Updating progress log message from Model to ViewModel in MVVM - wpf

Question

I have a method defined in the Model that would execute a long running script where I want to capture the output message when the script is in progress and output to the View via the ViewModel. I understand in order to get realtime update of the output message I should run the Model method in a backgroundworker and raise its ReportProgress event when it has output message to report in order to run the UI update and the script on two separate threads. The problem I have is the backgroundworker object is defined in the ViewModel, so using it to call the Model method is straight forward, but how do I raise the ReportProgress event from the Model method? The only way I can think of is passing in the backgroundworker as input parameter into the method but I feel uneasy about this. Can anyone tell me if this is the right approach in implementing the MVVM framework?
Here are my code stripped to the most bare bone. In my View xaml I have a TextBox bind to the Logger property and DeployCommand command in my ViewModel:
<TextBox Grid.Row="1 " Name="txtOutput" MinHeight="40"
Text="{Binding Logger}"
IsReadOnly="True" Margin="10,10" VerticalScrollBarVisibility="Auto"
IsEnabled="True" MaxLines="2000" TextWrapping="WrapWithOverflow"/>
<Button x:Name="BtnDeploy"
Command="{Binding DeployCommand}"
Content="Deploy"
Height="23"
Margin="5,2"
HorizontalAlignment="Right"
Width="125"
FontFamily="Kalinga"
AutomationProperties.AutomationId="DeployButton"/>
In my ViewModel, the DeployCommand command will trigger the method OnDeploy which in turn will call the Deploy method in Model using the backgroundworker object:
private string logger = string.Empty;
public string Logger
{
get { return logger; }
set
{
logger = value;
RaisePropertyChanged("Logger");
}
}
public ICommand DeployCommand { get; private set; }
public MainWindowViewModel()
{
_worker = new BackgroundWorker()
{
WorkerReportsProgress = true,
WorkerSupportsCancellation = true
};
_worker.DoWork += worker_DoWork;
// _worker.RunWorkerCompleted += worker_RunWorkerCompleted;
_worker.ProgressChanged += worker_ProgressChanged;
DeployController = new DeploymentModel();
this.DeployCommand = new DelegateCommand<object>(this.OnDeploy);
}
private void OnDeploy(object obj)
{
Logger += #"Offline Deployment Started" + System.Environment.NewLine;
if (!_worker.IsBusy)
{
_worker.RunWorkerAsync(DeployController);
}
}
private void worker_DoWork(object sender, DoWorkEventArgs e)
{
var worker = (BackgroundWorker)sender;
var deployModel = (DeploymentModel)e.Argument;
deployModel.Deploy(script);
}
private void worker_ProgressChanged(object sender, ProgressChangedEventArgs e)
{
Logger += e.UserState.ToString();
}
Finally in the Model:
public bool Deploy(string ScriptFile)
{
bool Success = true;
string strCmdText = string.Format(#"/c ""{0}""", ScriptFile);
try
{
var startInfo = new ProcessStartInfo
{
WindowStyle = ProcessWindowStyle.Hidden,
WorkingDirectory = kitFolder,
UseShellExecute = false,
RedirectStandardError = true,
RedirectStandardOutput = true,
FileName = "cmd.exe",
CreateNoWindow = true,
Arguments = strCmdText,
};
// Launch shell command to run powersheel script
using (Process myProcess = Process.Start(startInfo))
{
// capturing script output message
myProcess.OutputDataReceived += (s, e) =>
{
LogMessage("ExecuteDeploymentKit: " + e.Data);
};
myProcess.ErrorDataReceived += (s, e) =>
{
Success = false;
LogMessage("ExecuteDeploymentKit: ! > " + e.Data);
};
myProcess.BeginErrorReadLine();
myProcess.BeginOutputReadLine();
System.Threading.Thread.Sleep(5000);
myProcess.WaitForExit();
}
}
catch (Exception ex)
{
LogMessage("ExecuteDeploymentKit: " + ex.Message);
return false;
}
if (Success)
{
LogMessage("ExecuteDeploymentKit: Offline Deployment Kit executed successfully");
}
else
{
LogMessage("ExecuteDeploymentKit: Offline Deployment Kit failed");
}
return Success;
}
I have added workder_ProgressChanged to handle the ProgressChanged event of the backgroundworker in order to update the View in the UI thread but without the backgroundworker object in my Model, I can't raise the ProgressChanged event from the method Deploy()
Thanks

Answer 1

The standard way would be for your VM to implement the IProgress interface and pass your M the VM cast as an IProgress object. You shouldn't pass it the VM since that could be a reference nightmare.
But really, the background worker should be implemented in the VM, not the M. And you shouldn't use BackgroundWorker anymore and move onto the new async methods.

Answer 2

If I understand your question right, you might be breaking core principles of MVVM by letting the Model drive your viewmodel and view. Without really having much to go off of, I would suspect that the best approach to this would be to actually create a "service".
Keep your model dumb and let it only contain data. Think POCO. Then, utilize a service that implements a background worker. Have your View Model run the service. The View model can call the service and provide that service a reference to your instantiated model. This way, you aren't heavily coupling your model to your view model.