使用全局併發隊列從兩個不同線程中變換陣列時不會發生崩潰

Question

在下面的代碼中，我們有一個可變數組，它由兩個併發隊列進行變異。由於併發隊列不是線程安全的，因此這段代碼最好應該崩潰，但是這個代碼不會發生任何異常或崩潰。

請幫助我理解這種行爲。任何幫助將不勝感激:-)

@interface ViewController() 
    @property(nonatomic, strong) NSMutableArray *arr; 
    @end 

    @implementation ViewController 

- (void)viewDidLoad { 
    [super viewDidLoad]; 
    // Do any additional setup after loading the view, typically from a nib. 
    self.arr = [NSMutableArray new]; 
} 

-(void)viewDidAppear:(BOOL)animated{ 

     [super viewDidAppear:animated]; 

     __weak typeof(self) weakSelf = self; 

     dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ 
      for (int i = 0; i < 20000; i++) { 
        [weakSelf.arr addObject:[NSNumber numberWithInt:i]]; 
        NSLog(@"Added %@", [weakSelf.arr lastObject]); 
       } 

      NSLog(@"Final count %ld", [self.arr count]); 
     }); 

     [self performSelector:@selector(removeObjects) withObject:nil afterDelay:0.1]; 
    } 

    -(void)removeObjects{ 
     __weak typeof(self) weakSelf = self; 

     dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ 
      for (int i = 0; i < 1000; i++) { 
       if (weakSelf.arr.count > 1) { 
        [weakSelf.arr removeObjectAtIndex:0]; 
       } 
       NSLog(@"Remove object"); 
      } 
     }); 
    } 

    @end 

Answer 1

併發訪問一個資源並不意味着，保證崩潰。這與「有運氣」無關。也許你的代碼系統地運行在沒有崩潰發生的情況下。

此外，「不是線程安全的」並不意味着「會崩潰」。任何故障都可能發生。

Answer 2

併發訪問NSMutableArray不會明確地導致崩潰，但它可能導致數組損壞，從而導致您的應用程序因無效數據而崩潰。

考慮你的代碼的這個略微修改後的版本：

-(void)viewDidAppear:(BOOL)animated{ 

    [super viewDidAppear:animated]; 

    __weak typeof(self) weakSelf = self; 

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{ 
     for (int i = 0; i < 20000; i++) { 
      NSUInteger randomIndex = arc4random_uniform([weakSelf.arr count]); 
      [weakSelf.arr insertObject:[NSNumber numberWithInt:i] atIndex:randomIndex]; 
      NSLog(@"Added %@", weakSelf.arr[randomIndex]); 
     } 

     NSLog(@"Final count %ld", [self.arr count]); 
    }); 

    [self performSelector:@selector(removeObjects) withObject:nil afterDelay:0.1]; 
} 

-(void)removeObjects{ 
    __weak typeof(self) weakSelf = self; 

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{ 
     for (int i = 0; i < 1000; i++) { 
      if (weakSelf.arr.count > 1) { 
       NSUInteger randomIndex = arc4random_uniform([weakSelf.arr count]); 
       [weakSelf.arr removeObjectAtIndex:randomIndex]; 
      } 
      NSLog(@"Remove object"); 
     } 
    }); 
} 

每次你運行它，你得到的最終計數略有不同的值。

如果添加@synchronized使數組訪問線程安全的，那麼你總是得到的19000

-(void)viewDidAppear:(BOOL)animated{ 

    [super viewDidAppear:animated]; 

    __weak typeof(self) weakSelf = self; 

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{ 
     for (int i = 0; i < 20000; i++) { 
      @synchronized (self.arr) { 

      NSUInteger randomIndex = arc4random_uniform([weakSelf.arr count]); 
      [weakSelf.arr insertObject:[NSNumber numberWithInt:i] atIndex:randomIndex]; 
      NSLog(@"Added %@", weakSelf.arr[randomIndex]); 
      } 
     } 

     NSLog(@"Final count %ld", [self.arr count]); 
    }); 

    [self performSelector:@selector(removeObjects) withObject:nil afterDelay:0.1]; 
} 

-(void)removeObjects{ 
    __weak typeof(self) weakSelf = self; 

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{ 
     for (int i = 0; i < 1000; i++) { 
      if (weakSelf.arr.count > 1) { 
       @synchronized (self.arr) { 

       NSUInteger randomIndex = arc4random_uniform([weakSelf.arr count]); 
       [weakSelf.arr removeObjectAtIndex:randomIndex]; 
       } 
      } 
      NSLog(@"Remove object"); 
     } 
    }); 
} 

最終計數，而你正在使用快速列舉會給崩潰變異數組，但突變沒有按在這種情況下，甚至不得不從另一條線索來;只是在枚舉循環中改變數組會導致崩潰。