Am I misunderstanding events in C# WPF?

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I am working on an application in WPF using a printer with an SDK that says it has a StatusUpdate event. So far I can't get the status to come back even when I perform an action that I know should trigger one.

I have used the code in a windows forms application and it works like it should.
I suspect there is an issue with coding it in WPF using an MVVM design. I have tried putting the event handler code into the MainView code behind, then the MainViewModel. I created a separate clss to see if I could get it to fire from there too.

Does anyone have any suggestions on what I could do to the events to trigger in WPF?

Or am I missing something else about Events?

What I have tried:

I have tried putting the event handler code into the MainView code behind, then the MainViewModel. I created a separate clss to see if I could get it to fire from there too.

Posted 31-Aug-16 10:56am

FlywheelJack

Updated 1-Sep-16 3:41am

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Foothill 31-Aug-16 17:17pm

Can you specify which type of event you are trying to trigger on? WPF has two different types of events that can be loosely referred to Top Down (uses EventArgs) and Bottom Up (uses RoutedEventArgs). Top down events, like MouseDown, are raised from the view layer but bottom up events are raised from the Dispacher layer (Windows Message Pump), like PreviewMouseDown.

FlywheelJack 31-Aug-16 17:46pm

I couldn't begin to guess, but I can try :D
The application is using Microsoft.PointOfService to get the printer information etc. and implements its own StatusUpdateEventHandler to handle the events. This almost sounds like it would be the Bottom Up scenario since it is monitoring for status events, not necessarily triggered by the user in the UI.

Foothill 31-Aug-16 17:58pm

Took a minute to find but your Status event is raised by the application layer. Are you registering a delegate for the event? e.g. OnStatusChanged += HandleStatusChange; where the the handler's signature is HandleStatusChange(object sender, StatusUpdateEventArgs e){ // do something }

FlywheelJack 31-Aug-16 18:10pm

I tried different ways throughout the day;
mostly they look like this,

protected void AddStatusUpdateEvent(object eventSource)
{
EventInfo statusUpdateEvent = eventSource.GetType().GetEvent("StatusUpdateEvent");
if (statusUpdateEvent != null)
{
statusUpdateEvent.AddEventHandler(eventSource, new StatusUpdateEventHandler(OnStatusUpdateEvent));
}
}

FlywheelJack 31-Aug-16 18:11pm

Will have to leave now, Ill try this again tomorrow, thank you for your assistance.

Foothill 1-Sep-16 8:59am

Quick question, are you trying to respond to events coming from the printer?

FlywheelJack 1-Sep-16 9:23am

The status updates are coming from the printer but through an object created by Microsoft's Point of Service library. So a service object is created which then monitors and reports all things to do with the printer (a point of service printer is one that can print receipts, process checks etc.)

Philippe Mori 31-Aug-16 20:29pm

I never see code like that. What does that code should do?

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Foothill · Answer 1 · 2016-09-01T03:41:00

To listen for events from the printer, you need to work within the Event system.
Given my unfamiliarity to this part of .Net, I can only get you within the ballpark.

C#

using Microsoft.PointOfService;

namespace EventHandlers
{
  public class PrinterEventHandler
  {
    private PosPrinter _printer;

    public PrinterEventHandler(PosPrinter printer)
    {
      _printer = printer;
      _printer.OnStatusUpdate += HandleStatusUpdate;  // Add a listener function.  OnStatusUpdate should be a function that when called which raises that status updated event
    }

    public void HandleStatusUpdate(object sender, StatusUpdateEventArgs e)
    {
      // Respond to event...
    }
  }
}