How to return result to UI thread in Windows Phone? - windows-phone-7

Question

Does anyone know great idea How to return result to UI thread ?
I wrote this code, but It will be compile error because it can't return "img" in async.
public byte[] DownloadAsync2(Uri address)
{
byte[] img;
byte[] buffer = new byte[4096];
var wc = new WebClient();
wc.OpenReadCompleted += ((sender, e) =>
{
using (MemoryStream memoryStream = new MemoryStream())
{
int count = 0;
do
{
count = e.Result.Read(buffer, 0, buffer.Length);
memoryStream.Write(buffer, 0, count);
} while (count != 0);
Deployment.Current.Dispatcher.BeginInvoke(() =>
{
if (e.Error == null) img = memoryStream.ToArray();
});
}
}
);
wc.OpenReadAsync(address);
return img; //error : Use of unassigned local variable 'img'
}

Answer 1

Change your method to:
public void DownloadAsync2(Uri address, Action<byte[]> callback, Action<Exception> exception)
{
var wc = new WebClient();
wc.OpenReadCompleted += ((sender, e) =>
{
using (MemoryStream memoryStream = new MemoryStream())
{
int count = 0;
do
{
count = e.Result.Read(buffer, 0, buffer.Length);
memoryStream.Write(buffer, 0, count);
} while (count != 0);
Deployment.Current.Dispatcher.BeginInvoke(() =>
{
if (e.Error == null) callback(memoryStream.ToArray());
else exception(e.Error);
});
}
}
);
wc.OpenReadAsync(address);
}
Usage:
DownloadAsync2(SomeUri, (img) =>
{
// this line will be executed when image is downloaded,
// img - returned byte array
},
(exception) =>
{
// handle exception here
});
Or (old-style code without lambda expressions):
DownloadAsync2(SomeUri, LoadCompleted, LoadFailed);
// And define two methods for handling completed and failed events
private void LoadCompleted(byte[] img)
{
// this line will be executed when image is downloaded,
// img - returned byte array
}
private void LoadFailed(Exception exception)
{
// handle exception here
}