Question

我有一个列表，我需要加载产品名称，如：

List<string> prodNames= new List<string>();

我的系统上不存在产品数据。所以，我必须先下载它们。

将产品数据逐个下载到本地文件系统。下载产品会触发我处理的事件处理程序，处理程序会提供我需要添加到列表中的产品名称。

我知道有多少产品可以随时使用，比如5。

我想知道如何知道所有5个产品名称何时添加到我的列表中，以便我可以继续显示它们。这是伪代码

List ids = // list of product ids 
List<string> prodNames= new List<string>();  //list to fill with prod names once they are downloaded

this.EventHandler += OnDownloadedProduct; //event fired once each product is downloaded

void GetProducts()
{
  foreach (var id in ids)
  {
     // NOTE that I cannot modify neither downloader object nor GetProduct() 
     // method.  These are not my code but come from a component for which
     // I have no code access.
     downloader.GetProduct(id);  //fires OnDownloadedProduct once product is downloaded
  } 
}

//this event is fired once each product data is downloaded.
// ProductDataArgs is provided to me and I cannot modify it.
void OnDownloadedProduct(object sender, ProductDataArgs e)
{
   // this is where I get product name and add it to the list
   string prodName = e.getProdName();
   prodNames.Add(prodName);
   // <-- I could show prodName here in a message box but that
   // would show one message box for all 5 products.  I don't
   // want that.  I want to show all 5 product names in one
   // message box.
}

我需要在表单或消息框中显示所有5个产品名称。我可以在上面的活动中，但每个产品。这将显示5个消息框（或表单）。但我需要在一个消息框中显示所有5个。所以，我需要在调用者中这样的事情：

List<string> prodNames = await.GetProducts(ids);
var names = null;
foreach(var prod in prodNames)
{
  names += string.Format("{0}\n");
}
MesageBox.Show(names);

问题：我遇到的问题是操作可能需要一些时间，而且我不知道列表何时加载了所有5个产品名称（因为名称在GetProducts方法中不可用，但在OnDownloadedProduct中事件）。

经过我自己的一些研究后，Tasks / async / await似乎为此提供了解决方案，但到目前为止我看到的所有示例都没有解决我上面描述的情况，我依赖于事件处理程序来获取我需要的东西（在这种情况下，所有5个产品名称。）

更新感谢下面的@Kris评论，我已经能够将我的逻辑重写为此，但它无法正常工作：

public class ProductRetriver
{
    List<int> ids = new List<int>();  // product ids
    List<string> prodNames = new List<string>();  // product names

    private TaskCompletionSource<List<string>> prodsTask;

    this.EventHandler += OnDownloadedProduct; 

    Task<List<string>> async GetProducts(List<int> _ids)
    {
        ids = _ids;
        prodsTask = new TaskCompletionSource<List<string>>();
        foreach (var id in ids)
        {
            //PROBLEM: GetProduct() is not awaited, so method will finish
            //before list is filled.  Also, it seem to add duplicates for
            //some reason to the list.  I cannot modify the method as it is
            //not code available to me.
            downloader.GetProduct(id);  //fires OnDownloadedProduct 
        }
        return prodNames;
    }

    //this event is fired once each product data is downloaded
    void OnDownloadedProduct(object sender, ProductDataArgs e)
    {
        // this is where I get product name and add it to the list
        string prodName = e.getProdName();
        prodNames.Add(prodName);
        if (prodNames.Count == ids.Count)
        {
            prodsTask.SetResult(prodNames);
        }
    }
}

Answer 1

我不知道这个下载器是什么以及它来自哪里，如果eventhandler在外侧下载器试试这个

 public class ProductRetriver
{

    List<int> ids = new List<int>();
    List<string> prodNames = new List<string>();  //list to fill with prod names once they are downloaded
    private TaskCompletionSource<List<string>> prodsTask;
        this.EventHandler += OnDownloadedProduct; //event fired once each product is downloaded

        Task<List<string>> async GetProducts(List<int> _ids)
    {
        ids = _ids;
        prodsTask = new TaskCompletionSource<List<string>>();
        foreach (var id in ids)
        {
            downloader.GetProduct(id);  //fires OnDownloadedProduct once product is downloaded
        }

    }

    //this event is fired once each product data is downloaded
    void OnDownloadedProduct(object sender, ProductDataArgs e)
    {
        // this is where I get product name and add it to the list
        string prodName = e.getProdName();
        prodNames.Add(prodName);
        if (prodNames.Count == ids.Count)
        {
            prodsTask.SetResult(prodNames);
        }
    }
}

如果eventhandler是下载程序的一部分，则可以执行此操作

Task<List<string>> async GetProducts(List<int> ids)
    {
        List<string> prodNames = new List<string>();

        downloder.EventHandler += (s, e) => {

            string prodName = e.getProdName();
            prodNames.Add(prodName);
            if (prodNames.Count == ids.Count)
            {
                prodsTask.SetResult(prodNames);
            }
        };
        foreach (var id in ids)
        {
            downloader.GetProduct(id);  //fires OnDownloadedProduct once product is downloaded
        }

        return prodNames;

    }

你可以这样称呼

Products = await getproducts(ids);

Answer 2

如果我理解，你不需要收到downloader.GetProduct（id）的结果吗？如果是这样 - 你想要等待什么？只需创建任务列表并等待它们全部完成如下：

public class ProductRetriver
{
List<int> ids = new List<int>();  // product ids
List<string> prodNames = new List<string>();  // product names

private TaskCompletionSource<List<string>> prodsTask;

this.EventHandler += OnDownloadedProduct; 
public static List<Task> TaskList = new List<Task>();
Task<List<string>> async GetProducts(List<int> _ids)
{
    ids = _ids;
    prodsTask = new TaskCompletionSource<List<string>>();

    foreach (var id in ids)
    {

       TaskList.Add(downloader.GetProduct(id));
    }
    Task.WaitAll(TaskList.ToArray());
    return prodNames;
}

//this event is fired once each product data is downloaded
void OnDownloadedProduct(object sender, ProductDataArgs e)
{
    // this is where I get product name and add it to the list
    string prodName = e.getProdName();
    prodNames.Add(prodName);
    if (prodNames.Count == ids.Count)
    {
        prodsTask.SetResult(prodNames);
    }
}

}

使用async / await获取信息

2 个答案: