Question

I subscribe to an event like this:

private void CoupleOverlay(IMapVisual mapVisual)
{            
      overlay.DrawTilesProgressChanged += (s, e) => 
      {               
          mapVisual.DrawingProgress = e.ProgressPercentage;
      };
}

The application may call CoupleOverlay event many times so I need to unsubscribe DrawTilesProgressChanged event and subscribe again but because it is an anonymous event I can’t unsubscribe from it. I tried to convert to

private void CoupleOverlay(IMapVisual mapVisual)
{
    overlay.DrawTilesProgressChanged -=overlay_DrawTilesProgressChanged;                 
    overlay.DrawTilesProgressChanged +=overlay_DrawTilesProgressChanged;                 
}

private void overlay_DrawTilesProgressChanged(object sender, DrawTilesProgressChangedTileOverlayEventArgs e)
{       
    mapVisual.DrawingProgress = e.ProgressPercentage;
}

It is not working because mapVisual is not valid variable on overlay_DrawTilesProgressChanged method.

If I change the code to

private void CoupleOverlay(IMapVisual mapVisual)
{
    EventHandler<DrawTilesProgressChangedTileOverlayEventArgs> drawEvent = (s, e) =>
    {           
        mapVisual.DrawingProgress = e.ProgressPercentage;
    };

    overlay.DrawTilesProgressChanged -= drawEvent;
    overlay.DrawTilesProgressChanged += drawEvent;
}

It will not work cause drawEvent is a local variable and next time I call CoupleOverlay, it will create new one.

So How can I unsubscribe from this event handler? Or how can I know I’m subscribed to the event so I don’t need to subscribe again?

Answer 1

it will not work cause drawEvent is local variable and next time I call CoupleOverlay, it will create new one.

Indeed. You’d basically need to keep an instance variable instead of a local variable. The instance variable would effectively be “the currently subscribed event handler”. Then in CoupleOverlay you’d remove that event handler, create a new one (storing it in the variable) and resubscribing with it.

An alternative would be to use a single event handler, but keep the “current” overlay and map visual as instance variables, used by the event handler. The CoupleOverlay method would then just need to update those variables.