Executing a command from another command - reactiveui

Question

I have a 'hello world' form (Xamarin forms) with some fields and a submit button. There's an observable (CanSave) which controls when the SaveChangesCommand can execute. If the save button is pressed when the CanSave is false, I want to a display a message to the user.
With the code below, if I
enter incorrect data
click save (the error message shows)
then correct the data.
CanSave becomes true and SaveChangesCommand is executed - before the button is hit again. It's as though the previously blocked button press was queued until canExecute became true.
What am I missing?
Thanks :-)
public PersonalProfileModel()
{
this.SaveChangesCommand = ReactiveCommand.CreateAsyncTask(this.CanSave(), message => this.doAllThings(message as string));
this.ButtonClickedCommand = ReactiveCommand.Create(Observable.Return(true));
this.ButtonClickedCommand.InvokeCommand(this.SaveChangesCommand);
// ButtonClickedCommand.Subscribe(x => this.SaveChangesCommand.Execute("hello")); // tried this too
}
public ReactiveCommand<object> ButtonClickedCommand { get; set; }
public ReactiveCommand<string> SaveChangesCommand;
public IObservable<bool> CanSave()
{
var fieldsValid = this.WhenAnyValue(
x => x.Name,
x => x.Country,
(f1, f2) =>
f1 == "a"
&& f2 == "b");
return fieldsValid;
}
public Task<string> doAllThings(string message)
{
var result = Task.Run(() =>{return "hello " + message;});
return result;
}

Answer 1

How about this:
this.SaveChangesCommand = ReactiveCommand.CreateAsyncTask(
this.CanSave(),
message => this.doAllThings(message as string));
this.ButtonClickedCommand = ReactiveCommand.CreateAsyncObservable(
SaveChangesCommand.CanExecuteObservable.StartWith(true),
x => SaveChangesCommand.ExecuteAsync(x));
Now we're explicitly describing the relationship between the ButtonClicked's Command in terms of the SaveChangesCommand - "The ButtonClicked command is enabled when SaveChanges can be executed"

Answer 2

This turned out to be a misunderstanding in the behaviour of ReactiveCommands and canExecute.
See ReactiveCommand not respecting canExecute