GCD死锁，及同步、异步、串行和并行队列组合情形
程序员文章站
2022-04-09 18:33:39
一、概述 1）队列用来存储代码任务，线程用来运行代码任务； 2）main()函数作为程序入口，整个程序默认运行在主线程中，程序代码任务默认存放在主队列中； 3）以下所谓阻塞线程是针对主线程而言（子线程阻塞在所不问，自己手动管理）；队列阻塞主要是针对主队列（子队列阻塞在所不问，自己手动管理）； 4）在 ......
一、概述
1）队列用来存储代码任务，线程用来运行代码任务；
2）main()函数作为程序入口，整个程序默认运行在主线程中，程序代码任务默认存放在主队列中；
3）以下所谓阻塞线程是针对主线程而言（子线程阻塞在所不问，自己手动管理）；队列阻塞主要是针对主队列（子队列阻塞在所不问，自己手动管理）；
4）在主线中添加block任务（以下简称“b”）到某个队列中，添加b本身也是一个任务即dispatch_sync代码本身由主线程来运行（以下简称“a”）；
5）队列分三种：主队列、自定义串行队列（以下简称“自定串”）、并行队列；
6）线程阻塞原理（以主队列为例）：系统会从主队列中按顺序取出某个任务放到主线程中运行，一旦该任务运行完毕，该任务即从主队列中结束销毁，系统就会自动从主队列中取出下一个任务又放到主线程中运行（相当于前一个），如此反复，直到主队列中的任务全部运行完毕；
二、异步并发
1）示例图
2）代码
dispatch_queue_t queue2 = dispatch_get_global_queue(0, 0);
dispatch_async(queue2, ^{
for (int i = 0; i < 10; i++) {
nslog(@"执行任务3-----%@", [nsthread currentthread]);
}
});
for (int i = 0; i < 10; i++) {
nslog(@"执行任务1-----%@", [nsthread currentthread]);
}
//打印
2019-02-14 10:33:28.408678+0800 mj_ios_test[1864:38570] 执行任务1-----<nsthread: 0x600000c16980>{number = 1, name = main}
2019-02-14 10:33:28.408685+0800 mj_ios_test[1864:38623] 执行任务3-----<nsthread: 0x600000c7b780>{number = 3, name = (null)}
2019-02-14 10:33:28.408935+0800 mj_ios_test[1864:38570] 执行任务1-----<nsthread: 0x600000c16980>{number = 1, name = main}
2019-02-14 10:33:28.408935+0800 mj_ios_test[1864:38623] 执行任务3-----<nsthread: 0x600000c7b780>{number = 3, name = (null)}
2019-02-14 10:33:28.409076+0800 mj_ios_test[1864:38570] 执行任务1-----<nsthread: 0x600000c16980>{number = 1, name = main}
2019-02-14 10:33:28.409098+0800 mj_ios_test[1864:38623] 执行任务3-----<nsthread: 0x600000c7b780>{number = 3, name = (null)}
2019-02-14 10:33:28.409185+0800 mj_ios_test[1864:38570] 执行任务1-----<nsthread: 0x600000c16980>{number = 1, name = main}
2019-02-14 10:33:28.409232+0800 mj_ios_test[1864:38623] 执行任务3-----<nsthread: 0x600000c7b780>{number = 3, name = (null)}
2019-02-14 10:33:28.409301+0800 mj_ios_test[1864:38570] 执行任务1-----<nsthread: 0x600000c16980>{number = 1, name = main}
2019-02-14 10:33:28.409618+0800 mj_ios_test[1864:38623] 执行任务3-----<nsthread: 0x600000c7b780>{number = 3, name = (null)}
2019-02-14 10:33:28.409873+0800 mj_ios_test[1864:38570] 执行任务1-----<nsthread: 0x600000c16980>{number = 1, name = main}
2019-02-14 10:33:28.410141+0800 mj_ios_test[1864:38623] 执行任务3-----<nsthread: 0x600000c7b780>{number = 3, name = (null)}
2019-02-14 10:33:28.410480+0800 mj_ios_test[1864:38570] 执行任务1-----<nsthread: 0x600000c16980>{number = 1, name = main}
2019-02-14 10:33:28.410794+0800 mj_ios_test[1864:38623] 执行任务3-----<nsthread: 0x600000c7b780>{number = 3, name = (null)}
2019-02-14 10:33:28.411167+0800 mj_ios_test[1864:38570] 执行任务1-----<nsthread: 0x600000c16980>{number = 1, name = main}
2019-02-14 10:33:28.411486+0800 mj_ios_test[1864:38623] 执行任务3-----<nsthread: 0x600000c7b780>{number = 3, name = (null)}
2019-02-2019-02-14 10:33:28.411962+0800 mj_ios_test[1864:38623] 执行任务3-----<nsthread: 0x600000c7b780>{number = 3, name = (null)}
14 10:33:28.411763+0800 mj_ios_test[1864:38570] 执行任务1-----<nsthread: 0x600000c16980>{number = 1, name = main}
2019-02-14 10:33:28.422231+0800 mj_ios_test[1864:38570] 执行任务1-----<nsthread: 0x600000c16980>{number = 1, name = main}
2019-02-14 10:33:28.422250+0800 mj_ios_test[1864:38623] 执行任务3-----<nsthread: 0x600000c7b780>{number = 3, name = (null)}
3）分析
<1>首先添加gcd中的block任务（即“执行任务3”，相当于b）到并发队列queue2中，添加本身也是一个任务即a，该任务是存储在主队列中，由主线程运行；
<2>b被添加到queue2后，因为是异步，此时a不会在主线程中等待b执行完毕，而是由系统自动返回到主队列中取出下一个任务放到主线程中运行；
与此同时，系统又会单独开辟一个子线程来运行queue2中的b，所以即可由子线程来运行b；
<3>因此，此时系统有两条线程（主线程：number为1，name为main；子线程：number为3，name为null——因为没有取名字）在运行各自的任务，互不影响，交替执行；
三、几种情形
1）同步串行——主队列
结果：坏指令执行错误——死锁
//代码
dispatch_queue_t queue1 = dispatch_get_main_queue();
dispatch_sync(queue1, ^{
for (int i = 0; i < 10; i++) {
nslog(@"执行任务3-----%@", [nsthread currentthread]);
}
});
//错误
//分析
<1>a存储在主队列中，由主线来运行，b被添加到主队列中后，因为是同步，系统不会开辟一个子线程来运行b；
<2>a在主线程中运行完毕后，理应从主队列中销毁而执行下一个任务b，但因为是同步，a会依然停留在主队列中等待b运行完毕；又因为主队列是一个串行队列，因此b必须等到a执行完毕才能被系统取出放在主线程上运行（因为a排在b前面）；
<3>局面：a在主队列中一直等待b的执行完毕，而b在主队列中一直等待a的执行完毕——“你等我，我等你”，死锁；
2）同步串行——自定义串行队列
结果：顺序执行
//代码
dispatch_queue_t queue3 = dispatch_queue_create("myqueue3", dispatch_queue_serial);
dispatch_sync(queue3, ^{
for (int i = 0; i < 10; i++) {
nslog(@"执行任务3-----%@", [nsthread currentthread]);
}
});
for (int i = 0; i < 10; i++) {
nslog(@"执行任务1-----%@", [nsthread currentthread]);
}
//打印
2019-02-14 11:10:57.529267+0800 mj_ios_test[2440:59133] 执行任务3-----<nsthread: 0x60000252ce40>{number = 1, name = main}
2019-02-14 11:10:57.529503+0800 mj_ios_test[2440:59133] 执行任务3-----<nsthread: 0x60000252ce40>{number = 1, name = main}
2019-02-14 11:10:57.529627+0800 mj_ios_test[2440:59133] 执行任务3-----<nsthread: 0x60000252ce40>{number = 1, name = main}
2019-02-14 11:10:57.529771+0800 mj_ios_test[2440:59133] 执行任务3-----<nsthread: 0x60000252ce40>{number = 1, name = main}
2019-02-14 11:10:57.529903+0800 mj_ios_test[2440:59133] 执行任务3-----<nsthread: 0x60000252ce40>{number = 1, name = main}
2019-02-14 11:10:57.530018+0800 mj_ios_test[2440:59133] 执行任务3-----<nsthread: 0x60000252ce40>{number = 1, name = main}
2019-02-14 11:10:57.530147+0800 mj_ios_test[2440:59133] 执行任务3-----<nsthread: 0x60000252ce40>{number = 1, name = main}
2019-02-14 11:10:57.530270+0800 mj_ios_test[2440:59133] 执行任务3-----<nsthread: 0x60000252ce40>{number = 1, name = main}
2019-02-14 11:10:57.530418+0800 mj_ios_test[2440:59133] 执行任务3-----<nsthread: 0x60000252ce40>{number = 1, name = main}
2019-02-14 11:10:57.530547+0800 mj_ios_test[2440:59133] 执行任务3-----<nsthread: 0x60000252ce40>{number = 1, name = main}
2019-02-14 11:10:57.530666+0800 mj_ios_test[2440:59133] 执行任务1-----<nsthread: 0x60000252ce40>{number = 1, name = main}
2019-02-14 11:10:57.530898+0800 mj_ios_test[2440:59133] 执行任务1-----<nsthread: 0x60000252ce40>{number = 1, name = main}
2019-02-14 11:10:57.531152+0800 mj_ios_test[2440:59133] 执行任务1-----<nsthread: 0x60000252ce40>{number = 1, name = main}
2019-02-14 11:10:57.549535+0800 mj_ios_test[2440:59133] 执行任务1-----<nsthread: 0x60000252ce40>{number = 1, name = main}
2019-02-14 11:10:57.549711+0800 mj_ios_test[2440:59133] 执行任务1-----<nsthread: 0x60000252ce40>{number = 1, name = main}
2019-02-14 11:10:57.549853+0800 mj_ios_test[2440:59133] 执行任务1-----<nsthread: 0x60000252ce40>{number = 1, name = main}
2019-02-14 11:10:57.549989+0800 mj_ios_test[2440:59133] 执行任务1-----<nsthread: 0x60000252ce40>{number = 1, name = main}
2019-02-14 11:10:57.550113+0800 mj_ios_test[2440:59133] 执行任务1-----<nsthread: 0x60000252ce40>{number = 1, name = main}
2019-02-14 11:10:57.550247+0800 mj_ios_test[2440:59133] 执行任务1-----<nsthread: 0x60000252ce40>{number = 1, name = main}
2019-02-14 11:10:57.550381+0800 mj_ios_test[2440:59133] 执行任务1-----<nsthread: 0x60000252ce40>{number = 1, name = main}
//分析
<1>a存储在主队列中，由主线来运行，b被添加到自定义串行队列queue3中；因为是同步，系统不会开辟一个子线程来运行b；
<2>a在主线程运行完毕后会停留在主队列中一直等待b执行完毕，此时系统无法从主队列中取出下一个任务放到主线程上运行（因为被a堵住了），但会并且能从queue3中取出b放到主线程上运行；
<3>当主线程在运行“执行任务3”时，线程阻塞，当“执行任务3”运行完毕时，a收到消息则销毁，此时系统就会从主队列中取出下一个任务“执行任务1”放到主线程上运行；
3）同步并行
同“2）”；
补充：如果在该并行队列中有其他任务时异步执行的，则系统开启新线程，与主线程互不影响，交替执行；
4）异步串行——主队列
结果：先执行主队列中后面的任务，完毕后再执行a和b；
//代码
dispatch_queue_t queue1 = dispatch_get_main_queue();
dispatch_async(queue1, ^{
for (int i = 0; i < 10; i++) {
nslog(@"执行任务3-----%@", [nsthread currentthread]);
}
});
for (int i = 0; i < 10; i++) {
nslog(@"执行任务1-----%@", [nsthread currentthread]);
}
//打印
2019-02-14 15:18:24.484445+0800 mj_ios_test[6008:173463] 执行任务1-----<nsthread: 0x600002642940>{number = 1, name = main}
2019-02-14 15:18:24.484665+0800 mj_ios_test[6008:173463] 执行任务1-----<nsthread: 0x600002642940>{number = 1, name = main}
2019-02-14 15:18:24.484837+0800 mj_ios_test[6008:173463] 执行任务1-----<nsthread: 0x600002642940>{number = 1, name = main}
2019-02-14 15:18:24.484978+0800 mj_ios_test[6008:173463] 执行任务1-----<nsthread: 0x600002642940>{number = 1, name = main}
2019-02-14 15:18:24.485116+0800 mj_ios_test[6008:173463] 执行任务1-----<nsthread: 0x600002642940>{number = 1, name = main}
2019-02-14 15:18:24.485296+0800 mj_ios_test[6008:173463] 执行任务1-----<nsthread: 0x600002642940>{number = 1, name = main}
2019-02-14 15:18:24.485430+0800 mj_ios_test[6008:173463] 执行任务1-----<nsthread: 0x600002642940>{number = 1, name = main}
2019-02-14 15:18:24.485566+0800 mj_ios_test[6008:173463] 执行任务1-----<nsthread: 0x600002642940>{number = 1, name = main}
2019-02-14 15:18:24.485698+0800 mj_ios_test[6008:173463] 执行任务1-----<nsthread: 0x600002642940>{number = 1, name = main}
2019-02-14 15:18:24.485815+0800 mj_ios_test[6008:173463] 执行任务1-----<nsthread: 0x600002642940>{number = 1, name = main}
2019-02-14 15:18:24.507778+0800 mj_ios_test[6008:173463] 执行任务3-----<nsthread: 0x600002642940>{number = 1, name = main}
2019-02-14 15:18:24.507968+0800 mj_ios_test[6008:173463] 执行任务3-----<nsthread: 0x600002642940>{number = 1, name = main}
2019-02-14 15:18:24.508119+0800 mj_ios_test[6008:173463] 执行任务3-----<nsthread: 0x600002642940>{number = 1, name = main}
2019-02-14 15:18:24.508252+0800 mj_ios_test[6008:173463] 执行任务3-----<nsthread: 0x600002642940>{number = 1, name = main}
2019-02-14 15:18:24.508388+0800 mj_ios_test[6008:173463] 执行任务3-----<nsthread: 0x600002642940>{number = 1, name = main}
2019-02-14 15:18:24.508541+0800 mj_ios_test[6008:173463] 执行任务3-----<nsthread: 0x600002642940>{number = 1, name = main}
2019-02-14 15:18:24.508695+0800 mj_ios_test[6008:173463] 执行任务3-----<nsthread: 0x600002642940>{number = 1, name = main}
2019-02-14 15:18:24.509382+0800 mj_ios_test[6008:173463] 执行任务3-----<nsthread: 0x600002642940>{number = 1, name = main}
2019-02-14 15:18:24.509973+0800 mj_ios_test[6008:173463] 执行任务3-----<nsthread: 0x600002642940>{number = 1, name = main}
2019-02-14 15:18:24.510909+0800 mj_ios_test[6008:173463] 执行任务3-----<nsthread: 0x600002642940>{number = 1, name = main}
//分析
<1>异步会开启新线程，例外：如果是添加到主队列中，则不会开启子线程，依然由主线程来运行；
<2>因为是异步，所以a在主线程上运行完毕后会自动在主队列中销毁，此时系统会从主队列中取出下一个任务“执行任务1”放到主线程上运行，待“执行任务1”运行完毕后再来运行b——此时，三个任务都存储在主队列中，同时“执行任务1”存储在a和b之间；
5）异步串行——自定义串行队列
结果：同时交替执行；
//代码
dispatch_queue_t queue3 = dispatch_queue_create("myqueue3", dispatch_queue_serial);
dispatch_async(queue3, ^{
for (int i = 0; i < 10; i++) {
nslog(@"执行任务3-----%@", [nsthread currentthread]);
}
});
for (int i = 0; i < 10; i++) {
nslog(@"执行任务1-----%@", [nsthread currentthread]);
}
//打印
2019-02-14 15:35:12.843681+0800 mj_ios_test[6248:181875] 执行任务1-----<nsthread: 0x6000011aae40>{number = 1, name = main}
2019-02-14 15:35:12.843686+0800 mj_ios_test[6248:181935] 执行任务3-----<nsthread: 0x6000011c9e00>{number = 3, name = (null)}
2019-02-14 15:35:12.843933+0800 mj_ios_test[6248:181935] 执行任务3-----<nsthread: 0x6000011c9e00>{number = 3, name = (null)}
2019-02-14 15:35:12.843999+0800 mj_ios_test[6248:181875] 执行任务1-----<nsthread: 0x6000011aae40>{number = 1, name = main}
2019-02-14 15:35:12.844065+0800 mj_ios_test[6248:181935] 执行任务3-----<nsthread: 0x6000011c9e00>{number = 3, name = (null)}
2019-02-14 15:35:12.844208+0800 mj_ios_test[6248:181935] 执行任务3-----<nsthread: 0x6000011c9e00>{number = 3, name = (null)}
2019-02-14 15:35:12.844348+0800 mj_ios_test[6248:181935] 执行任务3-----<nsthread: 0x6000011c9e00>{number = 3, name = (null)}
2019-02-14 15:35:12.844474+0800 mj_ios_test[6248:181935] 执行任务3-----<nsthread: 0x6000011c9e00>{number = 3, name = (null)}
2019-02-14 15:35:12.844482+0800 mj_ios_test[6248:181875] 执行任务1-----<nsthread: 0x6000011aae40>{number = 1, name = main}
2019-02-14 15:35:12.844614+0800 mj_ios_test[6248:181935] 执行任务3-----<nsthread: 0x6000011c9e00>{number = 3, name = (null)}
2019-02-14 15:35:12.844743+0800 mj_ios_test[6248:181875] 执行任务1-----<nsthread: 0x6000011aae40>{number = 1, name = main}
2019-02-14 15:35:12.844769+0800 mj_ios_test[6248:181935] 执行任务3-----<nsthread: 0x6000011c9e00>{number = 3, name = (null)}
2019-02-14 15:35:12.845040+0800 mj_ios_test[6248:181875] 执行任务1-----<nsthread: 0x6000011aae40>{number = 1, name = main}
2019-02-14 15:35:12.845234+0800 mj_ios_test[6248:181935] 执行任务3-----<nsthread: 0x6000011c9e00>{number = 3, name = (null)}
2019-02-14 15:35:12.845559+0800 mj_ios_test[6248:181875] 执行任务1-----<nsthread: 0x6000011aae40>{number = 1, name = main}
2019-02-14 15:35:12.854636+0800 mj_ios_test[6248:181935] 执行任务3-----<nsthread: 0x6000011c9e00>{number = 3, name = (null)}
2019-02-14 15:35:12.855004+0800 mj_ios_test[6248:181875] 执行任务1-----<nsthread: 0x6000011aae40>{number = 1, name = main}
2019-02-14 15:35:12.855578+0800 mj_ios_test[6248:181875] 执行任务1-----<nsthread: 0x6000011aae40>{number = 1, name = main}
2019-02-14 15:35:12.855858+0800 mj_ios_test[6248:181875] 执行任务1-----<nsthread: 0x6000011aae40>{number = 1, name = main}
2019-02-14 15:35:12.856068+0800 mj_ios_test[6248:181875] 执行任务1-----<nsthread: 0x6000011aae40>{number = 1, name = main}
//分析
<1>开启了子线程，主线程对应主队列，运行“执行任务1”；子线程对应queue3队列，运行执行任务3；
<2>两条线程运行各自的任务，互不影响，交替执行；
6）异步并行
如上述示例图分析，同“5）”；
补充：6）和5）两种情况，相对于主线程效果都一样——交替执行；但是其内部任务执行有区别：
<1>顺序添加两个任务到自定义串行队列中——情况“5）”
//代码
dispatch_queue_t queue3 = dispatch_queue_create("myqueue3", dispatch_queue_serial);
dispatch_async(queue3, ^{
for (int i = 0; i < 10; i++) {
nslog(@"执行任务3-----%@", [nsthread currentthread]);
}
});
dispatch_async(queue3, ^{
for (int i = 0; i < 10; i++) {
nslog(@"执行任务3-2-----%@", [nsthread currentthread]);
}
});
for (int i = 0; i < 10; i++) {
nslog(@"执行任务1-----%@", [nsthread currentthread]);
}
//打印
2019-02-14 15:48:52.982315+0800 mj_ios_test[6484:189815] 执行任务3-----<nsthread: 0x600000320940>{number = 3, name = (null)}
2019-02-14 15:48:52.982310+0800 mj_ios_test[6484:189759] 执行任务1-----<nsthread: 0x600000356840>{number = 1, name = main}
2019-02-14 15:48:52.982607+0800 mj_ios_test[6484:189815] 执行任务3-----<nsthread: 0x600000320940>{number = 3, name = (null)}
2019-02-14 15:48:52.982607+0800 mj_ios_test[6484:189759] 执行任务1-----<nsthread: 0x600000356840>{number = 1, name = main}
2019-02-14 15:48:52.982759+0800 mj_ios_test[6484:189759] 执行任务1-----<nsthread: 0x600000356840>{number = 1, name = main}
2019-02-14 15:48:52.982759+0800 mj_ios_test[6484:189815] 执行任务3-----<nsthread: 0x600000320940>{number = 3, name = (null)}
2019-02-14 15:48:52.982871+0800 mj_ios_test[6484:189759] 执行任务1-----<nsthread: 0x600000356840>{number = 1, name = main}
2019-02-14 15:48:52.982917+0800 mj_ios_test[6484:189815] 执行任务3-----<nsthread: 0x600000320940>{number = 3, name = (null)}
2019-02-14 15:48:52.982970+0800 mj_ios_test[6484:189759] 执行任务1-----<nsthread: 0x600000356840>{number = 1, name = main}
2019-02-14 15:48:52.983061+0800 mj_ios_test[6484:189815] 执行任务3-----<nsthread: 0x600000320940>{number = 3, name = (null)}
2019-02-14 15:48:52.983401+0800 mj_ios_test[6484:189759] 执行任务1-----<nsthread: 0x600000356840>{number = 1, name = main}
2019-02-14 15:48:52.983668+0800 mj_ios_test[6484:189815] 执行任务3-----<nsthread: 0x600000320940>{number = 3, name = (null)}
2019-02-14 15:48:52.983945+0800 mj_ios_test[6484:189759] 执行任务1-----<nsthread: 0x600000356840>{number = 1, name = main}
2019-02-14 15:48:52.984186+0800 mj_ios_test[6484:189815] 执行任务3-----<nsthread: 0x600000320940>{number = 3, name = (null)}
2019-02-14 15:48:52.984453+0800 mj_ios_test[6484:189759] 执行任务1-----<nsthread: 0x600000356840>{number = 1, name = main}
2019-02-14 15:48:52.984710+0800 mj_ios_test[6484:189815] 执行任务3-----<nsthread: 0x600000320940>{number = 3, name = (null)}
2019-02-14 15:48:52.984928+0800 mj_ios_test[6484:189759] 执行任务1-----<nsthread: 0x600000356840>{number = 1, name = main}
2019-02-14 15:48:52.985130+0800 mj_ios_test[6484:189815] 执行任务3-----<nsthread: 0x600000320940>{number = 3, name = (null)}
2019-02-14 15:48:52.994169+0800 mj_ios_test[6484:189759] 执行任务1-----<nsthread: 0x600000356840>{number = 1, name = main}
2019-02-14 15:48:52.994181+0800 mj_ios_test[6484:189815] 执行任务3-----<nsthread: 0x600000320940>{number = 3, name = (null)}
2019-02-14 15:48:52.994355+0800 mj_ios_test[6484:189815] 执行任务3-2-----<nsthread: 0x600000320940>{number = 3, name = (null)}
2019-02-14 15:48:52.994502+0800 mj_ios_test[6484:189815] 执行任务3-2-----<nsthread: 0x600000320940>{number = 3, name = (null)}
2019-02-14 15:48:52.994671+0800 mj_ios_test[6484:189815] 执行任务3-2-----<nsthread: 0x600000320940>{number = 3, name = (null)}
2019-02-14 15:48:52.994804+0800 mj_ios_test[6484:189815] 执行任务3-2-----<nsthread: 0x600000320940>{number = 3, name = (null)}
2019-02-14 15:48:52.995173+0800 mj_ios_test[6484:189815] 执行任务3-2-----<nsthread: 0x600000320940>{number = 3, name = (null)}
2019-02-14 15:48:52.995663+0800 mj_ios_test[6484:189815] 执行任务3-2-----<nsthread: 0x600000320940>{number = 3, name = (null)}
2019-02-14 15:48:52.996280+0800 mj_ios_test[6484:189815] 执行任务3-2-----<nsthread: 0x600000320940>{number = 3, name = (null)}
2019-02-14 15:48:52.996518+0800 mj_ios_test[6484:189815] 执行任务3-2-----<nsthread: 0x600000320940>{number = 3, name = (null)}
2019-02-14 15:48:52.996819+0800 mj_ios_test[6484:189815] 执行任务3-2-----<nsthread: 0x600000320940>{number = 3, name = (null)}
2019-02-14 15:48:52.997497+0800 mj_ios_test[6484:189815] 执行任务3-2-----<nsthread: 0x600000320940>{number = 3, name = (null)}
//分析
<1>串行队列queue3中存储两个任务：“执行任务3”和“执行任务3-2”，主队列中存储三个任务：两个a和“执行任务1”；子线程运行queue3的任务，主线程运行主队列中的任务；
<2>“执行任务3”对应的a在主线程运行完毕后自动在主队列中销毁，系统就会从主队列中取出下一个任务即“执行任务3-2”对应的a；同时，“执行任务3”被添加到queue3中并在系统随即开辟的子线程中运行；
但同时，“执行任务3-2”也被添加到queue3中，并且其对应的a自动在主队列中销毁，此时，系统又会从主队列中取出下一个任务“执行任务1”；
<3>因此，此时有两条线程同时在运行：主线程运行“执行任务1”，子线程运行“执行任务3”；二者互不影响，交替执行；
<4>“执行任务3-2”需要等到“执行任务3”执行完毕后，才能被系统取出放到子线程中运行——因此，该任务最后运行；
<2>顺序添加两个任务到并行队列中——情况“6）”
//代码
dispatch_queue_t queue2 = dispatch_get_global_queue(0, 0);
dispatch_async(queue2, ^{
for (int i = 0; i < 10; i++) {
nslog(@"执行任务3-----%@", [nsthread currentthread]);
}
});
dispatch_async(queue2, ^{
for (int i = 0; i < 10; i++) {
nslog(@"执行任务3-2-----%@", [nsthread currentthread]);
}
});
for (int i = 0; i < 10; i++) {
nslog(@"执行任务1-----%@", [nsthread currentthread]);
}
//打印
2019-02-14 16:06:55.818176+0800 mj_ios_test[6737:199038] 执行任务1-----<nsthread: 0x600002ed1180>{number = 1, name = main}
2019-02-14 16:06:55.818227+0800 mj_ios_test[6737:199117] 执行任务3-2-----<nsthread: 0x600002eb23c0>{number = 4, name = (null)}
2019-02-14 16:06:55.818227+0800 mj_ios_test[6737:199118] 执行任务3-----<nsthread: 0x600002eb2380>{number = 3, name = (null)}
2019-02-14 16:06:55.818495+0800 mj_ios_test[6737:199118] 执行任务3-----<nsthread: 0x600002eb2380>{number = 3, name = (null)}
2019-02-14 16:06:55.818500+0800 mj_ios_test[6737:199038] 执行任务1-----<nsthread: 0x600002ed1180>{number = 1, name = main}
2019-02-14 16:06:55.818500+0800 mj_ios_test[6737:199117] 执行任务3-2-----<nsthread: 0x600002eb23c0>{number = 4, name = (null)}
2019-02-14 16:06:55.818630+0800 mj_ios_test[6737:199118] 执行任务3-----<nsthread: 0x600002eb2380>{number = 3, name = (null)}
2019-02-14 16:06:55.818657+0800 mj_ios_test[6737:199038] 执行任务1-----<nsthread: 0x600002ed1180>{number = 1, name = main}
2019-02-14 16:06:55.818753+0800 mj_ios_test[6737:199117] 执行任务3-2-----<nsthread: 0x600002eb23c0>{number = 4, name = (null)}
2019-02-14 16:06:55.818758+0800 mj_ios_test[6737:199118] 执行任务3-----<nsthread: 0x600002eb2380>{number = 3, name = (null)}
2019-02-14 16:06:55.819027+0800 mj_ios_test[6737:199038] 执行任务1-----<nsthread: 0x600002ed1180>{number = 1, name = main}
2019-02-14 16:06:55.819361+0800 mj_ios_test[6737:199117] 执行任务3-2-----<nsthread: 0x600002eb23c0>{number = 4, name = (null)}
2019-02-14 16:06:55.819644+0800 mj_ios_test[6737:199118] 执行任务3-----<nsthread: 0x600002eb2380>{number = 3, name = (null)}
2019-02-14 16:06:55.820062+0800 mj_ios_test[6737:199038] 执行任务1-----<nsthread: 0x600002ed1180>{number = 1, name = main}
2019-02-14 16:06:55.820389+0800 mj_ios_test[6737:199117] 执行任务3-2-----<nsthread: 0x600002eb23c0>{number = 4, name = (null)}
2019-02-14 16:06:55.820744+0800 mj_ios_test[6737:199118] 执行任务3-----<nsthread: 0x600002eb2380>{number = 3, name = (null)}
2019-02-14 16:06:55.821125+0800 mj_ios_test[6737:199038] 执行任务1-----<nsthread: 0x600002ed1180>{number = 1, name = main}
2019-02-14 16:06:55.821634+0800 mj_ios_test[6737:199117] 执行任务3-2-----<nsthread: 0x600002eb23c0>{number = 4, name = 2019-02-14 16:06:55.822389+0800 mj_ios_test[6737:199038] 执行任务1-----<nsthread: 0x600002ed1180>{number = 1, name = main}
(null)}
2019-02-14 16:06:55.821989+0800 mj_ios_test[6737:199118] 执行任务3-----<nsthread: 0x600002eb2380>{number = 3, name = (null)}
2019-02-14 16:06:55.833276+0800 mj_ios_test[6737:199038] 执行任务1-----<nsthread: 0x600002ed1180>{number = 1, name = main}
2019-02-14 16:06:55.833308+0800 mj_ios_test[6737:199118] 执行任务3-----<nsthread: 0x600002eb2380>{number = 3, name = (null)}
2019-02-14 16:06:55.833282+0800 mj_ios_test[6737:199117] 执行任务3-2-----<nsthread: 0x600002eb23c0>{number = 4, name = (null)}
2019-02-14 16:06:55.833419+0800 mj_ios_test[6737:199038] 执行任务1-----<nsthread: 0x600002ed1180>{number = 1, name = main}
2019-02-14 16:06:55.833462+0800 mj_ios_test[6737:199118] 执行任务3-----<nsthread: 0x600002eb2380>{number = 3, name = (null)}
2019-02-14 16:06:55.833503+0800 mj_ios_test[6737:199117] 执行任务3-2-----<nsthread: 0x600002eb23c0>{number = 4, name = (null)}
2019-02-14 16:06:55.833543+0800 mj_ios_test[6737:199038] 执行任务1-----<nsthread: 0x600002ed1180>{number = 1, name = main}
2019-02-14 16:06:55.833890+0800 mj_ios_test[6737:199118] 执行任务3-----<nsthread: 0x600002eb2380>{number = 3, name = (null)}
2019-02-14 16:06:55.834534+0800 mj_ios_test[6737:199117] 执行任务3-2-----<nsthread: 0x600002eb23c0>{number = 4, name = (null)}
2019-02-14 16:06:55.835481+0800 mj_ios_test[6737:199117] 执行任务3-2-----<nsthread: 0x600002eb23c0>{number = 4, name = (null)}
//分析
<1>此时有三条线程在跑：主线程运行“执行任务1”；number为3的子线程运行“执行任务3”；number为4的子线程运行“执行任务3-2”；
<2>各个线程间：运行各自的任务，互不影响，交替执行；
四、结论
1）a自始存储在主队列中并由主线程来运行；b初始并不必然存储在主队列中，取决于目标队列类型，是否由主线程来运行，取决于系统是否开启新线程；
2）同步：只运行在主线程中，a会在主队列中一直等待b执行完毕；异步：开启新线程，a不会等待；
3）线程阻塞：线程正在运行某个任务，下一个任务只能等待该任务运行完毕才能运行（前提：都运行在同一个线程中）；
4）添加b到主队列：同步，死锁；异步：运行在主线程中，不会开启新线程；
5）死锁本质：队列阻塞———添加任务a和被添加任务b都存储在主队列中，因为同步，系统不会开辟新线程，而a运行完毕后也不会从主队列中销毁，造成a等待b，b等待a；
<1>因此，任何一个队列，只要其中前后两个任务相互等待就会造成死锁；而此时对应的线程是闲置的，并没有阻塞；
<2>解决方案：要么两个任务放在不同队列中，要么添加任务（a）执行完毕后使其在队列中销毁（即异步）；
6）不同线程间，运行任务互不影响，交替执行；不同队列可共用同一个线程，同一个队列中的不同任务可运行在不同线程上；
7）主线程：number为1，name为main；子线程：number大于1，name自定义；
8）异步：队列性质决定线程性质（主线程还是子线程）及运行顺序——见“6）异步并行”；
说明：代码较为简单，主要是理论，就不上传github了；
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