有兩個(gè)參數(shù),第一個(gè)數(shù)分組,第二個(gè)是dispatch_time_t類型的參數(shù),而不要傳入1,2,或者1000,因?yàn)榇蛴〕鰜砜词且粋€(gè)很大的數(shù)字
dispatch_group_wait(mygroup, dispatch_time(DISPATCH_TIME_NOW, 4.0 * NSEC_PER_SEC));

//假設(shè)創(chuàng)建dispatch_group_wait里的dispatch_time等待時(shí)間是4秒,那么group里的內(nèi)容在4s內(nèi)完成的話,那wait直接結(jié)束,如果group在4s內(nèi)沒有執(zhí)行完,那么wait也會(huì)結(jié)束,結(jié)束之后,周末的任務(wù)就會(huì)被加進(jìn)來了

//設(shè)定總信號(hào)量
dispatch_semaphore_t source = dispatch_semaphore_create(1);

//消耗一個(gè)信號(hào)量
dispatch_semaphore_wait(source, dispatch_time(DISPATCH_TIME_NOW, 10.0 * NSEC_PER_SEC));

//發(fā)出一個(gè)信號(hào)量
dispatch_semaphore_signal(source);

//當(dāng)信號(hào)量為0的時(shí)候,wait下面的代碼不執(zhí)行