CommandManager.InvalidateRequerySuggested does not cause a requery on CanExecute in MVVM-Light

问题

I am using MVVM-Light RelayCommand

private ICommand myRevertCmd;
public ICommand Revert
    {
        get
        {
            if (myRevertCmd == null)
            {
                myRevertCmd = new RelayCommand(RevertExecute, CanRevertExecute);
            }

            return myRevertCmd;
        }
    }

    private void RevertExecute()
    {
        searchType = SearchType.Revert;
        SearchStart();
    }

    private bool CanRevertExecute()
    {
        return isRevertEnabled;
    }

I have some code that changes the value of isRevertEnabled but the linked button does not change. After some searching I found that you can use to force the re-evaluation of the button states

// force the GUI to re-evaluate the state of the buttons
CommandManager.InvalidateRequerySuggested();

But this doesn't work. Does any one have any suggestions?

回答1:

I would turn the isRevertEnabled flag into a property and call the OnPropertyChanged method from there (you need to implement the INotifyPropertyChanged interface). At the point where you change the flag, you need to use the property, e.g. IsRevertEnabled = true.

private bool isRevertEnabled;

public bool IsRevertEnabled
{
    get
    {
        return isRevertEnabled;
    }
    set
    {
        if (isRevertEnabled != value)
        {
            isRevertEnabled = value;
            OnPropertyChanged("IsRevertEnabled");
        }
    }
}

private bool CanRevertExecute()     
{         
    return IsRevertEnabled;     
}

回答2:

Just to add another possible solution, in my case I needed to call CommandManager.InvalidateRequerySuggested on the UI thread using Application.Current.Dispatcher.Invoke.

回答3:

There are a lot suggestions out there (here, here, here).

I use a simple but not so beautiful workaround. I simply call OnPropertyChanged("MyICommand") for my commands in my BackgroundWorker Completed Event.

回答4:

According to Josh Smith's article 'Allowing CommandManager to query your ICommand objects'. The problem is that the command is a non-routed command.

I have made a new implementation of the MVVM-Light RelayCommand as follows:

// original
//public event EventHandler CanExecuteChanged;


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

public void RaiseCanExecuteChanged()
{
    CommandManager.InvalidateRequerySuggested();
    //            var handler = CanExecuteChanged;
    //            if (handler != null)
    //            {
    //                handler(this, EventArgs.Empty);
    //            }
}

回答5:

I guess you can call "Revert.RaiseCanExecuteChanged()" method at "isRevertEnabled" INPC (or dependency property) set method. This will force the "CanRevertExecute" predicate.

回答6:

Just call Revert.RaiseCanExecuteChanged();

来源：https://stackoverflow.com/questions/6028561/commandmanager-invalidaterequerysuggested-does-not-cause-a-requery-on-canexecute

标签

wpf

mvvm-light