Use UIPasteboard to pass around data between the main app and app extensions

AppDelegate.m

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // Create a pasteboard, and pass some data to it
    // the created pasteboard can be specified as persistent as well
    UIPasteboard *pasteboard = [UIPasteboard pasteboardWithName:@"Widget" create:YES];
    pasteboard.persistent = YES;
    pasteboard.string = @"やっほー！";
    
    return YES;
}

WidgetViewController.m

- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];
    
    // Get some data from the created pasteboard
    UIPasteboard *pasteboard = [UIPasteboard pasteboardWithName:@"Widget" create:NO];
    if (pasteboard) {
        self.titleLabel.text = pasteboard.string;
    }
}