How to use CanExecute with Mvvmcross

Question

I have a Button

<Button 
      android:id="@+id/ButtonConnect"
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:text="Disconnect"
      local:MvxBind="{'Click':{'Path':'DisconnectCommand'}}" />

And I have a Command for it

public IMvxCommand DisconnectCommand
{
    get
    {
        return new MvxRelayCommand(this.GetService<IConnectionService>().Disconnect);
    }
}

Then I want to enable/disable the DisconnectCommand using

DisconnectCommand.CanExecute(this.GetService<IConnectionService>().UsbConnected);

But thats clearly wrong (It is not working), I put the check in as a a parameter, but normally I would do

DisconnectCommand.CanExecute = someBool;

But there is no property to set, so how to go about this?

Answer 1

To follow up on Stuart's answer, it's easy to support both ICommand.CanExecute along with exposing properties to support Android and iOS Mvx bindings.

To do this, convert your typical CanExecute() methods to properties, then add handlers to CanExecuteChanged that calls RaisePropertyChanged on the associated property. Then use RaiseCanExecuteChanged as normal and the PropertyChanged event gets fired as well.

    ...

    // constructor
    public SomeClass()
    {

        DoSomethingCommand = new MvxCommand(OnDoSomething, () => CanDoSomething);
        DoSomethingCommand .CanExecuteChanged += (sender, args) => RaisePropertyChanged(() => CanDoSomething);
    }

    public bool CanDoSomething
    {
        get { ... }
    }

    ...

Answer 2

To work out how to use CanExecute, take a look at Silverlight or WPF - there's lots of blogs out there which talk about how to use ICommand - e.g. http://weblogs.asp.net/nmarun/archive/2009/12/02/using-icommand-silverlight-4.aspx or http://blog.galasoft.ch/archive/2009/09/26/using-relaycommands-in-silverlight-and-wpf.aspx

An example would be something like:

private MvxRelayCommand _disconnectCommand;
public IMvxCommand DisconnectCommand
{
    get
    {
        if (_disconnectCommand == null)
            _disconnectCommand = new MvxRelayCommand(this.GetService<IConnectionService>().Disconnect, item => this.IsItemConnected(item));
        return _disconnectCommand;
    }
}

private void SomeServiceNotificationHandler()
{
    _disconnectCommand.RaisePropertyChanged();
}

private bool IsItemConnected(object thing)
{
    return /* your code */;
}

---

There is one small problem though....

CanExecute isn't really fully implemented across all the MvxBindings across all platforms... It will work for some of them, but for some of them it won't - and I don't really know which ones at present! If you come across issues, then please let me know (via GitHub issues) and they will get fixed...

---

Personally... I don't tend to use CanExecute - I tend instead to use a separate Boolean property which I then bind to whatever property is available on the control - e.g. most controls have something like Enabled, IsEnabled, Disabled, IsDisabled, etc.

I generally find it easier (and more readable) to set the Boolean property rather than to call RaiseCanExecuteChanged

e.g. I'd use something like:

<Button 
  android:id="@+id/ButtonConnect"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:text="Disconnect"
  local:MvxBind="{'Click':{'Path':'DisconnectCommand'},'Enabled':{'Path':'UsbConnected'}}" />

You can definitely argue that the CanExecute approach has advantages - because it keeps the Command logic all in one object, and because it can be used to prevent Execute calls happening within the RelayCommand. That's why I'm happy to try to fix CanExecute bugs in mvvmcross bindings as we find them.