c# - 等待异步操作完成

Question

我有一个关于异步方法如何在 c# 上工作的问题。我试图让 convertAddressToCoordinate 方法将 myLocation 变量设置为 GeoCoordinate。但是，甚至在设置 myLocation 值之前调用 compareDistance 方法。在调用 compareDistance() 之前，如何确保 myLocation 值不为空？

public GeoCoordinate myLocation = null;

public void returnClosestCurrent(string address)
 {

     convertAddressToCoordinate(address);
     compareDistance(myLocation);

 }

 public void convertAddressToCoordinate(string add)
 {
    WebClient wc = new WebClient();
     wc.DownloadStringCompleted += wc_DownloadStringCompleted;
     wc.DownloadStringAsync(new Uri("http://maps.googleapis.com/maps/api/geocode/json?address=1600+bay+st&sensor=false"));


 }

 void wc_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
 {

    XDocument xdoc = XDocument.Parse(e.Result, LoadOptions.None);

     var data = from query in xdoc.Descendants("location")
                 select new Location
                 {
                     lat = (string)query.Element("lat"),
                     lng = (string)query.Element("lng")
                 };

     GeoCoordinate destinationGeo = new GeoCoordinate(Convert.ToDouble(data.ElementAt(0).lat), Convert.ToDouble(data.ElementAt(0).lng));

     myLocation = destinationGeo;


 }

Answer 1

您可以使用 await 关键字来获取异步任务的值

http://msdn.microsoft.com/en-us/library/vstudio/hh156528.aspx

Answer 2

查看您的代码，在我看来，compareDistance(myLocation);应该只在方法结束时调用wc_DownloadStringCompleted。

更改后，一切都应该正常工作。

Answer 3

await你会想要wc.DownloadStringAsync

public async Task returnClosestCurrent(string address)
{
    await convertAddressToCoordinate(address)
              .ContinueWith(t => compareDistance(myLocation));
}

public async Task convertAddressToCoordinate(string add)
{
    WebClient wc = new WebClient();
    wc.DownloadStringCompleted += wc_DownloadStringCompleted;
    await wc.DownloadStringAsync(new Uri("http://maps.googleapis.com/maps/api/geocode/json?address=1600+bay+st&sensor=false"));
}

带有async/的模式await是代码倾向于将所有内容都变成async/await

它可以重构使用WebClient.DownloadStringTaskAsync

public async Task ReturnClosestCurrent(string address)
{
    await convertAddressToCoordinate(address)
            .ContinueWith(t => compareDistance(t.Result));
}

public async Task<GeoCoordinate> ConvertAddressToCoordinate(string add)
{
    WebClient wc = new WebClient();
    wc.DownloadStringCompleted += wc_DownloadStringCompleted;
    var content = await wc.DownloadStringTaskAsync(new Uri("http://maps.googleapis.com/maps/api/geocode/json?address=1600+bay+st&sensor=false"));

    return ParseContent(content);
}

private GeoCoordinate ParseContent(string content)
{
    XDocument xdoc = XDocument.Parse(content, LoadOptions.None);

    var data = from query in xdoc.Descendants("location")
             select new Location
             {
                 lat = (string)query.Element("lat"),
                 lng = (string)query.Element("lng")
             };

    GeoCoordinate destinationGeo = new GeoCoordinate(Convert.ToDouble(data.ElementAt(0).lat), Convert.ToDouble(data.ElementAt(0).lng));

    return destinationGeo;
}