Question

On Facebook’s iPhone app, the news feed refreshes every time the app becomes active. I would like to do something similar, but I’m concerned about a race condition. The general bootstrapping of my app is as follows:

UIApplicationDelegate

- (void)applicationDidFinishLaunching:(UIApplication*)application
{
  [window addSubview:[self.navigationController view];
  [window makeKeyAndVisible];
}

- (void)applicationDidBecomeActive:(UIApplication*)application
{
  [rootViewController refresh];
}

RootViewController

#pragma mark custom

- (void)refresh
{
  if (self.newsFeedModel == nil) {
   self.newsFeedModel = [[NewsFeedModel alloc] initWithDelegate:self];
  }
  [self.newsFeedModel request];
}

#pragma mark UIViewController

- (void)viewDidLoad
{
  // initialize the table
  // add subviews and whatnot
}

#pragma mark NewsFeedDelegate

- (void)newsFeedSucceeded:(NSMutableArray*)feed
{
  // reload table view with new feed data
}

After sprinkling NSLog everywhere, I determined the order of operations to be:

1. applicationDidFinishLaunching
2. applicationDidBecomeActive
3. refresh
4. viewDidLoad
5. newsFeedSucceeded

Notice how refresh is called before the root view has been loaded. While we’re busy querying the server, the root view loads. When the server responds, the root view is populated with the feed. This works in most cases because the network operation takes a long time. However, if the network operation finishes faster than view can be loaded, then I will be attempting to construct the news feed before the view has been loaded. This would be bad. What is the best Cocoa Touch practice for solving this race condition? I would just set a bunch of flags to determine what state we’re in and refresh the news feed depending on the state, but I’m wondering if there were built in events in Cocoa Touch to handle this for me.

Answer 1

I think you want to take a look at applicationWillEnterForeground: instead.

applicationDidBecomeActive: can be called while your app is still running in the foreground. For instance if a text message comes while your app is in the foreground and the user dismisses it, applicationDidBecomeActive: will get called.

You can subscribe to the UIApplicationWillEnterForegroundNotification event in your RootViewController using NSNotificationCenter. I would do this in RootViewController initWithNibName: or whichever init method you are using.

Now you just need to call refresh in 2 places. Once at the end of viewDidLoad and again whenever applicationWillEnterForeground: is called.

This should solve your race condition problem. Since RootViewController is handling it’s own refreshing when it knows it is ok to do so.

RootViewController

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil {
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if(self) {
        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(applicationWillEnterForeground:) name:UIApplicationWillEnterForegroundNotification object:nil];
    }
    return self;
}

- (void)viewDidLoad
{
    // initialize the table
    // add subviews and whatnot
    [self refresh];
}


- (void)applicationWillEnterForeground:(UIApplication *)application {
    [self refresh];
}

- (void)dealloc
{
    [[NSNotificationCenter defaultCenter] removeObserver:self];
    [super dealloc];
}