<h2>IOS多线程编程简介</h2>    <h2>基本概念</h2>    <p>线程：线程是操作系统能够进行运算调度的最小单位。它被包含在进程之中，是进程中的实际运作单位。一条线程指的是进程中一个单一顺序的控制流，一个进程中可以并发多个线程，每条线程并行执行不同的任务。线程是独立调度和分派的基本单位。同一进程中的多条线程将共享该进程中的全部系统资源，但是自有调用堆栈和寄存器环境。</p>    <p>进程：进程是计算机中已运行程序的实体。其本身并不是几部运行单位，是线程的容器。</p>    <p>任务：任务(task)用于指代抽象的概念,表示需要执行工作，具体可以是一个函数或者一个block。</p>    <h2>IOS常用的多线程编程技术</h2>    <p>IOS常用的多线程编程技术包括：NSThread、Cocoa NSOperation、GCD（grand central dispatch）。其优缺点对比如下表所示：</p>    <table>     <thead>      <tr>       <th>多线程技术</th>       <th>优点</th>       <th>缺点</th>      </tr>     </thead>     <tbody>      <tr>       <td>NSThread</td>       <td>轻量级最低，相对简单</td>       <td>需要手动管理所有的线程活动（生命周期，休眠，同步等）线程同步对数据的加锁会有一定的系统开销</td>      </tr>      <tr>       <td>Cocoa NSOperation</td>       <td>自带线程周期管理，可只关注自己处理逻辑</td>       <td>NSOperation是面向对象的抽象类，实现只能是其子类（NSInvocationOperation和NSBlockOperation），对象需要添加到NSOperationQueue队列里执行</td>      </tr>      <tr>       <td>GCD</td>       <td>效率高，可避免并发陷阱</td>       <td>基于C而非OC实现</td>      </tr>     </tbody>    </table>    <p>如果对性能效率要求较高，处理大量并发时用GCD，简单而安全的可用NSOPeration或者NSThread。Apple推荐使用GCD</p>    <h2>NSThread</h2>    <p>NSThread具体的底层实现机制是Mach线程，实现技术有三种Cocoa threads、POSIX threads（UNIX）、Multiprocessing Services。</p>    <h3>使用方式</h3>    <ol>     <li>显式创建方式： <pre>  <code class="language-objectivec">-(id)initWithTarget:(id)target selector:(SEL)selector object:(id)argument   +(void)detachNewThreadSelector:(SEL)aSelector toTarget:(id)aTarget withObject:(id)anArgument</code></pre> </li>     <li>隐式创建方式： <pre>  <code class="language-objectivec">[Object performSelectorInBackground:@selector(doSomething)withObject:nil];</code></pre> <h3>代码示例</h3> </li>    </ol>    <pre>  <code class="language-objectivec">- (void)testNSThread{      _lock = [[NSLock alloc] init];      _condition = [[NSCondition alloc] init];      self.total = 20;        NSThread *thread1 = [[NSThread alloc] initWithTarget:self selector:@selector(run) object:nil];      [thread1 setName:@"Thread--1"];      [thread1 start];        NSThread *thread2 = [[NSThread alloc] initWithTarget:self selector:@selector(run) object:nil];      [thread2 setName:@"Thread--2"];      [thread2 start];    //    [NSThread detachNewThreadSelector:@selector(run) toTarget:self withObject:nil];  //    [self performSelectorInBackground:@selector(run) withObject:nil];  }      - (void)run{      while (YES) {          [self.lock lock];          if(self.total >= 0){              [NSThread sleepForTimeInterval:0.09];              NSInteger count = 20 - self.total;              NSLog(@"total is :%zd,left is:%zd, thread name is:%@", self.total, count,[[NSThread currentThread] name]);              self.total--;          }else{              break;          }          [self.lock unlock];      }  }</code></pre>    <h3>分析说明：</h3>    <p>NSThread中线程内存管理，是否循环引用，同步问题都需要手动管理。除了代码中的lock还可以用@synchronized来简化NSLock的使用。</p>    <pre>  <code class="language-objectivec">- (void)doSomeThing:(id)anObj   {       @synchronized(anObj)       {           // Everything between the braces is protected by the @synchronized directive.       }   }</code></pre>    <p>还可以用NSCondition中的wait消息设置等待线程，然后通过signal消息 发送信号的方式，在一个线程唤醒另外一个线程的等待。</p>    <p>线程间通信：</p>    <pre>  <code class="language-objectivec">//在指定线程上执行操作  //在主线程上执行操作  [self performSelectorOnMainThread:@selector(run) withObject:nil waitUntilDone:YES];   [self performSelector:@selector(run) onThread:threadName withObject:nil waitUntilDone:YES];   //在当前线程执行操作  [self performSelector:@selector(run) withObject:nil];</code></pre>    <p>获取线程：</p>    <pre>  <code class="language-objectivec">NSThread *current = [NSThread currentThread];  NSThread *main = [NSThread mainThread];</code></pre>    <h2>Cocoa NSOperation</h2>    <h3>使用方式</h3>    <p>NSOperation的使用方式有两种：</p>    <ul>     <li>使用NSInvocationOperation和NSBlockOperation</li>     <li>自定义NSOperation子类。 <h3>代码示例</h3> </li>    </ul>    <pre>  <code class="language-objectivec">-(void)testNSOPeration{      self.total = 20;      NSInvocationOperation *operation1 = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(run2) object:nil];      NSOperationQueue *queue = [[NSOperationQueue alloc] init];      [queue addOperation:operation1];          NSBlockOperation *operation2 = [NSBlockOperation blockOperationWithBlock:^{          [self run2];      }];  //    [operation1 start];//当前线程直接执行      [operation2 addDependency:operation1];  //    [operation2 addExecutionBlock:^{  //        [self run];  //    }];      [queue addOperation:operation2];  //    queue.maxConcurrentOperationCount = 1;  }    - (void)run2{      while (YES) {          if(self.total >= 0){  //            [NSThread sleepForTimeInterval:0.09];              NSInteger count = 20 - self.total;              NSLog(@"total is :%zd,left is:%zd, thread name is:%@", self.total, count,[[NSThread currentThread] name]);              self.total--;          }else{              break;          }      }  }</code></pre>    <p>自定义NSOperation子类</p>    <p>自定义NSOperation子类需要实现start、main、isExecuting、isFinish、isConcurrnet（asynchronous）方法。</p>    <pre>  <code class="language-objectivec">@interface MyOperation : NSOperation {      BOOL        executing;      BOOL        finished;  }  - (void)completeOperation;  @end    @implementation MyOperation  - (id)init {      self = [super init];      if (self) {          executing = NO;          finished = NO;      }      return self;  }    - (BOOL)isConcurrent {      return YES;  }    - (BOOL)isExecuting {      return executing;  }    - (BOOL)isFinished {      return finished;  }  @end</code></pre>    <h3>分析说明</h3>    <p>NSOperationQueue 是一个并行队列，可以通过设置maxConcurrentOperationCount来设定最大并行操作数，对于自定义实现的NSOperation子类，可以通过实现isConcurrent(isAsynchronous)方法来制定具体的操作是否同步执行。</p>    <h2>GCD</h2>    <h3>使用方式</h3>    <p>GCD的工作原理是让程序平行排队的特定任务，根据可用的处理资源，安排他们在任何可用的处理器核心上执行任务。</p>    <p>队列的创建</p>    <ul>     <li>主线程队列（串行）：<br> dispatch_get_main_queue()</li>     <li>全局队列（并行）：<br> dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)<br> 第一个参数说明队列的优先级，第二个参数暂未用到，默认0。根据优先级划分，共有四个全局队列 <pre>  <code class="language-objectivec">#define DISPATCH_QUEUE_PRIORITY_HIGH 2  #define DISPATCH_QUEUE_PRIORITY_DEFAULT 0  #define DISPATCH_QUEUE_PRIORITY_LOW (-2)  #define DISPATCH_QUEUE_PRIORITY_BACKGROUND INT16_MIN</code></pre> </li>     <li>自定义队列：<br> dispatch_queue_create(queuename, attr)<br> 第一个参数制定队列名称，自定义队列可以是串行也可以是并行的，由第二个参数attr决定。 <pre>  <code class="language-objectivec">DISPATCH_QUEUE_SERIAL //(NULL) 串行  DISPATCH_QUEUE_SERIAL_INACTIVE  DISPATCH_QUEUE_CONCURRENT //并行</code></pre> <strong>多线程执行</strong></li>     <li>同步执行：<br> dispatch_sync(dispatch_queue_t _Nonnull queue, ^(void)block)</li>     <li>异步执行：<br> dispatch_async(dispatch_queue_t _Nonnull queue, ^(void)block)</li>     <li>一次性执行：</li>    </ul>    <p>tatic dispatch_once_t onceToken; dispatch_once(&amp;onceToken, ^{ code to be executed once });</p>    <ul>     <li>延时执行：</li>    </ul>    <p>dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{ code to be executed after a specified delay });</p>    <ul>     <li>循环迭代执行：<br> dispatch_apply(size_t iterations, dispatch_queue_t _Nonnull queue, ^(size_t)block)<br> 能并发地执行不同的迭代。这个函数是同步的，所以和普通的 for 循环一样，它只会在所有工作都完成后才会返回。</li>     <li>栅栏(barrier)执行：<br> dispatch_barrier_sync(dispatch_queue_t _Nonnull queue, ^(void)block) dispatch_barrier_async(dispatch_queue_t _Nonnull queue, ^(void)block)<br> 栅栏执行的意思是：在barrier前面的任务执行结束后它才执行，而且它后面的任务等它执行完成之后才会执行。</li>     <li>分组执行：<br> dispatch_group_async(dispatch_group_t _Nonnull group, dispatch_queue_t _Nonnull queue, ^(void)block) dispatch_group_notify(dispatch_group_t _Nonnull group, dispatch_queue_t _Nonnull queue, ^(void)block)<br> dispatch_group_async可以实现监听一组任务是否完成，完成后得到通知执行其他的操作<br> <strong>信号量</strong> <pre>  <code class="language-objectivec">dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);  dispatch_semaphore_signal(semaphore);   dispatch_time_t timeoutTime = dispatch_time(DISPATCH_TIME_NOW, kDefaultTimeoutLengthInNanoSeconds); dispatch_semaphore_wait(semaphore, timeoutTime)</code></pre> <strong>Dispatch Source</strong> <pre>  <code class="language-objectivec">dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, dispatchQueue);    dispatch_source_set_timer(timer, DISPATCH_TIME_NOW, intervalInSeconds * NSEC_PER_SEC, leewayInSeconds * NSEC_PER_SEC);    dispatch_source_set_event_handler(timer, ^{        code to be executed when timer fires    });    dispatch_resume(timer);</code></pre> <h3>代码示例</h3> <strong>栅栏执行：</strong></li>    </ul>    <pre>  <code class="language-objectivec">- (void)testBarrier{      dispatch_queue_t queue = dispatch_queue_create("com.zyw.test", DISPATCH_QUEUE_CONCURRENT);      dispatch_async(queue, ^{          [NSThread sleepForTimeInterval:1];          NSLog(@"dispatch_async1");      });      dispatch_async(queue, ^{          [NSThread sleepForTimeInterval:2];          NSLog(@"dispatch_async2");      });      dispatch_barrier_async(queue, ^{          NSLog(@"dispatch_barrier_async");          [NSThread sleepForTimeInterval:2];        });      dispatch_async(queue, ^{          [NSThread sleepForTimeInterval:1];          NSLog(@"dispatch_async3");         });  }</code></pre>    <p>输出：</p>    <pre>  <code class="language-objectivec">2016-12-02 17:17:24.710 TestThread[1434:164682] dispatch_async1  2016-12-02 17:17:25.713 TestThread[1434:164681] dispatch_async2  2016-12-02 17:17:25.713 TestThread[1434:164681] dispatch_barrier_async  2016-12-02 17:17:28.720 TestThread[1434:164681] dispatch_async3</code></pre>    <p>分组执行：</p>    <pre>  <code class="language-objectivec">- (void)testGroup{      dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);      dispatch_group_t group = dispatch_group_create();      dispatch_group_async(group, queue, ^{          [NSThread sleepForTimeInterval:1];          NSLog(@"group1");      });      dispatch_group_async(group, queue, ^{          [NSThread sleepForTimeInterval:2];          NSLog(@"group2");      });      dispatch_group_async(group, queue, ^{          [NSThread sleepForTimeInterval:3];          NSLog(@"group3");      });      dispatch_group_notify(group, dispatch_get_main_queue(), ^{          NSLog(@"EndAllgroupTask");      });  //    第二种使用方法  //    dispatch_group_enter(group);  //    dispatch_group_leave(group);  //    dispatch_group_notify(group, dispatch_get_main_queue(), ^{  //          //    });  }</code></pre>    <p>输出：</p>    <pre>  <code class="language-objectivec">2016-12-02 17:11:50.592 TestThread[1407:161853] group1  2016-12-02 17:11:51.591 TestThread[1407:161852] group2  2016-12-02 17:11:52.589 TestThread[1407:161855] group3  2016-12-02 17:11:52.590 TestThread[1407:161635] EndAllgroupTask</code></pre>    <p>信号量：</p>    <pre>  <code class="language-objectivec">- (void)testSemaphore{      dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);      dispatch_semaphore_t semaphore = dispatch_semaphore_create(5);      for (NSInteger i = 0 ; i < 10; i++) {          dispatch_async(queue, ^{              dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);              NSLog(@"*****%zd", i);              [NSThread sleepForTimeInterval:1];              dispatch_semaphore_signal(semaphore);          });      }  }</code></pre>    <p>输出：</p>    <pre>  <code class="language-objectivec">2016-12-02 17:36:11.071 TestThread[1496:173204] *****1  2016-12-02 17:36:11.071 TestThread[1496:173207] *****2  2016-12-02 17:36:11.071 TestThread[1496:173203] *****0  2016-12-02 17:36:11.071 TestThread[1496:173242] *****3  2016-12-02 17:36:11.071 TestThread[1496:173243] *****4    2016-12-02 17:36:12.074 TestThread[1496:173244] *****5  2016-12-02 17:36:12.074 TestThread[1496:173250] *****9  2016-12-02 17:36:12.074 TestThread[1496:173246] *****7  2016-12-02 17:36:12.074 TestThread[1496:173245] *****6  2016-12-02 17:36:12.074 TestThread[1496:173247] *****8</code></pre>    <p>Dispatch Source</p>    <pre>  <code class="language-objectivec">- (void)testSource{      dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);      dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);      dispatch_time_t start = dispatch_time(DISPATCH_TIME_NOW, 5);      dispatch_source_set_timer(timer, start, 3 * NSEC_PER_SEC, 0 * NSEC_PER_SEC);      dispatch_source_set_event_handler(timer, ^{          NSLog(@"******Hello");      });      //启动      dispatch_resume(timer);  }</code></pre>    <p>同步异步串行并行简单常用未列出</p>    <h3>分析说明</h3>    <p>同步异步和并行串行</p>    <table>     <thead>      <tr>       <th> </th>       <th>并行队列</th>       <th>串行队列</th>       <th>主队列</th>      </tr>     </thead>     <tbody>      <tr>       <td><strong>异步执行</strong></td>       <td>开启多个新的线程，任务同时执行</td>       <td>开启一个新的线程，任务按顺序执行</td>       <td>不开启新的线程，任务按顺序执行</td>      </tr>      <tr>       <td><strong>同步执行</strong></td>       <td>不开启新的线程，任务按顺序执行</td>       <td>不开启新的线程,任务按顺序执行</td>       <td>死锁</td>      </tr>     </tbody>    </table>    <p>对于GCD中的block执行，都有对应的function执行函数，比如：</p>    <pre>  <code class="language-objectivec">dispatch_sync_f(dispatch_queue_t  _Nonnull queue, void * _Nullable context, dispatch_function_t  _Nonnull work)  dispatch_async_f(dispatch_queue_t  _Nonnull queue, void * _Nullable context, dispatch_function_t  _Nonnull work)</code></pre>    <p>信号量</p>    <p>dispatch_semaphore_create(N);N>=0时可以正常执行，当N小于0时一直等待</p>    <p>代码示例的解释：创建了一个初使值为5的semaphore，每一次for循环都会创建一个新的线程（具体看GCD的线程池，此处认为是新建），线程结束的时候会发送一个信号，线程创建之前会信号等待，所以当同时创建了5个线程之后，for循环就会阻塞，等待有线程结束之后会增加一个信号才继续执行，如此就形成了对并发的控制，如上就是一个并发数为5的一个线程队列。</p>    <p>Dispatch Source</p>    <p>它基本上就是一个低级函数的 grab-bag，可监听事件如下：</p>    <p>DISPATCH_SOURCE_TYPE_DATA_ADD：属于自定义事件，可以通过dispatch_source_get_data函数获取事件变量数据，在我们自定义的方法中可以调用dispatch_source_merge_data函数向Dispatch Source设置数据，下文中会有详细的演示。</p>    <p>DISPATCH_SOURCE_TYPE_DATA_OR：属于自定义事件，用法同上面的类型一样。</p>    <p>DISPATCH_SOURCE_TYPE_MACH_SEND：Mach端口发送事件。</p>    <p>DISPATCH_SOURCE_TYPE_MACH_RECV：Mach端口接收事件。</p>    <p>DISPATCH_SOURCE_TYPE_PROC：与进程相关的事件。</p>    <p>DISPATCH_SOURCE_TYPE_READ：读文件事件。</p>    <p>DISPATCH_SOURCE_TYPE_WRITE：写文件事件。</p>    <p>DISPATCH_SOURCE_TYPE_VNODE：文件属性更改事件。</p>    <p>DISPATCH_SOURCE_TYPE_SIGNAL：接收信号事件。</p>    <p>DISPATCH_SOURCE_TYPE_TIMER：定时器事件。</p>    <p>DISPATCH_SOURCE_TYPE_MEMORYPRESSURE：内存压力事件。</p>    <p> </p>    <p>来自：http://www.jianshu.com/p/af0a221896ba</p>    <p> </p>