Forms Android System Download

不用自己写下载文件的功能，直接调用系统下载服务
 forms 接口

using System;
using System.Collections;
using System.Collections.Generic;

namespace
{
    public interface ISystemDownloadFile
    {
        //下载文件
        void  DownloadThread (string strUrl,string str1);
    }
}

平台实现
using System;
using System.Collections;
using System.Collections.Generic;
using System.Threading.Tasks;
using System.Threading;

using Xamarin.Forms;

using Android.Net;
using Android.App;
using Android.Content;
using Android.Widget;
using Android.Runtime;
using Android.OS;
using Android.Telephony;
using Android.Util;
using Java.Net;
using Java.IO;
using;
using.TDroid;

[assembly:Dependency(typeof(SystemDownloadFile))]
namespace.TDroid
{
    public class SystemDownloadFile : Java.Lang.Object,ISystemDownloadFile
    {

        private DownloadManager downloadManager;
        private long downloadId = 0;
        private string filename;

        public SystemDownloadFile()
        {
            initData ();
        }
            
        public void DownloadThread(string strUrl,string str1)
        {
            getFilename (strUrl);

            string strFolder = Android.OS.Environment.ExternalStorageDirectory + "/Download/";
            File folder = new File (strFolder);
            if (!folder.Exists () || !folder.IsDirectory) {
                folder.Mkdirs ();
            }
            DownloadManager.Request request = new DownloadManager.Request (Android.Net.Uri.Parse (strUrl));
            request.SetDestinationInExternalPublicDir (strFolder, filename);
            request.SetTitle (filename);
            request.SetDescription (filename);
            request.SetNotificationVisibility (DownloadVisibility.VisibleNotifyCompleted);
            request.SetVisibleInDownloadsUi (false);
            downloadId = downloadManager.Enqueue (request);
        }

        /*
        public async Task SaveTextAsync (string filename, string text)
        {
            var docsPath = Environment.GetFolderPath(Environment.SpecialFolder.Personal);
            var path = Path.Combine(docsPath, filename);

            using (StreamWriter sw = File.CreateText(path))
            {
                await sw.WriteAsync(text);
            }
        }
        */

        private void getFilename(string strUrl)
        {
            filename = strUrl.Substring (strUrl.LastIndexOf("/")+1);

        }

        private void initData()
        {
            downloadManager = (DownloadManager)((Activity)Forms.Context).GetSystemService(Android.Content.Context.DownloadService);
        }
        /*
        public void DownloadThread(string strUrl,string str1)
        {
            string strUrl1 = null;
            string str11 = null;
            Thread thread = new Thread (delegate(){DownloadFile(strUrl,str1);});
            strUrl1 = strUrl;
            str11 = str1;
            thread.Start ();
        }*/

        /*
        public void DownloadFile (string strUrl,string str1)
        {
            try {
                Java.Net.URL url = new Java.Net.URL (strUrl);
                connection = url.OpenConnection ();
                if (connection.ConnectTimeout == 5) {
                    Log.Info ("---->", "网络有问题");
                }
                //inputStream = connection.InputStream;
                inputStream = (InputStream)connection.Content;
            } catch (Exception ex) {
                //Log.Info ("---->", ex.Data);
            }
            string filename = strUrl.Substring (strUrl.LastIndexOf ("/") + 1);
            string savePath = Android.OS.Environment.ExternalStorageDirectory + "/DownloadFile";
            File file1 = new File (savePath);
            if (!file1.Exists()) {
                file1.Mkdir ();
            }
            string savePathString =  Android.OS.Environment.ExternalStorageDirectory + "/DownloadFile/" + filename;
            File file = new File (savePathString);
            if (!file.Exists()) {
                try{
                    file.CreateNewFile();
                }catch(IOEaxception e){
                }
            }
            try{
                outputStream = new FileOutputStream(file);
                byte[] buffer = new byte[1024*4];
                FileLength = connection.ContentLength;
                while(DownedFileLength < FileLength){
                    outputStream.Write(buffer);
                    DownedFileLength += inputStream.Read(buffer);
                }
            }catch(FileNotFoundException e){
            }catch(IOException e1){
        }

        }*/
    }        
}