以下代码段将在默认优先级并发队列(即后台线程)上2秒后执行一个块.如果您认为合适,可以更改队列的优先级,但除非您在并发队列上处理大量不同的操作,否则默认就足够了.

double delayInSeconds = 2.0;
dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW,(int64_t)(delayInSeconds * NSEC_PER_SEC));

dispatch_after(popTime,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0),^(void){
    // Your code here
});

如果您想重复调用它,则可以使用dispatch_source_set_timer设置重复执行.它的主旨如下：

// Create a dispatch source that'll act as a timer on the concurrent queue
// You'll need to store this somewhere so you can suspend and remove it later on
dispatch_source_t dispatchSource = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER,0)); 

// Setup params for creation of a recurring timer
double interval = 2.0;
dispatch_time_t startTime = dispatch_time(DISPATCH_TIME_NOW,0);
uint64_t intervalTime = (int64_t)(interval * NSEC_PER_SEC);
dispatch_source_set_timer(dispatchSource,startTime,intervalTime,0);

// Attach the block you want to run on the timer fire
dispatch_source_set_event_handler(dispatchSource,^{
    // Your code here
});

// Start the timer
dispatch_resume(dispatchSource);

// ----

// When you want to stop the timer,you need to suspend the source
dispatch_suspend(dispatchSource);

// If on iOS5 and/or using MRC,you'll need to release the source too
dispatch_release(dispatchSource);