ios 中的定时器功能

在Object-C中，有三种方法可以实现定时器的功能。

1 使用NSObject对象的performSelector:withObject:afterDelay:的方法。
2 使用GCD的Block Objects方法。
3 使用GCD的C Functions方法。

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-  ( void ) printString : ( NSString  * )paramString {      NSLog ( @ "%@", paramString ); } -  ( BOOL ) application : (UIApplication  * )application didFinishLaunchingWithOptions : ( NSDictionary  * )launchOptions {      [self performSelector : @selector (printString : ) withObject : @ "Grand Central Dispatch" afterDelay : 3.0 ];     self.window  =  [ [UIWindow alloc ] initWithFrame :  [ [UIScreen mainScreen ] bounds ] ];      // Override point for customization after application launch.     self.window.backgroundColor  =  [UIColor whiteColor ];       [self.window makeKeyAndVisible ];      return  YES; }

使用GCD的dispatch_after方法和dispatch_after_f方法。
第一个方法三个参数：Delay in nanoseconds，Dispatch queue，Block object
第二个方法四个参数：Delay in nanoseconds，Dispatch queue，Context，C function

第一种方法的实现：

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double delayInSeconds  =  2.0; dispatch_time_t delayInNanoSeconds  = dispatch_time (DISPATCH_TIME_NOW, delayInSeconds  * NSEC_PER_SEC ); dispatch_queue_t concurrentQueue  = dispatch_get_global_queue (DISPATCH_QUEUE_PRIORITY_DEFAULT,  0 ); dispatch_after (delayInNanoSeconds, concurrentQueue,  ^ ( void ) {       /* Perform your operations here */ } );

第二种实现：

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void processSomething ( void  *paramContext ) {      /* Do your processing here */     NSLog ( @ "Processing..." );  } -  ( BOOL ) application : (UIApplication  * )application didFinishLaunchingWithOptions : ( NSDictionary  * )launchOptions {      double delayInSeconds  =  2.0;     dispatch_time_t delayInNanoSeconds  = dispatch_time (DISPATCH_TIME_NOW, delayInSeconds  * NSEC_PER_SEC );     dispatch_queue_t concurrentQueue  = dispatch_get_global_queue (DISPATCH_QUEUE_PRIORITY_DEFAULT,  0 );     dispatch_after_f (delayInNanoSeconds, concurrentQueue, NULL, processSomething );     self.window  =  [ [UIWindow alloc ] initWithFrame :  [ [UIScreen mainScreen ] bounds ] ];     self.window.backgroundColor  =  [UIColor whiteColor ];      [self.window makeKeyAndVisible ];      return  YES; }