等待CLGeocoder.GeocodeAddressAsync永远不会返回

Question

我正在尝试测试Apple的前向地理编码服务的准确性,以涵盖我们的数据模型上的纬度/经度缺失或无效的罕见情况,但我们仍然知道物理地址.MonoTouch的提供CLGeocoder.GeocodeAddress(String, CLGeocodeCompletionHandler)以及GeocodeAddressAsync(String)但是当我打电话给他们的completionHandler不会被调用和异步方法永远不会返回.

我的应用程序日志中没有任何内容表明存在问题,并且将调用封装在try-catch块中并未显示任何异常.在项目选项中启用了地图集成.没有捕获网络流量(无论如何可能是SSL)我没有想法.

这是加载地标并尝试地理编码地址的代码:

    private void ReloadPlacemarks()
    {
        // list to hold any placemarks which come back with empty/invalid coordinates
        List<ServiceCallWrapper> geoList = new List<ServiceCallWrapper> ();

        mapView.ClearPlacemarks ();

        List<MKPlacemark> placemarks = new List<MKPlacemark>();
        if (serviceCallViewModel.ActiveServiceCall != null) {
            var serviceCall = serviceCallViewModel.ActiveServiceCall;
            if (serviceCall.dblLatitude != 0 && serviceCall.dblLongitude != 0) {
                placemarks.Add (serviceCall.ToPlacemark ());
            } else {
                // add it to the geocode list
                geoList.Add (serviceCall);
            }
        }

        foreach (var serviceCall in serviceCallViewModel.ServiceCalls) {
            if (serviceCall.dblLatitude != 0 && serviceCall.dblLongitude != 0) {
                placemarks.Add (serviceCall.ToPlacemark ());
            } else {
                //add it to the geocode list
                geoList.Add (serviceCall);
            }
        }

        if (placemarks.Count > 0) {
            mapView.AddPlacemarks (placemarks.ToArray ());
        }

        if (geoList.Count > 0) {

            // attempt to forward-geocode the street address
            foreach (ServiceCallWrapper s in geoList) {
                ServiceCallWrapper serviceCall = GeocodeServiceCallAddressAsync (s).Result;
                mapView.AddPlacemark (serviceCall.ToPlacemark());
            }
        }

    }

    private async Task<ServiceCallWrapper> GeocodeServiceCallAddressAsync(ServiceCallWrapper s)
    {
        CLGeocoder geo = new CLGeocoder ();
        String addr = s.address + " " + s.city + " " + s.state + " " + s.zip;
        Console.WriteLine ("Attempting forward geocode for service call UID: " + s.call_uid + " with address: " + addr);

                    //app hangs on this
        CLPlacemark[] result = await geo.GeocodeAddressAsync(addr);

                    //code updating latitude and longitude (omitted)

        return s;
    }

Answer 1

你的问题是这一行:

ServiceCallWrapper serviceCall = GeocodeServiceCallAddressAsync (s).Result;

通过调用Task<T>.Result,您正在导致死锁.我在我的博客上完整地解释了这一点,但它的要点是await(默认情况下)它会在产生控制时捕获"上下文",并将使用该上下文来完成该async方法.在这种情况下,"上下文"是UI上下文.因此,Result当响应进入时,UI线程被阻塞(等待),并且该async方法无法继续,因为它正在等待在UI线程上运行.

解决方案是一直使用async.换句话说,每更换Task<T>.Result和Task.Wait使用await:

private async Task ReloadPlacemarksAsync()
{
  ...
  ServiceCallWrapper serviceCall = await GeocodeServiceCallAddressAsync (s);
  ...
}

请注意,void ReloadPlacemarks现在是Task ReloadPlacemarksAsync,所以此更改会影响您的来电者.async将通过代码库"增长",这是正常的.有关详细信息,请参阅有关异步最佳实践的MSDN文章.