Raise custom event in multi-project

Question

it concerns following: I have two projects which should exists more or less independently from each other. Project one is a kind File System Watcher. The other one cosnists of my UI. The file watcher raises an event, if there is a new file. After that, the data from the file should be added to a database. That's coarsely the background story. The actual problem is that after the file watcher raised an event, I want to notify the UI to update the view of the data. That means, the event should be raised by the file watcher and the event should be registered in the implementation of the UI. The main problem ist now that I need instances of classes from both projects. Obviuosly this results in the circular dependency problem. Of course there is the solution of interfaces for the CP problem, but this won't solve the problem, that I need the same object for data creation and event registration. Hopefully you can help me with this problem. Thanks.

Answer 1

Why do you think you need an UI instance in the business logic assembly?

To register an event handler, you usually need only the instance from the calling assembly (observer, already contained in the calling assembly) and an instance of the referenced assembly (your assembly containing the filesystem watcher).

Then you have e.g. the following structure:

Assembly with logic

public class MyCustomWatcher
{   
    public event EventHandler Event;

    private void RaiseEventForWhateverReason()
    {
        if (Event != null)
        {
            Event(this, new Args());
        }
    }
   public Data GetData()
   {
    //return the data
   }
}

Assembly with UI: - both form and the controller types are declared here.

class Form : System.Windows.Forms.Form
{
 public void DisplayNotification(Data data)
 {
   //actual code here
 }
}

class Controller 
{
    private Form form;
    private MyCustomWatcher watcher;

    public void Init()
    {
      this.watcher = CreateWatcher();
      RegisterEvents();
      ShowForm();
    }
    void ShowForm()
    {
     //show
    }
    void RegisterEvents()
    {
        this.watcher.Event += HandleChange;
    }

    void HandleChange(object sender /*this will be the instance that raised the event*/, SomeEventArgs e)
    {
        //BTW: this.watcher == sender; //the same instance

        form.DisplayNotification(this.watcher.GetData());
    }
}

Assembly with UI references the assembly with logic. No circular dependency here.

Answer 2

I'm not sure I understand why the FileWatcher would have any dependency on you UI, but since you say it does you could add a third project to work as an event aggregator between the two. This would give both projects a dependency on the aggregator, but will remove the dependencies on each other.