5.11 Chaining Requests

The GoF book (Gamma et al. 1995) gives an example of a MacroCommand class, a command that holds a linear collection (such as a list or a vector) of Commands. When a MacroCommand is executed, it executes in sequence each of the commands that it holds.

This feature can be very useful. For instance, let's refer to the undo/redo example again. One "do" operation might be accompanied by multiple "undo" operations. For example, inserting a character may automatically scroll the text window (some editors do this to ensure better text visibility). On undoing that operation, you'd like the window to "unscroll" back to where it was. (Most editors don't unscroll correctly. What a nuisance.) To unscroll, you need to store multiple Commands in a single Functor object and execute them as a single unit. The Document::InsertChar member function pushes a MacroCommand onto the undo stack. The MacroCommand would be composed of Document::Delete-Char and Window::Scroll. The latter member function would be bound to an argument that holds the old position. (Again, binding comes in very handy.)

Loki defines a class FunctorChain and the helper function Chain. Chain's declaration looks like this:

template <class Fun1, class Fun2>


Fun2 Chain(


   const Fun1& fun1,


   const Fun2& fun2);

FunctorChain's class implementation is trivial—it stores the two functors, and FunctorChain::operator() calls them in sequence. You can chain multiple functors by issuing repeated calls to Chain.

Let's have Chain conclude the support for macro commands. One thing about "nice to have" features is that the list can go on forever. However, there's plenty of room to grow. Neither BindFirst nor Chain incurred any change to Functor, which is proof that you can build similar facilities on your own.