请帮我解决问题。我正在尝试下载文件，同时更新BTProgressHUD进度显示。我知道它不起作用的原因是使用主线程进行下载并且不允许我更新UI但我无法弄清楚如何正确使用线程池以允许我更新BTProgressHUD而文件正在下载。请帮忙!!
`
BTProgressHUD.Show("Downloading...", progress);
string this_file = "example.pdf";
string file_url = "http://our_server.com/files/" + this_file;
Uri url = new Uri(file_url);
var documents = Environment.GetFolderPath (Environment.SpecialFolder.MyDocuments);
var folder = Path.Combine (documents, "", "PDF");
System.Net.HttpWebRequest request = (System.Net.HttpWebRequest)System.Net.WebRequest.Create(url);
System.Net.HttpWebResponse response = (System.Net.HttpWebResponse)request.GetResponse();
response.Close();
Int64 iSize = response.ContentLength;
// keeps track of the total bytes downloaded so we can update the progress bar
Int64 iRunningByteTotal = 0;
// use the webclient object to download the file
using (System.Net.WebClient client = new System.Net.WebClient())
{
// open the file at the remote URL for reading
using (System.IO.Stream streamRemote = client.OpenRead(new Uri(file_url)))
{
// using the FileStream object, we can write the downloaded bytes to the file system
using (Stream streamLocal = new FileStream(folder + "/" + this_file, FileMode.Create, FileAccess.Write, FileShare.None))
{
// loop the stream and get the file into the byte buffer
int iByteSize = 0;
byte[] byteBuffer = new byte[iSize];
while ((iByteSize = streamRemote.Read(byteBuffer, 0, byteBuffer.Length)) > 0)
{
// write the bytes to the file system at the file path specified
streamLocal.Write(byteBuffer, 0, iByteSize);
iRunningByteTotal += iByteSize;
// calculate the progress out of a base "100"
double dIndex = (double)(iRunningByteTotal);
double dTotal = (double)byteBuffer.Length;
double dProgressPercentage = (dIndex / dTotal);
int iProgressPercentage = (int)(dProgressPercentage * 100);
if (iProgressPercentage == 100)
{
var z = new UIAlertView ("Download Complete", this_file + " downloaded.", null, "OK", null);
z.Show();
BTProgressHUD.Dismiss();
}
if (iProgressPercentage % 10 == 0)
{
// THIS BUT NEVER HAPPENS!!! Cannot update the progress display
progress += 0.1f;
BTProgressHUD.Show("XXX", progress);
}
} // while..
streamLocal.Close(); // clean up the file stream
} // using stream
streamRemote.Close(); // close the connection to the remote server
} // using I.O
} // using system.net
`
非常感谢任何帮助。
答案 0 :(得分：1)
我使用TPL来启动后台线程，然后使用InvokeOnMainThread回调到UI。我已将BTProgressHUD替换为UILabel，但它应该可以正常工作。这是工作：
private void DownloadCoffeePDF()
{
Task.Factory.StartNew (() => {
InvokeOnMainThread(() => {
this.TheLabel.Text = string.Format("Downloading...{0}", progress);
});
string file_url = "http://www.pnf.org/coffeeedited041001.pdf";
Uri url = new Uri(file_url);
var documents = Environment.GetFolderPath (Environment.SpecialFolder.MyDocuments);
var folder = Path.Combine (documents, "", "PDF");
System.Net.HttpWebRequest request = (System.Net.HttpWebRequest)System.Net.WebRequest.Create(url);
System.Net.HttpWebResponse response = (System.Net.HttpWebResponse)request.GetResponse();
response.Close();
Int64 iSize = response.ContentLength;
// keeps track of the total bytes downloaded so we can update the progress bar
Int64 iRunningByteTotal = 0;
// use the webclient object to download the file
using (System.Net.WebClient client = new System.Net.WebClient())
{
// open the file at the remote URL for reading
using (System.IO.Stream streamRemote = client.OpenRead(new Uri(file_url)))
{
// using the FileStream object, we can write the downloaded bytes to the file system
using (Stream streamLocal = new FileStream(folder + "/" + "Coffee.pdf", FileMode.Create, FileAccess.Write, FileShare.None))
{
// loop the stream and get the file into the byte buffer
int iByteSize = 0;
byte[] byteBuffer = new byte[iSize];
while ((iByteSize = streamRemote.Read(byteBuffer, 0, byteBuffer.Length)) > 0)
{
// write the bytes to the file system at the file path specified
streamLocal.Write(byteBuffer, 0, iByteSize);
iRunningByteTotal += iByteSize;
// calculate the progress out of a base "100"
double dIndex = (double)(iRunningByteTotal);
double dTotal = (double)byteBuffer.Length;
double dProgressPercentage = (dIndex / dTotal);
int iProgressPercentage = (int)(dProgressPercentage * 100);
if (iProgressPercentage == 100)
{
InvokeOnMainThread(() => {
var z = new UIAlertView ("Download Complete", "Coffee.pdf" + " downloaded.", null, "OK", null);
z.Show();
this.TheLabel.Text = "Download Complete";
});
}
if (iProgressPercentage % 10 == 0)
{
InvokeOnMainThread(() => {
// THIS BUT NEVER HAPPENS!!! Cannot update the progress display
progress += 0.1f;
this.TheLabel.Text = string.Format("{0}", progress);
});
}
} // while..
streamLocal.Close(); // clean up the file stream
} // using stream
streamRemote.Close(); // close the connection to the remote server
} // using I.O
} // using system.net
});
}