IOS開發（64）之GCD任務最多隻執行一次

1 前言

 

使用 dispatch_once 函數 在 APP 的生命週期內來保證你想確保每段代碼只執行一次,即使它在代碼的不同地方多次調用(比如單例的初始化)。

 

2 代碼執行個體

ZYAppDelegate.m

 

//一個用於調度一次函數的標識static dispatch_once_t onceToken;//Block Objectvoid (^executedOnlyOnce)(void) = ^{    static NSUInteger numberOfEntries = 0;    numberOfEntries++;    NSLog(@"Executed %lu time(s)", (unsigned long)numberOfEntries);};- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{    //聲明一個隊列    dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);    //執行一次的隊列    dispatch_once(&onceToken, ^{ dispatch_async(concurrentQueue,                                                executedOnlyOnce);    });    dispatch_once(&onceToken, ^{ dispatch_async(concurrentQueue,                                                executedOnlyOnce);    });    self.window = [[[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]] autorelease];    // Override point for customization after application launch.    self.viewController = [[[ZYViewController alloc] initWithNibName:@"ZYViewController" bundle:nil] autorelease];    self.window.rootViewController = self.viewController;    [self.window makeKeyAndVisible];    return YES;}

 

ZYViewController.m

 

- (void)viewDidLoad{    [super viewDidLoad];    ZYMySingleton *test = [[ZYMySingleton alloc] init];    //迴圈單例方法    for (int i=0; i<5; i++) {        [test sharedInstance];    }    [test release];}

 

ZYMySingleton.m

 

- (id) sharedInstance{    static ZYMySingleton *SharedInstance = nil;    //一個用於調度一次函數的標識    static dispatch_once_t onceToken;    dispatch_once(&onceToken, ^{        SharedInstance = [ZYMySingleton new];        NSLog(@"SharedInstance is ======>%@",SharedInstance);    });    return SharedInstance;}

 

運行後控制台顯示結果

 

2013-05-10 17:30:49.334 GCDSingleTest[2717:1303] Executed 1 time(s)

2013-05-10 17:30:49.336 GCDSingleTest[2717:c07] SharedInstance is ======><ZYMySingleton: 0x7195990>

3 結語

以上是所有內容，希望對大家有所協助

Demo代碼下載：http://download.csdn.net/detail/u010013695/5353668