Question

我正在使用Xamarin学习C＃和Android编程。在一些教程之后，我做了一个简单的程序，通过Async进程从URL下载图像。我想改进我的项目添加一个可以删除该过程的停止按钮。我已按照本指南https://msdn.microsoft.com/enus/library/jj155759(v=vs.110).aspx 我添加了一个令牌，但问题出在我的代码的第90行。我的方法“GetByteArrayAsync（”“）不能使用多个参数。

你有什么建议吗？

using Android.App;
using Android.Widget;
using Android.OS;
using System.IO;
using Android.Graphics;
using System.Threading.Tasks;
using System.Net.Http;
using System;
using System.Threading;
namespace App4
{
[Activity(Label = "App4", MainLauncher = true, Icon = "@drawable/icon")]
public class MainActivity : Activity
{
    //Declare a Cancellation Token Source 
    CancellationTokenSource cts;

    Button startbutton;
    Button stopbutton;
    TextView textView;
    TextView textView1;
    ImageView image;

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

        startbutton = FindViewById<Button>
            (Resource.Id.startbutton);
        stopbutton = FindViewById<Button>
            (Resource.Id.stopbutton);
        textView = FindViewById<TextView>
            (Resource.Id.textView);
        image = FindViewById<ImageView>
            (Resource.Id.image);
        textView1 = FindViewById<TextView>
            (Resource.Id.textView1);
        image.SetImageResource(Resource.Drawable.Icon);


        //BASIC ASYNC START TASK
        // click the startbutton to start the process
        startbutton.Click += async (sender, e) =>
        {
            textView1.Text += " Loading...";

            // Instantiate the CancellationTokenSource
            cts = new CancellationTokenSource();

            try
            {                
                // **** GET ****
                await DownloadImageAsync(cts.Token);
            }
            catch (System.OperationCanceledException)
            {
                textView1.Text += " Download deleted ";
            }
            catch (Exception)
            {
                textView1.Text += "Generic Error";
            }

            // ***Set the CancellationTokenSource to null when the download is complete.
            cts = null;
        };

        //STOP BUTTON 
        stopbutton.Click += (sender,e) =>
        {
            if (cts != null)
            {
                cts.Cancel();
            }
        };
    }

    //ASYNC METHOD called by startbutton
    public async Task DownloadImageAsync(CancellationToken ct)
    {
        var httpClient = new HttpClient();

        // You might need to slow things down to have a chance to cancel.
        await Task.Delay(500);

        // byte[] imageBytes = await httpClient.GetByteArrayAsync("https://xamarin.com/content/images/pages/about/team-h.jpg",ct);
        byte[] imageBytes = await httpClient.GetByteArrayAsync("https://pbs.twimg.com/profile_images/604644048/sign051.gif",ct);

        //// THIS CODE STORE THE IMAGE IN THE LOCAL MEMORY OF THE APP////
        string documentsPath = System.Environment.GetFolderPath(System.Environment.SpecialFolder.Personal);
        string localFilename = "image.jpg";
        string localPath = System.IO.Path.Combine(documentsPath, localFilename);
        File.WriteAllBytes(localPath, imageBytes);
        textView1.Text = " Image downloaded.";

        var localImage = new Java.IO.File(localPath);
        if (localImage.Exists())
        {
            var teamBitmap = BitmapFactory.DecodeFile(localImage.AbsolutePath);
            image.SetImageBitmap(teamBitmap);
        }
        ///////////////////////////////////////////////////////////////////////////
    }
}
}

删除C＃中的异步任务

0 个答案: