documentsDirectory];
//save content to the documents directory
[string writeToFile:fileName
atomically:YES
encoding:NSUTF8StringEncoding
error:nil];