mvvm - Can execute question using delegate commands in prism -

this seems dumb question have looked through docs prism , searched internet , can't find example... here deal.

i using delegatecommand in prism, working fine except when assign delegate can execute canexecute method. in view model have event takes bool publishing , can see event firing , bool getting passed view model command in no problem don't understand... how can execute know state has changed? here code example.

from view models ctor

eventaggregator.getevent<navigationenabledevent>().subscribe(onnavigationenabledchange, threadoption.uithread);  navigatecommand = new delegatecommand(onnavigate, () => nextbuttonenabled); 

now - here onnavigationenablechange event.

private void onnavigationenabledchange(bool navigationstate)     {       nextbuttonenabled = navigationstate;     } enter code here 

like - totally missing here - how command know nextbuttonenabled no true?

if point me working example awesome.

ok - thanks!

this why don't use implementation of delegatecommand in prism. i've hated callback-based approach enabling/disabling commands. it's entirely unnecessary, , far can tell, (and rather doubtful) 'benefit' it's consistent how execution handled. has seemed pointless me because execution , enabling/disabling different: button knows when wants execute command doesn't know when command's status might have changed.

so end writing this:

public class relaycommand : icommand {     private bool _isenabled;     private action _onexecute;      public relaycommand(action executehandler)     {         _isenabled = true;         _onexecute = executehandler;     }      public bool isenabled     {         { return _isenabled; }         set         {             _isenabled = value;             if (canexecutechanged != null)             {                 canexecutechanged(this, eventargs.empty);             }         }     }       public bool canexecute(object parameter)     {         return _isenabled;     }      public event eventhandler canexecutechanged;      public void execute(object parameter)     {         _onexecute();     } } 

(if necessary modify use weak references execute change event handlers, prism does.)

but answer question: how callback approach meant work? prism's delegatecommand offers raisecanexecutechanged method can invoke ask raise event that'll cause command invokers query command's canexecute. given have tell delegatecommand time enabled status changes, don't see meaningful benefit of callback-based approach. (sometimes see broadcast model though - arranging change in status anywhere notifies all command invokers! in case, callback useful because means doesn't matter if don't know changed. requerying every single command seems unpleasant me.)