MVVM Enable/Disable button via a PropertyChanged event on an ObservableCollection

Question

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.NET3.5

Hello guys,
I'm clearly missing something within a binding of a command on a button in MVVM WPF project.
I have a DataGrid which is bound to a View Model property Paths of type ObservableCollection<folders></folders> (where Folder is just a custom type with Path and Count property)

<WpfToolkit:DataGrid ItemsSource="{Binding Paths}" IsReadOnly="True" AutoGenerateColumns="False" CanUserAddRows="False"
 <WpfToolkit:DataGrid.Columns>
    <WpfToolkit:DataGridTextColumn x:Uid="Custom:DataGridTextColumn_1" Header="Path" Binding="{Binding Path}"  />
        <WpfToolkit:DataGridTextColumn x:Uid="Custom:DataGridTextColumn_2" Header="Count" Binding="{Binding Count}" />
 </WpfToolkit:DataGrid.Columns>
</WpfToolkit:DataGrid>

The user can add items to this collection only using a button which prompts him to select a folder on the system.

<Button Command="{Binding AddMoreCommand}" Content="Add more" />

The command is executed (RelayCommand technique used):

C#

public void AddMoreFolders()
 {
  IFolderBrowserDialogService fbg = GetService<IFolderBrowserDialogService>();
     if (fbg != null)
     {
      if (DialogResult.OK == fbg.Show())
        {
          string selectedPath = fbg.SelectedPath;
          Paths.Add(new Folder { Path = selectedPath, Count = 0 });
          OnPropertyChanged("Paths");
        }
      }
 }

The thing that I would like to achieve is the following: Once at least 1 folder is added in the DataGrid the Start button should become Enabled. Following is XAML for Start button.

<Button Command="{Binding StartCommand}" />

Initially it is disabled because it's underlying CanStartProcessing method returns False.

C#

public bool CanStartProcessing()
{
    return !_processing && Paths.Count > 0;
}

The thing which I'm not able to achieve is as follows: Once a folder is added to the Paths collection from the Datagrid (and OnPropertyChanged is called, I'm expecting that the CanStartProcessing will execute, as the button's command is bound to it. But it does not. I've tried almost everything including Dispatcher.CurrentDispatcher.BeginInvoke(new Action(CommandManager.InvalidateRequerySuggested)); which tries to force the UI to invalidate it's content. Any help or suggestion is greatly appreciated.

Posted 8-Apr-11 10:48am

Ciumac Sergiu

Updated 8-Apr-11 11:42am

v2

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3 solutions

Solution 2

Hi,

you can also do your logik to enable/disable the Button in the Command.

C#

private RelayCommand _StartCommand;
public RelayCommand StartCommand
      {
          get { return _StartCommand ?? (_StartCommand = new RelayCommand(() => Starting(), () => CanStartProcessing())); }
      }

The first Mehod will be called if the Command is executing. The second describes if the Command isExecutable. The Button will be automatically disabled.

The update of the CanStartProcessing can be forced with:

CommandManager.InvalidateRequerySuggested()

Hope it helps

Posted 8-Apr-11 22:37pm

DrNimnull

Updated 9-Apr-11 0:40am

v2

Comments

Ciumac Sergiu 9-Apr-11 6:28am

This logic is already implemented, the problem is that it doesn't work as described in the question.

DrNimnull 9-Apr-11 6:45am

ohh sorry was a little remiss that i saw it too late, that you have already done these things. Can you post your Code of your RelayCommand.

Ciumac Sergiu 9-Apr-11 6:47am

Thank you DrNimnull, I've figured out the problem, it was a simple miss within the RelayCommand class. Please see my solution below.

Solution 1

Hi Ciumac!

It doesn't look like your Start button Enabled property is bound to anything. I would make the following changes:

1. Convert your CanStartProcessing method to a Property.

C#

public bool CanStartProcessing
{
    get { return !_processing && Paths.Count > 0; }
}

2. Bind the Start button to that property.

XML

<Button Command="{Binding StartCommand}" IsEnabled="{Binding CanStartProcessing}" />

3. Raise the PropertyChanged event for your CanStartProcessing property in addition to raising it for the Paths property.

C#

if (DialogResult.OK == fbg.Show())
        {
          string selectedPath = fbg.SelectedPath;
          Paths.Add(new Folder { Path = selectedPath, Count = 0 });
          OnPropertyChanged("Paths");
          OnPropertyChanged("CanStartProcessing");
        }

Note that you will also need to raise PropertyChanged when the _processing flag changes, so that the button becomes disabled during processing.

Posted 8-Apr-11 22:00pm

Jonathan Cardy

Updated 8-Apr-11 22:01pm

v2

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Ciumac Sergiu · Accepted Answer · 2011-04-09T00:45:00

Thank you all for suggestions, I've figured out what was the problem. There was a need to add in the RelayCommand class the following code for CanExecuteChanged event:

C#

public event EventHandler CanExecuteChanged
{
     add { CommandManager.RequerySuggested += value; }
     remove { CommandManager.RequerySuggested -= value; }
}

Initially it was:

public event EventHandler CanExecuteChanged;

As MSDN states RequerySuggested occurs when the CommandManager detects conditions that might change the ability of a command to execute.
MVVM can be very annoying sometimes.
Regards