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;
}