6.8 Performing Tasks After a Delay with GCD

延迟执行代码

在Core Foundation中，我们可以performSelector:withObject:afterDelay:来延迟调用方法，GCD中如何实现呢？

使用dispatch_after，dispatch_after_f

来看个例子

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 */

});

第一个参数delayInNanoSeconds， 他是一个绝对时间absolute time，可以由dispatch_time获得，DISPATCH_TIME_NOW 代表当前时间now,结果返回的就是2秒后的时间。

同理，3秒钟的时间这样获得

dispatch_time_t delay = dispatch_time(DISPATCH_TIME_NOW, 3.0f * NSEC_PER_SEC);

半秒后的时间这样获得

dispatch_time_t delay =dispatch_time(DISPATCH_TIME_NOW, (1.0 / 2.0f) * NSEC_PER_SEC);

再来看看dispatch_after_f如何使用

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]];

// Override point for customization after application launch.

self.window.backgroundColor = [UIColor whiteColor]; [self.window makeKeyAndVisible];
return YES;

}

第三个参数NULL是传递给processSomething的内存地址