RelayCommand.CanExecute not updating IsEnabled in UI - button

Question

I have a Windows Phone 8 app and I have a RelayCommand Instance called DiscoverExpansionModulesCommand. I have a button with the Command property bound to DiscoverExpansionModulesCommand. When the app first loads, the button is enabled or disabled properly. However, when on the page and I want to change whether the command can execute, the method CanExecuteDiscoverExpansionModulesCommand() properly fires and it returns the proper true or false value, but the button does not reflect it. Why isn't button updating it's UI? I found another article on this issue here http://social.msdn.microsoft.com/Forums/en-US/silverlightarchieve/thread/48a341e4-f512-4c33-befd-b614404b4920/
My ViewModel:
using GalaSoft.MvvmLight;
using GalaSoft.MvvmLight.Command;
using GalaSoft.MvvmLight.Messaging;
using MAL.Portable.Commands;
using MAL.Portable.Message;
using MAL.Portable.Model;
using System;
using System.Collections.Generic;
using System.Windows.Input;
namespace MAL.Portable.ViewModel
{
public class SettingsViewModel : ViewModelBase
{
// Define an observable collection property that controls can bind to.
private List<Setting> settings;
private String controllerUrl;
private String controllerPort;
private String temperature;
private Wifi wifi;
private Boolean connected;
private Boolean checkingConnection;
public SettingsViewModel()
{
DiscoverExpansionModulesCommand = new RelayCommand(OnDiscoverExpansionModules, CanExecuteDiscoverExpansionModulesCommand);
Messenger.Default.Register<RetrieveSettingsMessage>
(
this, (action) => RetrievedListsMessage(action)
);
Messenger.Default.Send<GetSettingsMessage>(new GetSettingsMessage());
}
public ICommand DiscoverExpansionModulesCommand
{
get;
private set;
}
public String ConnectionStatus
{
get
{
if (checkingConnection)
return "checking";
else
return connected ? "connected" : "not connnected";
}
}
private Boolean CanExecuteDiscoverExpansionModulesCommand()
{
return connected;
}
private void OnDiscoverExpansionModules()
{
}
private void CheckConnection()
{
wifi = null;
if (!String.IsNullOrWhiteSpace(ControllerUrl) && !String.IsNullOrWhiteSpace(ControllerPort) && !checkingConnection)
{
checkingConnection = true;
wifi = new ReefAngelWifi(controllerUrl, controllerPort);
wifi.TestConnectionComplete += wifi_TestConnectionComplete;
wifi.RequestFail += wifi_RequestFail;
wifi.BeginTestConnection();
}
}
private void wifi_RequestFail(object sender, RequestExceptionEventArgs e)
{
connected = false;
checkingConnection = false;
RaisePropertyChanged("ConnectionStatus");
}
private void wifi_TestConnectionComplete(object sender, TestConnectionEventArgs e)
{
connected = e.TestSuccessful;
checkingConnection = false;
DiscoverExpansionModulesCommand.CanExecute(null);
RaisePropertyChanged("ConnectionStatus");
RaisePropertyChanged("DiscoverExpansionModulesCommand");
}
private object RetrievedListsMessage(RetrieveSettingsMessage action)
{
settings = action.Settings;
CheckConnection();
return null;
}
private String GetStringValue(String key)
{
if (settings == null) return String.Empty;
var item = settings.Find(x => x.Key == key);
if (item == null) return String.Empty;
else return item.Value;
}
private Boolean GetBooleanValue(String key)
{
if (settings == null) return false;
var item = settings.Find(x => x.Key == key);
if (item == null) return false;
else return Boolean.Parse(item.Value);
}
}
}
And the XAML
<phone:PhoneApplicationPage
xmlns:ReefAngel="clr-namespace:MAL.WindowsPhone8"
xmlns:Controls="clr-namespace:Telerik.Windows.Controls;assembly=Telerik.Windows.Controls.Input"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:phone="clr-namespace:Microsoft.Phone.Controls;assembly=Microsoft.Phone"
xmlns:shell="clr-namespace:Microsoft.Phone.Shell;assembly=Microsoft.Phone"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
xmlns:cmd="clr-namespace:GalaSoft.MvvmLight.Command;assembly=GalaSoft.MvvmLight.Platform.WP8"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:telerikPrimitives="clr-namespace:Telerik.Windows.Controls;assembly=Telerik.Windows.Controls.Primitives"
x:Class="MAL.WindowsPhone8.ReefAngel.SettingsPage"
xmlns:converter="clr-namespace:MAL.WindowsPhone8.Converters"
FontFamily="{StaticResource PhoneFontFamilyNormal}"
FontSize="{StaticResource PhoneFontSizeNormal}"
DataContext="{Binding Settings, Source={StaticResource Locator}}"
SupportedOrientations="Portrait" Orientation="Portrait"
mc:Ignorable="d"
shell:SystemTray.IsVisible="True">
<phone:PhoneApplicationPage.Resources>
<converter:BooleanToStringConverter x:Key="temperatureConverter" TrueString="Celsius" FalseString="Fahrenheit" />
<converter:BooleanToStringConverter x:Key="timeFormatConverter" TrueString="24 hour" FalseString="12 hour" />
<converter:BooleanToStringConverter x:Key="dateFormatConverter" TrueString="dd/mm/yyyy" FalseString="mm/dd/yyyy" />
</phone:PhoneApplicationPage.Resources>
<!--LayoutRoot is the root grid where all page content is placed-->
<Grid x:Name="LayoutRoot" >
<phone:Pivot Title="{Binding LocalizedResources.ApplicationTitle, Source={StaticResource LocalizedStrings}, StringFormat='\{0\} Settings'}">
<phone:PivotItem Header="connection">
<Grid>
<StackPanel Margin="12,0,0,0">
<TextBlock Margin="0,20,0,0" TextWrapping="Wrap" Text="Reef Angel Wifi Address"/>
<TextBox Height="72" TextWrapping="Wrap" Text="{Binding ControllerUrl, Mode=TwoWay}"/>
<TextBlock Margin="0,20,0,0" TextWrapping="Wrap" Text="Reef Angel Wifi Port"/>
<TextBox Height="72" TextWrapping="Wrap" Text="{Binding ControllerPort, Mode=TwoWay}"/>
<StackPanel Orientation="Horizontal">
<TextBlock Margin="0,20,0,0" TextWrapping="Wrap" Text="Reef Angel Wifi Status : "/>
<TextBlock Margin="0,20,0,0" TextWrapping="Wrap" Text="{Binding ConnectionStatus, Mode=OneWay}"/>
</StackPanel>
</StackPanel>
</Grid>
</phone:PivotItem>
<phone:PivotItem Header="expansion">
<Grid>
<Button Content="Discover Expansion Modules" x:Name="DiscoverButton" Command="{Binding DiscoverExpansionModulesCommand, Mode=OneWay}" />
</Grid>
</phone:PivotItem>
</phone:Pivot>
</Grid>
</phone:PhoneApplicationPage>
I am using the MVVM Light Portable Class Libraries.

Answer 1

You need to call RelayCommand.RaiseCanExecuteChanged() when the conditions you evaluate inside your CanExecute method change.
Edit
private void wifi_RequestFail(object sender, RequestExceptionEventArgs e)
{
connected = false;
checkingConnection = false;
RaisePropertyChanged("ConnectionStatus");
DiscoverExpansionModulesCommand.RaiseCanExecuteChanged();
}
private void wifi_TestConnectionComplete(object sender, TestConnectionEventArgs e)
{
connected = e.TestSuccessful;
checkingConnection = false;
DiscoverExpansionModulesCommand.CanExecute(null);
RaisePropertyChanged("ConnectionStatus");
RaisePropertyChanged("DiscoverExpansionModulesCommand");
DiscoverExpansionModulesCommand.RaiseCanExecuteChanged();
}
This will not cause a loop as it only tells the RelayCommand to re-execute the specified CanExecute method. In your case this only means that the property CanExecuteDiscoverExpansionModulesCommand is read.

Answer 2

It appears to be a cross threading issue. And figuring out how to call a Dispatcher in the PCL was tricky, but I found it here: Update UI thread from portable class library