It turns out there was a lovely little race with View.ActionTarget, where
either the View.ActionTarget or the corresponding {s:Action} could be
evaluated first. This meant setting View.ActionTarget was risky.
It happened to work in previous releases because the bootstrapper accessed
View.ViewManager, which caused its class ctor to run and somehow caused
View.ActionTarget to be evaluated earlier.
I don't think there's a decent way of getting around this: we can't
guarentee the order in which things in Xaml are going to be evaluated.
That's not the WPF way.
Therefore, only throw exceptions if someone actually clicks the button.
The previous behaviour was to raise an exception unless Execute.Dispatcher
had been explicitly defined. This was to detect cases where the user hadn't
set up Execute correctly, and treat them as errors rather than simply going
ahead with some possibly-unexpected behaviour.
However, since BootstrapperBase sets Execute.Dispatcher automatically, it's
highly unlikely that itwould ever not be set when it needed to be. Exceptions
are design mode and unit tests, both of which want (or can cope with) a
synchronous dispatcher.
Hence the behaviour change. Execute.Dispatcher is a synchronous dispatcher
by default, but is overridden to one that uses Application.Current.Dispatcher
by the bootstrapper if we're in a real application.
Antony Male