ios - 如何接收异步任务完成的通知

Question

我在 prepareForSegue 回调中有 NSOperationQueue 和 3 NSInvocationOperation ，我需要在完成所有异步任务后移动到另一个 View Controller 。
只有在移动到另一个屏幕之后，我才能收到关于完成所有异步任务的通知？

我试过这个没有成功:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

});

编辑:

似乎我不明白一些事情:

-(void)initPurchase{

    [operationQueue cancelAllOperations];

    NSInvocationOperation *downloadImageOperation = [[NSInvocationOperation alloc] initWithTarget:[BSImageDownloader getInstance] selector:@selector(downloadImageSync:) object:@"http://.........jpg"];


    NSInvocationOperation *createImageOperation = [[NSInvocationOperation alloc] initWithTarget:[BSImageCreator getInstance] selector:@selector(createImage:) object:@"dsadsadsa"];


    NSInvocationOperation *saveImageOperation = [[NSInvocationOperation alloc] initWithTarget:[BSImageSaver getInstance] selector:@selector(saveImageAsPng:) object:[BSSharedObject getInstance].createdImage];

    [createImageOperation addDependency:downloadImageOperation];
    [saveImageOperation addDependency:createImageOperation];


    [operationQueue addOperation:downloadImageOperation];
    [operationQueue addOperation:createImageOperation];
    [operationQueue addOperation:saveImageOperation];


}


-(void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender{

    [[BSPopupManager getInstance]showWaitingPopup];


    dispatch_group_t group = dispatch_group_create();

    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), 
                         ^{
                             Purchase *purchase = [[Purchase alloc] init];
                             [purchase initPurchase];
                         });

    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        [[BSPopupManager getInstance] closeWaitingPopup];
        BSPurchaseViewController *purchaseViewController = [segue destinationViewController];
        purchaseViewController.pngImage = [BSSharedObject getInstance].createdImage;


 NSLog(@"2");
    });

    dispatch_release(group);

}
}

我仍然得到 NSLog(@"2");早于我在第三个 NSInvocationOperation 中保存图像

Answer 1

您可以使用 dispatch_group_async()如果你打算使用 GCD 来执行你的异步任务，函数:Grand Central Dispatch (GCD) Reference

dispatch_group_t group = dispatch_group_create();

dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), 
^{
     //Code here is executed asynchronously
});

dispatch_group_notify(group, dispatch_get_main_queue(), 
^{
   //Do something when async has completed
   //Note: You are not required to use the main 
   //queue if you aren't performing any UI work.
});

dispatch_release(group);