iOS多线程操作时一些要注意的安全问题

   <p>这次STMAssembleView <a href="/misc/goto?guid=4959713578873730443" rel="nofollow,noindex">https://github.com/ming1016/STMAssembleView</a> 加入异步解析上线后发现一些线程安全方面的问题，现总结下。 先看看这段代码</p>    <pre>  <code class="language-objectivec">- (void)viewDidLoad {      [super viewDidLoad];        self.asStr = @"string is very first";        [self performSelector:@selector(doSomething) withObject:nil afterDelay:0.5];        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{          NSLog(@"before sleep %@",self.asStr);          sleep(3);          NSLog(@"after sleep %@",self.asStr);      });  }    - (void)doSomething {      self.asStr = @"string has changed";  }</code></pre>    <p>执行结果</p>    <pre>  <code class="language-objectivec">2016-08-31 20:58:43.927 HomePageTest[68018:535737] before sleep string is very first  2016-08-31 20:58:46.927 HomePageTest[68018:535737] after sleep string has changed</code></pre>    <p>会发现在异步执行中如果asStr改变了，那么异步线程里的asStr也会改变这样就没法保证异步对资源独占操作。</p>    <p>如果在异步block里创建一个str赋值如下代码</p>    <pre>  <code class="language-objectivec">- (void)viewDidLoad {      [super viewDidLoad];        self.asStr = @"string is very first";        [self performSelector:@selector(doSomething) withObject:nil afterDelay:0.5];        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{          NSString *str = self.asStr;          NSLog(@"before sleep %@",str);          sleep(3);          NSLog(@"after sleep %@",str);      });  }    - (void)doSomething {      self.asStr = @"string has changed";  }</code></pre>    <p>执行结果</p>    <pre>  <code class="language-objectivec">2016-08-31 20:59:50.094 HomePageTest[68075:537624] before sleep string is very first  2016-08-31 20:59:53.097 HomePageTest[68075:537624] after sleep string is very first</code></pre>    <p>这样新的string就不会受到外部改变的影响，但是如果在这个赋值时刻self.asStr已变成野指针那么后面的操作还是会出错，虽然这样情况不是那么容易出现。</p>    <p>如何防止这种情况呢，可以看看下面的代码</p>    <pre>  <code class="language-objectivec">- (void)viewDidLoad {      [super viewDidLoad];        self.asStr = @"string is very first";        [self performSelector:@selector(doSomething) withObject:nil afterDelay:0.5];        __weak __typeof(self.asStr) weakString = self.asStr;      dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{          __strong __typeof(weakString) strongString = weakString;          if(strongString) {              NSLog(@"before sleep %@",strongString);              sleep(3);              NSLog(@"after sleep %@",strongString);          }      });  }    - (void)doSomething {      self.asStr = @"string has changed";  }</code></pre>    <p>执行结果</p>    <pre>  <code class="language-objectivec">2016-08-31 21:00:24.457 HomePageTest[68131:538976] before sleep string is very first  2016-08-31 21:00:27.461 HomePageTest[68131:538976] after sleep string is very first</code></pre>    <p>weakString会在self.asStr释放时置为nil，如果不是nil时，能够确保对象在block调用的完整周期里面被retain，如果被抢占对strongString的执行会继续并且会产生一样的值，如果strongString执行到时是nil，那么block不能正确执行前已经返回，这样就不会出现先前那样的问题。</p>    <p>还可以用锁来保证多个线程对一份资源在操作时不会被更改</p>    <pre>  <code class="language-objectivec">@interface HomeViewController () {      pthread_mutex_t _mutex;  }  @end    @implementation HomeViewController    - (void)viewDidLoad {      [super viewDidLoad];      pthread_mutex_init(&_mutex, NULL);      self.asStr = @"string is very first";        [self performSelector:@selector(doSomething) withObject:nil afterDelay:0.5];        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{          pthread_mutex_lock(&_mutex);          self.asStr = @"string has changed in global queue";          NSLog(@"before sleep %@",self.asStr);          sleep(3);          NSLog(@"after sleep %@",self.asStr);          pthread_mutex_unlock(&_mutex);        });    }    - (void)doSomething {      pthread_mutex_lock(&_mutex);      self.asStr = @"string has changed";      pthread_mutex_unlock(&_mutex);  }    - (void)dealloc  {      pthread_mutex_destroy(&_mutex);  }</code></pre>    <p>执行结果</p>    <pre>  <code class="language-objectivec">2016-08-31 21:01:00.351 HomePageTest[68174:540038] before sleep string has changed in global queue  2016-08-31 21:01:03.354 HomePageTest[68174:540038] after sleep string has changed in global queue</code></pre>    <p>在RAC中使用的是OSSpinLock来保证线程安全的，不过几位苹果工程师在swift-dev邮件列表中讨论weak属性的线程安全问题的邮件里爆出自旋锁有bug，邮件地址： <a href="/misc/goto?guid=4958978317716582850" rel="nofollow,noindex">https://lists.swift.org/pipermail/swift-dev/Week-of-Mon-20151214/000372.html</a> 。大概就是不同优先级线程调度算法会有优先级反转问题，比如低优先级获锁访问资源，高优先级尝试访问时会等待，这时低优先级又没法争过高优先级导致任务无法完成lock释放不了。也可以看看ReactiveCo社区的讨论 <a href="/misc/goto?guid=4958978318411046818" rel="nofollow,noindex">https://github.com/ReactiveCocoa/ReactiveCocoa/issues/2619</a></p>    <p>本来OSSpinLock是性能最高的锁，但是由于如果不在同一个优先级线程进行锁操作就不能保证安全，那么dispatch_semaphore和pthread_mutex这种仅次于自旋锁的可以作为替代方案。我注意到非死book的KVOController在2016年5月17日时的一个Commit里将所有OSSpinLock替换成了pthread_mutex，可参看这个commit <a href="/misc/goto?guid=4959713579015913265" rel="nofollow,noindex">https://github.com/非死book/KVOController/commit/4f5c329b26f48b151eed82da085288763e2e1761</a> 。pthread_mutex会在新系统中性能得到很大的提升，所以可以考虑这个方案。</p>    <p> </p>    <p>来自：http://www.starming.com/index.php?v=index&view=102</p>    <p> </p>