Performer handles a specific Change.
It's a key component in the relationship between WidgetModel
that requests some data, and the source of these data.
R - type of result
C - type of change on which performer triggers
WidgetModel
WM is logical representation of widget and its state.
WidgetModelDependencies - is pack of dependencies for WidgetModel. Most often it is ErrorHandler.
Model - optionally, but recommended, manager for connection with business layer