一、簡單介紹下將會(huì)用到的一些東西
英語不好就不翻譯官方文檔了..
1、dispatch_group_async
Submits a block to a dispatch queue and associates the block with the givendispatch group
//將一個(gè)block(代碼塊)加入到dispatch_queue_t queue中并和dispatch_group_t group相關(guān)聯(lián)
void
dispatch_group_async(dispatch_group_t group,
dispatch_queue_t queue,
dispatch_block_t block);
個(gè)人理解：將代碼塊dispatch_block_t block放入隊(duì)列dispatch_queue_t queue中執(zhí)行；并和調(diào)度組dispatch_group_t group相互關(guān)聯(lián)；如果提交到dispatch_queue_t queue中的block全都執(zhí)行完畢會(huì)調(diào)用dispatch_group_notify并且dispatch_group_wait會(huì)停止等待；
2、dispatch_group_enter(group)、dispatch_group_leave(group)
Calling this function indicates another block has joined the group througha means other than dispatch_group_async(). Calls to this function must be
* balanced with dispatch_group_leave().
調(diào)用這個(gè)方法標(biāo)志著一個(gè)代碼塊被加入了group，和dispatch_group_async功能類似；
需要和dispatch_group_enter()、dispatch_group_leave()成對(duì)出現(xiàn)；
void
dispatch_group_enter(dispatch_group_t group);
個(gè)人理解：和內(nèi)存管理的引用計(jì)數(shù)類似，我們可以認(rèn)為group也持有一個(gè)整形變量(只是假設(shè))，當(dāng)調(diào)用enter時(shí)計(jì)數(shù)加1，調(diào)用leave時(shí)計(jì)數(shù)減1，當(dāng)計(jì)數(shù)為0時(shí)會(huì)調(diào)用dispatch_group_notify并且dispatch_group_wait會(huì)停止等待；
3、dispatch_group_notify
void
dispatch_group_notify(dispatch_group_t group,dispatch_queue_t queue,
dispatch_block_t block);
個(gè)人理解：當(dāng)提交到隊(duì)列dispatch_queue_t queue上的所有任務(wù)執(zhí)行完畢時(shí)會(huì)執(zhí)行dispatch_group_notify里的dispatch_block_t block的代碼
4、dispatch_group_wait
long
dispatch_group_wait(dispatch_group_t group, dispatch_time_t timeout);
個(gè)人理解：和dispatch_group_notify功能類似(多了一個(gè)dispatch_time_t參數(shù)可以設(shè)置超時(shí)時(shí)間)，在group上任務(wù)完成前，dispatch_group_wait會(huì)阻塞當(dāng)前線程(所以不能放在主線程調(diào)用)一直等待；當(dāng)group上任務(wù)完成，或者等待時(shí)間超過設(shè)置的超時(shí)時(shí)間會(huì)結(jié)束等待；
二、dispatch_group_async代碼示例
- (void)groupSync
{
dispatch_queue_t disqueue =? dispatch_queue_create("com.shidaiyinuo.NetWorkStudy", DISPATCH_QUEUE_CONCURRENT);
dispatch_group_t disgroup = dispatch_group_create();
dispatch_group_async(disgroup, disqueue, ^{
NSLog(@"任務(wù)一完成");
});
dispatch_group_async(disgroup, disqueue, ^{
sleep(8);
NSLog(@"任務(wù)二完成");
});
dispatch_group_notify(disgroup, disqueue, ^{
NSLog(@"dispatch_group_notify 執(zhí)行");
});
dispatch_async(dispatch_get_global_queue(0, 0), ^{
dispatch_group_wait(disgroup, dispatch_time(DISPATCH_TIME_NOW, 5 * NSEC_PER_SEC));
NSLog(@"dispatch_group_wait 結(jié)束");
});
}
向group中放入兩個(gè)任務(wù)(準(zhǔn)確講是將任務(wù)加入到了并行隊(duì)列disqueue中執(zhí)行，然后隊(duì)列和group關(guān)聯(lián)當(dāng)隊(duì)列上任務(wù)執(zhí)行完畢時(shí)group會(huì)進(jìn)行同步)，第二個(gè)任務(wù)會(huì)等待8秒所以第一個(gè)任務(wù)會(huì)先完成；會(huì)先打印任務(wù)一完成再打印任務(wù)二完成，當(dāng)兩個(gè)任務(wù)都完成時(shí)dispatch_group_notify中的block會(huì)執(zhí)行；會(huì)接著打印dispatch_group_notify 執(zhí)行；dispatch_group_wait設(shè)置了超時(shí)時(shí)間為5秒所以它會(huì)在5秒后停止等待打印dispatch_group_wait 結(jié)束(任務(wù)二會(huì)等待8秒所以它會(huì)在任務(wù)二完成前打印)；
測試輸出結(jié)果
需要注意的：dispatch_group_wait是同步的所以不能放在主線程執(zhí)行。
補(bǔ)充：dispatch_group會(huì)等和它關(guān)聯(lián)的所有的dispatch_queue_t上的任務(wù)都執(zhí)行完畢才會(huì)發(fā)出同步信號(hào)(dispathc_group_notify的代碼塊block會(huì)被執(zhí)行,group_wati會(huì)結(jié)束等待)。也就是說一個(gè)group可以關(guān)聯(lián)多個(gè)任務(wù)隊(duì)列；下面給出示例：
- (void)groupSync2
{
dispatch_queue_t dispatchQueue = dispatch_queue_create("ted.queue.next1", DISPATCH_QUEUE_CONCURRENT);
dispatch_queue_t globalQueue = dispatch_get_global_queue(0, 0);
dispatch_group_t dispatchGroup = dispatch_group_create();
dispatch_group_async(dispatchGroup, dispatchQueue, ^(){
sleep(5);
NSLog(@"任務(wù)一完成");
});
dispatch_group_async(dispatchGroup, dispatchQueue, ^(){
sleep(6);
NSLog(@"任務(wù)二完成");
});
dispatch_group_async(dispatchGroup, globalQueue, ^{
sleep(10);
NSLog(@"任務(wù)三完成");
});
dispatch_group_notify(dispatchGroup, dispatch_get_main_queue(), ^(){
NSLog(@"notify：任務(wù)都完成了");
});
}
上面的代碼里有兩個(gè)隊(duì)列一個(gè)是我自己創(chuàng)建的并行隊(duì)列dispatchQueue，另一個(gè)是系統(tǒng)提供的并行隊(duì)列g(shù)lobalQueue；dispatch_group_notify會(huì)等這兩個(gè)隊(duì)列上的任務(wù)都執(zhí)行完畢才會(huì)執(zhí)行自己的代碼塊。
多個(gè)隊(duì)列執(zhí)行結(jié)果
三、dispatch_group_enter、dispatch_group_level示例
和dispatch_async相比，當(dāng)我們調(diào)用n次dispatch_group_enter后再調(diào)用n次dispatch_group_level時(shí)，dispatch_group_notify和dispatch_group_wait會(huì)收到同步信號(hào)；這個(gè)特點(diǎn)使得它非常適合處理異步任務(wù)的同步當(dāng)異步任務(wù)開始前調(diào)用dispatch_group_enter異步任務(wù)結(jié)束后調(diào)用dispatch_group_leve；
下面是代碼示例：
- (void)groupSync
{
dispatch_group_t group = dispatch_group_create();
dispatch_group_enter(group);
dispatch_async(dispatch_get_global_queue(0, 0), ^{
sleep(5);
NSLog(@"任務(wù)一完成");
dispatch_group_leave(group);
});
dispatch_group_enter(group);
dispatch_async(dispatch_get_global_queue(0, 0), ^{
sleep(8);
NSLog(@"任務(wù)二完成");
dispatch_group_leave(group);
});
dispatch_group_notify(group, dispatch_get_global_queue(0, 0), ^{
NSLog(@"任務(wù)完成");
});
}
示例代碼中在global_queue上執(zhí)行sleep任務(wù)模擬網(wǎng)絡(luò)請求。
控制臺(tái)打印結(jié)果
補(bǔ)充：如果像最后一個(gè)示例那樣，block里執(zhí)行的是同步類型的代碼那么用dispatch_group_async一樣可以達(dá)到同步的效果，但是異步任務(wù)就不行了如下：
- (void)groupSync2
{
dispatch_queue_t dispatchQueue = dispatch_queue_create("ted.queue.next1", DISPATCH_QUEUE_CONCURRENT);
dispatch_queue_t globalQueue = dispatch_get_global_queue(0, 0);
dispatch_group_t dispatchGroup = dispatch_group_create();
dispatch_group_async(dispatchGroup, dispatchQueue, ^(){
dispatch_async(globalQueue, ^{
sleep(5);
NSLog(@"任務(wù)一完成");
});
});
dispatch_group_async(dispatchGroup, dispatchQueue, ^(){
dispatch_async(globalQueue, ^{
sleep(8);
NSLog(@"任務(wù)二完成");
});
});
dispatch_group_notify(dispatchGroup, dispatch_get_main_queue(), ^(){
NSLog(@"notify：任務(wù)都完成了");
});
}
如果dispatch_group_async里執(zhí)行的是異步代碼dispatch_group_notify會(huì)直接觸發(fā)而不會(huì)等待異步任務(wù)完成，而dispatch_group_enter、和dispatch_group_leave則不會(huì)有這個(gè)問題，它們只需要在任務(wù)開始前enter結(jié)束后leave即可達(dá)到線程同步的效果。
