Question

我最近开始使用Xamarin而且我对c＃很新。我想创建一个异步HTTP请求，所以我开始使用async和await与HttpWebRequest。但我没有得到异步调用，UI正在被阻止。这是我测试的代码。我有两个实现，RequestCitas（）应该是异步的，SRequestCitas（）是同步的，有什么问题？。

namespace mc
{
[Activity (Label = "mc", MainLauncher = true, Theme="@android:style/Theme.Holo.Light")]
public class MainActivity : Activity
{

    List<Cita> citas;
    CitaAdapter adapter;

    protected override void OnCreate (Bundle savedInstanceState)
    {
        base.OnCreate (savedInstanceState);
        SetContentView (Resource.Layout.Main);


        citas = new List<Cita> ();
        adapter = new CitaAdapter (this, citas);
        ListView listView = FindViewById<ListView> (Resource.Id.main_citas);
        listView.Adapter = adapter;

        var c1 = new Cita (0, "ernesto", "De los Santos", "19/2/14", "19/2/14", "09:28", "10:45");
        var c2 = new Cita (1, "ernesto", "De los Santos", "19/2/14", "19/2/14", "09:28", "10:45");
        var c3 = new Cita (2, "ernesto", "De los Santos", "19/2/14", "19/2/14", "09:28", "10:45");
        var c4 = new Cita (3, "ernesto", "De los Santos", "19/2/14", "19/2/14", "09:28", "10:45");
        var c5 = new Cita (4, "ernesto", "De los Santos", "19/2/14", "19/2/14", "09:28", "10:45");

        citas.Add (c1);
        citas.Add (c2);
        citas.Add (c3);
        citas.Add (c4);
        citas.Add (c5);

        Console.WriteLine ("Notificando al adaptador");
        adapter.NotifyDataSetChanged ();

        string result;

        FindViewById<Button> (Resource.Id.button1).Click += async (sender, e) => {
            result =  await RequestCitas ();
        };
        FindViewById<Button> (Resource.Id.button2).Click += delegate {
            result = SRequestCitas ();
        };
    }

    private async Task<string>  RequestCitas ()
    {

        // var url = "http://192.168.1.126:8081";
        var url = "http://192.168.1.124";
        //var method = "/citas/get";
        var method = "";

        var encoding = new UTF8Encoding ();
        byte[] parametros = encoding.GetBytes ("usersID=97");

        var request = (HttpWebRequest)WebRequest.Create (url + method);
        request.Method = "POST";
        request.ContentType = "application/x-www-form-urlencoded";
        request.ContentLength = parametros.Length;

        var dataStream = request.GetRequestStream ();
        dataStream.Write (parametros, 0, parametros.Length);
        dataStream.Close ();

        string str = ""; 

        Task<WebResponse> task = request.GetResponseAsync ();

        WebResponse newResponse = await task;

        var responseStream = newResponse.GetResponseStream ();
        var streamReader = new StreamReader (responseStream, System.Text.Encoding.GetEncoding ("utf-8"));
        char[] read = new Char[256];
        int count = streamReader.Read (read, 0, 256);

        while (count > 0) {
            str += new string (read, 0, count);
            count = streamReader.Read (read, 0, 256);
        }
        responseStream.Close ();

        return str;
    }


    private string SRequestCitas ()
    {

        var url = "http://192.168.1.126:8081";
        var method = "/citas/get";

        var encoding = new UTF8Encoding ();
        byte[] parametros = encoding.GetBytes ("usersID=97");

        var request = (HttpWebRequest)WebRequest.Create (url + method);
        request.Method = "POST";
        request.ContentType = "application/x-www-form-urlencoded";
        request.ContentLength = parametros.Length;

        var dataStream = request.GetRequestStream ();
        dataStream.Write (parametros, 0, parametros.Length);
        dataStream.Close ();

        string str = ""; 

        HttpWebResponse newResponse = (HttpWebResponse)request.GetResponse ();

        var responseStream = newResponse.GetResponseStream ();
        var streamReader = new StreamReader (responseStream, System.Text.Encoding.GetEncoding ("utf-8"));
        char[] read = new Char[256];
        int count = streamReader.Read (read, 0, 256);

        while (count > 0) {
            str += new string (read, 0, count);
            count = streamReader.Read (read, 0, 256);
        }
        responseStream.Close ();

        return str;
    }

}
}

Answer 1

您正在使用async方法获得响应，但您正在使用同步流操作读取流。请尝试使用streamReader.ReadAsync。

或者，如果您感到雄心勃勃，可以尝试完全替换整个while循环并使用：

var str = await streamReader.ReadToEndAsync();

在旁注上，StreamReader实施了IDisposable，并且当您完成该实例时，您并没有正确处理该实例。你可能会泄漏原生资源。

Answer 2

当您创建使用await关键字的方法时，必须将我标记为async，以便编译器允许使用关键字。此外，您应该遵循指南并使用异步前缀

结束您的方法

您的async方法正在从导致您的UI阻止的StreamReader中同步读取。

这是使用HttpClient的异步实现：

private async Task<string> RequestCitasAsync()
{
    var url = "http://192.168.1.126:8081/citas/get";
    byte[] parametros = Encoding.UTF8.GetBytes("usersID=97");

    var httpClient = new HttpClient();
    var response = await httpClient.PostAsync(url, new ByteArrayContent(parametros));

    return await response.Content.ReadAsStringAsync();
}

使用async等待C＃Xamarin

2 个答案: