深入GCD（四）：使用串行队列实现简单的预加载

其主要思路是使用gcd创建串行队列，然后在此队列中先后执行两个任务：1.预加载一个viewController 2.将这个viewController推入
代码如下：
@implementation DWAppDelegate
{
    dispatch_queue_t _serialQueue;
    UINavigationController *_navController;
}

- (dispatch_queue_t)serialQueue
{
    if (!_serialQueue) {
        _serialQueue = dispatch_queue_create("serialQueue", DISPATCH_QUEUE_SERIAL);//创建串行队列
    }
    return _serialQueue;
}

- (void)prepareViewController
{
    dispatch_async([self serialQueue], ^{//把block中的任务放入串行队列中执行，这是第一个任务
        self.viewController = [[[DWViewController alloc] init] autorelease];
        sleep(2);//假装这个viewController创建起来很花时间。。其实view都还没加载，根本不花时间。
        NSLog(@"prepared");
    });
}

- (void)goToViewController
{
    dispatch_async([self serialQueue], ^{//第二个任务，推入viewController
        NSLog(@"go");
        dispatch_async(dispatch_get_main_queue(), ^{//涉及UI更新的操作，放入主线程中
            [_navController pushViewController:self.viewController animated:YES];
        });
    });
}

- (void)dealloc
{
    dispatch_release(_serialQueue);
    [_navController release];
    [_window release];
    [_viewController release];
    [super dealloc];
}

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    [self prepareViewController];
   
    self.window = [[[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]] autorelease];
    // Override point for customization after application launch.
   
    DWViewController *viewController = [[[DWViewController alloc] initWithNibName:@"DWViewController" bundle:nil] autorelease];
    viewController.view.backgroundColor = [UIColor blueColor];
    _navController = [[UINavigationController alloc] initWithRootViewController:viewController];
    self.window.rootViewController = _navController;
   
    [self goToViewController];
   
    [self.window makeKeyAndVisible];
    return YES;
}