關于GCD的最最基本的知識，往往很多初學者都被忽略。理解上造成了偏差，而不注重去實踐， 所有我們需要認真的總結一下。
dispatch_asyn和dispatch_sync添加任務到dispatch隊列時，是否創建線程呢，那么創建線程是創建一個呢還是多個呢，如果你自己能直接分的清楚就不必看我下文的實踐代碼了， 下文的代碼做了詳細的說明， 不僅僅是代碼，也包含了總結， 如果你想實踐的話， 也可以跟著實踐一遍哦，這樣最好哦
以下主要總結了隊列的類型和隊列的堵塞
- (void)viewDidLoad {
[super viewDidLoad];
//********
[self blockIntroduced];
//********通過以上測試我們得出如下幾個結論
/*
1.dispatch_sync添加任務到隊列，不會創建新的線程都是在當前線程中處理的。無論添加到串行隊列里或者并行隊列里，都是串行效果，因為這個方法是等任務執行完成以后才會返回。
2.dispatch_async添加任務到
2-1：mainQueue不創建線程，在主線程中串行執行
2-2：globalQueue 和 并行隊列：根據任務系統決定開辟線程個數
2-3：串行對列：創建一個線程：串行執行。
*/
[self blockedMainThread0];
[self blockedMainThread1];
[self blockedMainThread2];
[self blockedMainThread3];
// [self blockedMainThread];
}
-(void)blockIntroduced
{
//隊列一般就是系統的主隊列和全局隊列還有自己手動創建的串行隊列和并行隊列
dispatch_queue_t chuanxingduilie = dispatch_queue_create("chuanxingduilie", DISPATCH_QUEUE_SERIAL);
dispatch_queue_t bingxingduilie = dispatch_queue_create("bingxingduilie", DISPATCH_QUEUE_CONCURRENT);
dispatch_queue_t mainQueue = dispatch_get_main_queue();
dispatch_queue_t globalQueue = dispatch_get_global_queue(0, 0);
//方法有兩個 dispatch_sync 和 dispatch_async dispatch_async方法是立刻返回的，也就是說把block內容加到相應隊列里后會立馬返回，而dispatch_sync要等到加到隊列里執行完之后才會返回。
//這時候總共有2*4 = 8種組合，接下來我們來說一下這八種組合
//一：
for(int i=0;i<3;i++){
dispatch_sync(chuanxingduilie, ^{
NSLog(@"dispatch_sync-chuanxingduilie-%d\n mainThread:%@",i,[NSThread currentThread]);
});
}
/*
2016-02-05 21:07:12.622 GcdTest[7229:164997] dispatch_sync-chuanxingduilie-0
mainThread:<NSThread: 0x7fad93407dd0>{number = 1, name = main}
2016-02-05 21:07:12.623 GcdTest[7229:164997] dispatch_sync-chuanxingduilie-1
mainThread:<NSThread: 0x7fad93407dd0>{number = 1, name = main}
2016-02-05 21:07:12.623 GcdTest[7229:164997] dispatch_sync-chuanxingduilie-2
mainThread:<NSThread: 0x7fad93407dd0>{number = 1, name = main}
*/
//第一種方式可見，串行隊列是在主線程里完成的，因為是串行隊列，所以打印%d是有順序的。
//二：
for(int i=0;i<3;i++){
dispatch_sync(bingxingduilie, ^{
NSLog(@"dispatch_sync-bingxingduilie-%d\n mainThread:%@",i,[NSThread currentThread]);
});
}
/*
2016-02-05 21:09:26.422 GcdTest[7241:165846] dispatch_sync-bingxingduilie-0
mainThread:<NSThread: 0x7face9d01d20>{number = 1, name = main}
2016-02-05 21:09:26.422 GcdTest[7241:165846] dispatch_sync-bingxingduilie-1
mainThread:<NSThread: 0x7face9d01d20>{number = 1, name = main}
2016-02-05 21:09:26.422 GcdTest[7241:165846] dispatch_sync-bingxingduilie-2
mainThread:<NSThread: 0x7face9d01d20>{number = 1, name = main}
*/
//第二種方式可以發現同樣還是在主線程里執行并行隊列，(雖然是并行隊列，但這時候依然在同一個線程里執行)
//三：
/*
for(int i=0;i<3;i++){
NSLog(@"dispatch_sync-mainQueue");
dispatch_sync(mainQueue, ^{
NSLog(@"dispatch_sync-mainQueue-%d\n mainThread:%@",i,[NSThread currentThread]);
});
}
//打印結果：dispatch_sync-mainQueue，發現主線程被堵塞。原因在堵塞中分析。此種方式暫時先屏蔽掉
*/
//四：
for(int i=0;i<3;i++){
dispatch_sync(globalQueue, ^{
NSLog(@"dispatch_sync-globalQueue-%d\n mainThread:%@",i,[NSThread currentThread]);
});
}
/*
2016-02-05 21:22:41.107 GcdTest[7338:172870] dispatch_sync-globalQueue-0
mainThread:<NSThread: 0x7feadad050b0>{number = 1, name = main}
2016-02-05 21:22:41.107 GcdTest[7338:172870] dispatch_sync-globalQueue-1
mainThread:<NSThread: 0x7feadad050b0>{number = 1, name = main}
2016-02-05 21:22:41.107 GcdTest[7338:172870] dispatch_sync-globalQueue-2
mainThread:<NSThread: 0x7feadad050b0>{number = 1, name = main}
*/
//打印結果可以看出依然是主線程。
//五：
for (int i=0; i<3; i++) {
dispatch_async(chuanxingduilie, ^{
if (i==1) {
sleep(2);
}
NSLog(@"dispatch_async-chuanxingduilie-%d\n Thread:%@",i,[NSThread currentThread]);
});
}
/*
2016-02-05 22:32:14.957 GcdTest[7499:191789] dispatch_async-chuanxingduilie-0
Thread:<NSThread: 0x7fd7eaf10c70>{number = 3, name = (null)}
2016-02-05 22:32:16.962 GcdTest[7499:191789] dispatch_async-chuanxingduilie-1
Thread:<NSThread: 0x7fd7eaf10c70>{number = 3, name = (null)}
2016-02-05 22:32:16.963 GcdTest[7499:191789] dispatch_async-chuanxingduilie-2
Thread:<NSThread: 0x7fd7eaf10c70>{number = 3, name = (null)}
*/
//可以看出創建了一個線程， 在串行隊列里串行執行的
//六：
for (int i=0; i<3; i++) {
dispatch_async(bingxingduilie, ^{
if (i==1) {
sleep(3);
}
NSLog(@"dispatch_async-bingxingduilie-%d\n Thread:%@",i,[NSThread currentThread]);
});
}
/*
2016-02-05 22:10:07.083 GcdTest[7425:184003] dispatch_async-bingxingduilie-2
Thread:<NSThread: 0x7fcda24bcd00>{number = 4, name = (null)}
2016-02-05 22:10:07.083 GcdTest[7425:183982] dispatch_async-bingxingduilie-0
Thread:<NSThread: 0x7fcda2502310>{number = 3, name = (null)}
2016-02-05 22:10:10.083 GcdTest[7425:184002] dispatch_async-bingxingduilie-1
Thread:<NSThread: 0x7fcda2436180>{number = 5, name = (null)}
*/
//分析可以看出創建了多個線程，任務執行順序不一定（時間差不多的話）
//七：
for (int i=0; i<3; i++) {
dispatch_async(mainQueue, ^{
if (i==1) {
sleep(3);
}
NSLog(@"dispatch_async-mainQueue-%d\n Thread:%@",i,[NSThread currentThread]);
});
}
/*
Thread:<NSThread: 0x7fbb28704510>{number = 5, name = (null)}
2016-02-05 22:15:03.709 GcdTest[7460:186485] dispatch_async-mainQueue-0
Thread:<NSThread: 0x7fbb28703fb0>{number = 1, name = main}
2016-02-05 22:15:06.710 GcdTest[7460:186485] dispatch_async-mainQueue-1
Thread:<NSThread: 0x7fbb28703fb0>{number = 1, name = main}
2016-02-05 22:15:06.710 GcdTest[7460:186485] dispatch_async-mainQueue-2
Thread:<NSThread: 0x7fbb28703fb0>{number = 1, name = main}
*/
//可以看出這種情況還是在當前線程環境中執行，并不創建線程，因為是在主隊列里，順序執行
//八：
for (int i=0; i<3; i++) {
dispatch_async(globalQueue, ^{
NSLog(@"dispatch_async-globalQueue-%d\n Thread:%@",i,[NSThread currentThread]);
});
}
/*
2016-02-05 22:17:58.306 GcdTest[7482:188067] dispatch_async-globalQueue-1
Thread:<NSThread: 0x7fa281e24580>{number = 4, name = (null)}
2016-02-05 22:17:58.306 GcdTest[7482:188099] dispatch_async-globalQueue-2
Thread:<NSThread: 0x7fa281e321c0>{number = 5, name = (null)}
2016-02-05 22:17:58.307 GcdTest[7482:188069] dispatch_async-globalQueue-0
Thread:<NSThread: 0x7fa281e262b0>{number = 3, name = (null)}
*/
//可以看出創建了多個線程，執行順序并不一定
}
-(void)blockedMainThread
{
//打印結果是1 分析：mainQueue里存在任務1，同步線程任務，這兩個任務，當執行dispatch_sync時，把打印任務2加入主隊列，想要打印2必須等之前所有的任務都執行完成，這時候因為主隊列里有同步線程任務，這時候相當于自己在等自己執行完成，進入死循環。
NSLog(@"1"); // 任務1
dispatch_sync(dispatch_get_main_queue(), ^{
NSLog(@"2"); // 任務2
});
NSLog(@"3"); // 任務3
}
-(void)blockedMainThread0
{
//打印結果123 分析：mainQueue里存在任務1，同步線程任務，當執行dispatch_sync時，拿到全局隊列，
NSLog(@"1"); // 任務1
dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
NSLog(@"2"); // 任務2
});
NSLog(@"3"); // 任務3
}
-(void)blockedMainThread1
{
/*
//執行結果：
2016-02-05 22:42:30.661 GcdTest[7511:194820] 1
2016-02-05 22:42:30.662 GcdTest[7511:194820] 5
2016-02-05 22:42:30.662 GcdTest[7511:194929] 2 <NSThread: 0x7f9ca8709990>{number = 3, name = (null)}
*/
dispatch_queue_t queue = dispatch_queue_create("com.demo.serialQueue", DISPATCH_QUEUE_SERIAL);
NSLog(@"1"); // 任務1
dispatch_async(queue, ^{
NSLog(@"2 %@",[NSThread currentThread]); // 任務2
dispatch_sync(queue, ^{
NSLog(@"3"); // 任務3
});
NSLog(@"4"); // 任務4
});
NSLog(@"5"); // 任務5
}
-(void)blockedMainThread2
{
//1,5,2,3,4 可以看出在全局隊列里拿到住隊列同步執行是沒有問題的。
NSLog(@"****************");
NSLog(@"1"); // 任務1
dispatch_async(dispatch_get_global_queue(0, 0), ^{
NSLog(@"2"); // 任務2
dispatch_sync(dispatch_get_main_queue(), ^{
NSLog(@"3 %@",[NSThread currentThread]); // 任務3
//2016-02-05 22:51:00.044 GcdTest[7548:198417] 3 <NSThread: 0x7feb3b701be0>{number = 1, name = main}
});
NSLog(@"4"); // 任務4
});
NSLog(@"5"); // 任務5
}
-(void)blockedMainThread3
{
//執行結果：11，44
dispatch_async(dispatch_get_global_queue(0, 0), ^{
NSLog(@"11"); // 任務1
dispatch_sync(dispatch_get_main_queue(), ^{
NSLog(@"22"); // 任務2
});
NSLog(@"33"); // 任務3
});
NSLog(@"44"); // 任務4
while (1) {
}
NSLog(@"55"); // 任務5
}
- (void)didReceiveMemoryWarning {
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}
文章未完待續：待續內容為dispatch里的大部分常用的方法，等我有時間了一定會更新上去， 如果大家喜歡文章， 就關注哦~
有什么不明白的，可以直接評論回復，交流。
1.dispath_once
dispatch_once的作用就是只執行一次，我們在寫單例的時候可以用到
+ (BWStatusBarOverlay *)shared {
static dispatch_once_t pred = 0;
__strong static id _sharedObject = nil;
dispatch_once(&pred, ^{
_sharedObject = [[self alloc] init];
});
return _sharedObject;
}
2.dispatch_after
//延遲0.5s以后執行
dispatch_time_t time = dispatch_time(DISPATCH_TIME_NOW, NSEC_PER_SEC*0.5);
dispatch_after(time, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSLog(@"doSomething");
});
3.dispatch_group
dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, globalQueue, ^{
sleep(1);
NSLog(@"1");
});
dispatch_group_async(group, globalQueue, ^{
sleep(2);
NSLog(@"2");
});
dispatch_group_async(group, globalQueue, ^{
sleep(3);
NSLog(@"3");
});
dispatch_group_notify(group, globalQueue, ^{
NSLog(@"Over!");
});
4.dispatch_barrier_async
dispatch_queue_t concurrentQueue = dispatch_queue_create("my.concurrent.queue", DISPATCH_QUEUE_CONCURRENT);
dispatch_async(concurrentQueue, ^(){
NSLog(@"dispatch-1");
});
dispatch_async(concurrentQueue, ^(){
NSLog(@"dispatch-2");
});
dispatch_barrier_async(concurrentQueue, ^(){
NSLog(@"dispatch-barrier-1+2");
});
dispatch_async(concurrentQueue, ^(){
NSLog(@"dispatch-3");
});
dispatch_barrier_async(concurrentQueue, ^{
NSLog(@"dispatch-barrier-1+2+3");
});
dispatch_async(concurrentQueue, ^(){
NSLog(@"dispatch-4");
});
最后不多說，需要看看蘋果官方文檔這里截圖看下關于GCD的東西
gcd-1.png
gcd-2.png
gcd-3.png
看官方的東西其實不會的東西太多太多，任重而道遠， 明天就回家過年嘍， 祝愿每一個人新的一年都能對自己的目標更近一步。
