為什么用鎖：

多線程編程中知纷，如果對同一數(shù)據(jù)源進行讀寫操作就會造成不可預知的結(jié)果，所以我們應(yīng)該盡量避免并發(fā)操作資源在線程之間共享是辕，以減少線程間的相互作用眉反，就需要一些同步工具狞谱，來確保當它們交互的時候是安全的。

鎖的種類：

iOS開發(fā)中常用的鎖有如下幾種：

1. @synchronized 同步鎖
2. NSLock 對象鎖
3. NSRecursiveLock 遞歸鎖
4. NSConditionLock 條件鎖
5. pthread_mutex 互斥鎖（C語言）
6. dispatch_semaphore 信號量實現(xiàn)加鎖（GCD）
7. OSSpinLock 自旋鎖 （暫不建議使用禁漓，原因參見這里）

鎖的性能對比：

work_time.png

那根據(jù)這張圖所描述的耗時對別正確嗎？我們自己實際測試一下播歼，看下邊的測試代碼：

#import <pthread.h>
#import <libkern/OSAtomic.h>

#define CycleTime (1024*1024*32)

- (void)testAction {
  NSTimeInterval time = [NSDate date].timeIntervalSince1970;
  
  //同步鎖
  for (int i = 0; i < CycleTime; i++) {
      @synchronized(self) {
          
      }
  }
  NSLog(@"%f : work time of Synchronized",[NSDate date].timeIntervalSince1970-time);
  time = [NSDate date].timeIntervalSince1970;
  
  //條件鎖
  NSConditionLock *conditionLock = [[NSConditionLock alloc] init];
  for (int i = 0; i < CycleTime; i++) {
      [conditionLock lock];
      [conditionLock unlock];
  }
  NSLog(@"%f : work time of NSConditionLock",[NSDate date].timeIntervalSince1970-time);
  time = [NSDate date].timeIntervalSince1970;
  
  //遞歸鎖
  NSRecursiveLock *rsLock = [[NSRecursiveLock alloc] init];
  for (int i = 0; i < CycleTime; i++) {
      [rsLock lock];
      [rsLock unlock];
  }
  NSLog(@"%f : work time of NSRecursiveLock",[NSDate date].timeIntervalSince1970-time);
  time = [NSDate date].timeIntervalSince1970;
  
  //互斥鎖
  NSLock *mutexLock = [[NSLock alloc] init];
  for (int i = 0; i < CycleTime; i++) {
      [mutexLock lock];
      [mutexLock unlock];
  }
  NSLog(@"%f : work time of NSLock",[NSDate date].timeIntervalSince1970-time);
  time = [NSDate date].timeIntervalSince1970;
  
  //互斥鎖
  pthread_mutex_t mutex;
  pthread_mutex_init(&mutex, NULL);
  for (int i = 0; i < CycleTime; i++) {
      pthread_mutex_lock(&mutex);
      pthread_mutex_unlock(&mutex);
  }
  NSLog(@"%f : work time of pthread_mutex",[NSDate date].timeIntervalSince1970-time);
  time = [NSDate date].timeIntervalSince1970;
  
  //信號量
  dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);
  for (int i = 0; i < CycleTime; i++) {
      dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
      dispatch_semaphore_signal(semaphore);
  }
  NSLog(@"%f : work time of dispatch_semaphore",[NSDate date].timeIntervalSince1970-time);
  time = [NSDate date].timeIntervalSince1970;
  
  //自旋鎖
  OSSpinLock spinlock = OS_SPINLOCK_INIT;
  for (int i = 0; i < CycleTime; i++) {
      OSSpinLockLock(&spinlock);
      OSSpinLockUnlock(&spinlock);
  }
  NSLog(@"%f : work time of OSSpinLock",[NSDate date].timeIntervalSince1970-time);
}

輸出：

2018-03-29 18:51:14.638390+0800 Lock[97416:4313533] 3.498765 : work time of Synchronized
2018-03-29 18:51:16.987938+0800 Lock[97416:4313533] 2.349282 : work time of NSConditionLock
2018-03-29 18:51:18.480868+0800 Lock[97416:4313533] 1.492687 : work time of NSRecursiveLock
2018-03-29 18:51:19.286578+0800 Lock[97416:4313533] 0.805475 : work time of NSLock
2018-03-29 18:51:20.029690+0800 Lock[97416:4313533] 0.742872 : work time of pthread_mutex
2018-03-29 18:51:20.556899+0800 Lock[97416:4313533] 0.526966 : work time of dispatch_semaphore
2018-03-29 18:51:20.909306+0800 Lock[97416:4313533] 0.352188 : work time of OSSpinLock

看輸出結(jié)果一目了然了伶跷，跟上圖的評測結(jié)果一致掰读。

如何使用鎖：

1. @synchronized賣票例子：

- (void)sycnAction {
  __block int ticketsCount = 5;
  
  void(^saleTickets)(void) = ^() {
      while (YES) {
          /*
           注意點:
           1.加鎖的代碼盡量少
           2.添加的OC對象必須在多個線程中都是同一對象
           3.優(yōu)點是不需要顯式的創(chuàng)建鎖對象，便可以實現(xiàn)鎖的機制叭莫。
           4.@synchronized塊會隱式的添加一個異常處理例程來保護代碼蹈集，該處理例程會在異常拋出的時候自動的釋放互斥鎖雇初。所以如果不想讓隱式的異常處理例程帶來額外的開銷，你可以考慮使用鎖對象靖诗。
           */
          
          //這里參數(shù)添加一個OC對象，一般使用self
          @synchronized(self) {
              [NSThread sleepForTimeInterval:1];
              if (ticketsCount > 0) {
                  ticketsCount--;
                  NSLog(@"Tickets = %d, Thread:%@",ticketsCount,[NSThread currentThread]);
              }
              else {
                  NSLog(@"Clear  Thread:%@",[NSThread currentThread]);
                  break;
              }
          }
      }
  };
  
  NSLog(@"Start tickets count = %d, Thread:%@",ticketsCount,[NSThread currentThread]);

  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
      saleTickets();
  });
  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
      saleTickets();
  });
}

2. NSLock互斥鎖賣票例子：

- (void)lockAction {
  __block int ticketsCount = 5;
  
  NSLock *lock = [[NSLock alloc] init];
  
  void(^lockSaleTickets)(void) = ^() {
      while (1) {
          [lock lock];
          
          [NSThread sleepForTimeInterval:1];
          if (ticketsCount > 0) {
              ticketsCount--;
              NSLog(@"Tickets= %d, Thread:%@",ticketsCount,[NSThread currentThread]);
          }
          else {
              NSLog(@"Clear  Thread:%@",[NSThread currentThread]);
              break;
          }
          
          [lock unlock];
      }
  };
  
  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
      lockSaleTickets();
  });
  
  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
      lockSaleTickets();
  });
}

3. NSRecursiveLock遞歸鎖賣票例子：

- (void)recursiveLockAction {
//    NSLock *lock = [[NSLock alloc] init];
  NSRecursiveLock *lock = [[NSRecursiveLock alloc] init];
  
  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
      static void(^TestMethod)(int);
      
      TestMethod = ^(int value) {
          [lock lock];
          if (value > 0) {
              NSLog(@"value = %d",value);
              [NSThread sleepForTimeInterval:1];
              TestMethod(--value);
          }
          [lock unlock];
      };
      
      NSLog(@"Begain Test");
      TestMethod(5);
  });
}

4. NSConditionLock條件鎖賣票例子：

- (void)conditionLockActive {
  NSConditionLock *lock = [[NSConditionLock alloc] init];
  
  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
      for (int i = 0; i <= 5; i++) {
          [lock lock];
          NSLog(@"1.thread1 condition = %ld, i = %d",(long)lock.condition,i);
          [lock unlockWithCondition:i];
          NSLog(@"2.thread1 condition = %ld, i = %d",(long)lock.condition,i);
      }
  });
  
  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
      [lock lockWhenCondition:2];
      NSLog(@"thread2");
      [lock unlock];
  });
}

5. pthread_mutex_t互斥鎖例子：

- (void)pthreadMutexAction {
  __block pthread_mutex_t mutex;
  pthread_mutex_init(&mutex, NULL);
  
  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
      sleep(2);
      pthread_mutex_lock(&mutex);
      NSLog(@"任務(wù)2");
      
      pthread_mutex_unlock(&mutex);
  });
  
  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
      pthread_mutex_lock(&mutex);
      NSLog(@"任務(wù)1");
      pthread_mutex_unlock(&mutex);
  });
}

6. dispatch_semaphore_t信號量例子：

- (void)semaphoreAction {
  dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);
  
  __block int count = 10;
  
  void(^TestMethod)(void) = ^() {
      while (YES) {
          dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
          if (count > 0) {
              count --;
              NSLog(@"value = %d, %@",count,[NSThread currentThread]);
              [NSThread sleepForTimeInterval:1];
          }
          else {
              NSLog(@"Done %@",[NSThread currentThread]);
              break;
          }
          dispatch_semaphore_signal(semaphore);
      }
  };
  
  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
      TestMethod();
  });
  
  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
      TestMethod();
  });
}

7. OSSpinLock自旋鎖例子：

- (void)spinAction {
  __block int tickets = 5;
  
  OSSpinLock spinlock = OS_SPINLOCK_INIT;
  
  void(^TestMethod)(void) = ^() {
      while (YES) {
          OSSpinLockLock(&spinlock);
          if (tickets > 0) {
              tickets --;
              NSLog(@"value = %d, %@",tickets,[NSThread currentThread]);
              [NSThread sleepForTimeInterval:1];
          }
          else {
              NSLog(@"Done %@",[NSThread currentThread]);
              break;
          }
          OSSpinLockUnlock(&spinlock);
      }
  };
  
  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
      TestMethod();
  });
  
  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
      TestMethod();
  });
}

死鎖：待續(xù)....