等待 dispatch_semaphore 以等待许多异步任务完成的正确方法

Question

我有一个异步方法longRunningMethodOnObject:completion:

此方法接收“对象”类型的对象 - 使用其数据，然后调用完成处理程序。

我需要调用许多不同的“longRunningMethods”并等待全部完成。

我希望所有“longRunningMethodOnObject”在“for”循环中彼此异步（并行）运行。 （我不确定“longRunningMethodOnObject”是否彼此串行运行，但这是一个更普遍的问题）

我不确定我是否创建了正确的信号量，希望能解释一下在它们之间同步的正确方法。

包装函数的代码如下：

-(void)applyLongRunningOperationOnObjectInArray:(NSArray *)theObjects completion:(completion)block
{
// offsetting to worker thread in my object
  dispatch_async(self.myQueue,^{
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0); //Not sure if this should be here or in the for loop
    for(Object *ob in theObjects)
    {
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0); // the semaphore for the "for" loop - should this be here? or above the for loop
      [GlobalObject longRunningMethodOnObject:ob completion:^{ // would like each call to be independant of previous call in for loop
        dispatch_semaphore_signal(semaphore); // signaling that it completed
      }];
    }
    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER); // would like to wait here for each semaphore 
    block(); // invoke the completion handler
  });
}

Answer 1

我想你可以在这种情况下使用dispatch_semaphore，但是调度组可能会使应用程序逻辑更简单：

NSArray *theObjects = @[@"Apple",@"Orange",@"Peach"];
dispatch_group_t group = dispatch_group_create();
dispatch_queue_t _myQueue = dispatch_queue_create("com.cocoafactory.DispatchGroupExample",
                                                      0);
for( id ob in theObjects ) {
    dispatch_group_async(group, _myQueue, ^{
        //  do some long running task.
    });
}
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

//  now do whatever should be done after
//  all of your objects are processed
//  in your case block()

Concurrency Programming Guide 有点像这个话题。向下滚动到“等待队列任务组”

要回答有关任务是否在队列中同时执行的问题，这取决于。在上面的示例中，_myQueue 是一个串行队列。全局命名队列是并发的。从 iOS 5 开始，您还可以使用 DISPATCH_QUEUE_CONCURRENT 队列类型创建并发队列。