对于异步循环，然后等待它完成（的OBJ-C）

Question

因此，让我们假设我有以下代码

-(void)doSomething{ 
    [self expensiveMethod]; 
    [self otherMethod]; //Depends on above method to have finished 
} 

-(void)expensiveMethod{ 
for(int i = 0; i<[someArray count]; i++{ 
    [self costlyOperation:someArray[i]]; 
    } 
} 

理想我想[self costlyOperation]分拆其他线程使每一个为已完成接近并行（当然，我意识到这不是完全可能的）。一旦对每个数组对象完成了[self costlyOperation]，我希望它返回，以便[self otherMethod可以利用处理。

Answer 1

您可以使用默认队列在后台使用调度异步运行任务。

编辑

您可以并行执行ASYC组异步任务。您可能需要按照您的要求稍微调整一下。

-(void)doSomething{ 
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void){ 
     //Background Thread 
     [self expensiveMethod:^{ 
      dispatch_async(dispatch_get_main_queue(), ^(void){ 
       //Run UI Updates 
       [self otherMethod]; //Depends on above method to have finished 
      }); 
     }]; 
    }); 
} 

-(void)expensiveMethod:(void (^)(void))callbackBlock { 

    dispatch_group_t group = dispatch_group_create(); 

    for(int i = 0; i<[someArray count]; i++) { 
     __block int index = i; 
     dispatch_group_async(group,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0),^{ 
      [self costlyOperation:someArray[index]]; 
     }); 
    } 

    dispatch_group_notify(group,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0),^{ 
     if (callbackBlock) { 
      callbackBlock(); 
     } 
    }); 
}