Backing Up the Local Database
,To transfer the local database to the server, it must first be copied to a directory in isolated storage. This prevents the file from being modified by the SQL CE engine while the transfer is under way. The viewmodel’s BackupDatabase method creates a temporary directory and then copies the local .sdf database file to the directory, as shown:
string uploadUrl = "http://localhost:60182/BackupService/UploadFile/";
const string localDatabaseName = "Todo.sdf";
const string transferDirectory = "/shared/transfers";
string uploadPath = transferDirectory + "/" + localDatabaseName;
using (IsolatedStorageFile isolatedStorageFile
= IsolatedStorageFile.GetUserStoreForApplication())
{
if (!isolatedStorageFile.FileExists(localDatabaseName))
{
throw new InvalidOperationException(
"Database file does not exist in isolated storage.");
}
if (!isolatedStorageFile.DirectoryExists(transferDirectory))
{
isolatedStorageFile.CreateDirectory(transferDirectory);
}
isolatedStorageFile.CopyFile(localDatabaseName, uploadPath, true);
}
The BackupDatabase method then constructs a destination URL for the upload. The remote URL is constructed using the base URL of the upload file path on the server. This URL is rerouted when it arrives at the server, and its segments are passed as arguments to the SaveFile WCF service method. See the following excerpt:
string deviceId = GetUserId();
string remoteUrl = string.Format("{0}{1}/{2}",
uploadUrl,
deviceId,
localDatabaseName);
Uri remoteUri = new Uri(remoteUrl, UriKind.Absolute);
A BackgroundTransferRequest is constructed, which causes the file to be uploaded to the server. Uploads use the HTTP POST method, as shown:
BackgroundTransferRequest request
= new BackgroundTransferRequest(remoteUri)
{
TransferPreferences = TransferPreferences.AllowBattery,
Method = "POST",
UploadLocation = new Uri(uploadPath, UriKind.Relative)
};
To monitor the progress of the transfer request, while the app is running in the foreground, we subscribe to the TransferStatusChanged and the TransferProgressChanged events. The transfer request is then added to the BackgroundTransferService, which queues the upload. See the following:
request.TransferStatusChanged += HandleUploadTransferStatusChanged;
request.TransferProgressChanged += HandleUploadTransferProgressChanged;
BackgroundTransferService.Add(request);
Message = "Backing up data to cloud.";
ProgressVisible = true;
When the viewmodel’s ProgressVisible property is set to true, it causes a progress indicator to be displayed in the view. This occurs via a custom ProgressIndicatorProxy class, first discussed in Chapter 5, “Using Content Controls, Items Controls, and Range Controls.” In addition, the progress indicator also displays the progress of the operation via the viewmodel’s Progress property. The Progress property is updated whenever the TransferProgressChanged event is raised, as shown:
void HandleTransferProgressChanged(
object sender, BackgroundTransferEventArgs e)
{
if (e.Request.BytesSent > 0)
{
Progress = (double)e.Request.TotalBytesToSend / e.Request.BytesSent;
}
else
{
Progress = 0;
}
}
When the request’s TransferStatusChanged event is raised, if the request has completed, it is removed from the BackgroundTransferStatus, and the progress indicator is hidden.
Note
A TransferStatus of Completed does not necessarily mean that the transfer completed successfully, but rather that the operation has ended for whatever reason. It is therefore critical to test for the presence of an error contained in the Request.TransferError property.
The ViewModelBase class’s MessageService is used to display the result to the user, as shown:
void HandleUploadTransferStatusChanged(
object sender, BackgroundTransferEventArgs e)
{
if (e.Request.TransferStatus == TransferStatus.Completed)
{
BackgroundTransferService.Remove(e.Request);
ProgressVisible = false;
if (e.Request.TransferError != null)
{
MessageService.ShowError("An error occured during backup.");
}
else
{
MessageService.ShowMessage("Backup successful.");
}
}
}
After the local database file has been transferred to the server, the user can nominate to restore the database from the backup.