如何跨线程跟踪 NSError 对象？

Question

我有一组使用NSInvocationOperation 产生的异步调用：

- (void)listRequestQueue:(StoreDataListRequest *)request {
    [openListRequests addObject:request];

    NSInvocationOperation *requestOp = [[NSInvocationOperation alloc]
                                        initWithTarget:self
                                        selector:@selector(listRequestStart:)
                                        object:request];
    [opQueue addOperation:requestOp];
    [requestOp release];
}

- (void)listRequestStart:(StoreDataListRequest *)request {
    if(self.resourceData == nil) {
        //TODO fail appropriately...
        return;
    }

    StoreDataListResponse *response = [self newListResponseForProductID:request.productID];

    [self performSelectorOnMainThread:@selector(listRequestFinish:)
                           withObject:response waitUntilDone:NO];
    [response release];

    [self performSelectorOnMainThread:@selector(cleanUpListRequest:)
                           withObject:request waitUntilDone:NO];
}

- (void)listRequestFinish:(StoreDataListResponse *)response {
    [self.delegate storeData:self didReceiveListResponse:response];
}

- (StoreDataListResponse *)newListResponseForProductID:(NSString *)productID {
    CollectionData *data = [self.resourceData dataForProduct:productID];
    if(data == nil) {
        //TODO do something
    }

    StoreDataListResponse *response = [[StoreDataListResponse alloc] init];
    response.productID = productID;
    if(productID != data.productID) {
        //TODO fail; remove product from list
    }
    response.name = NSLocalizedString(@"Loading...", @"Loading message");
    response.blurb = NSLocalizedString(@"Waiting for response from server", @"Waiting for website to respond");
    return response;
}

对于上述代码中的每个TODOs，我应该解决问题并让任何处理程序知道事情已经失败以及原因。查看NSError 类和文档，它似乎是合适的答案，但我无法弄清楚如何使用NSInvocationOperation 和performSelectorOnMainThread:withObject:waitUntilDone:。我当然可以通过将NSErrors 从newListResponseForProductID: 方法中更改为以下内容：

- (StoreDataListResponse *)newListResponseForProductID:(NSString *)productID error:(NSError **)error;

如何将生成的错误返回到主线程，以便处理失败的请求？

Answer 1

在主线程上运行任何代码的最简单方法是在 iOS 4 及更高版本上使用 GCD 和块。像这样：

dispatch_async(dispatch_get_main_queue(), ^(void) {
    // Your code to run on the main thread here, 
    // any variables in scope like NSError is captured.
});

如果您知道自己在后台线程上并希望在继续之前在主线程上完成块，请使用dispatch_sync()。