sâmbătă, august 20, 2005

Din blog-ul lui Martin Fowler despre framework-uri.

There's a big difference now in the flow of control between these programs - in particular the control of when the process_name and process_quest methods are called. In the command line form I control when these methods are called, but in the window example I don't. Instead I hand control over to the windowing system (with the Tk.mainloop command). It then decides when to call my methods, based on the bindings I made when creating the form. The control is inverted - it calls me rather me calling the framework. This phenomenon is Inversion of Control (also known as the Hollywood Principle - "Don't call us, we'll call you").

One important characteristic of a framework is that the methods defined by the user to tailor the framework will often be called from within the framework itself, rather than from the user's application code. The framework often plays the role of the main program in coordinating and sequencing application activity. This inversion of control gives frameworks the power to serve as extensible skeletons. The methods supplied by the user tailor the generic algorithms defined in the framework for a particular application.

--Ralph Johnson and Brian Foote

Inversion of Control is a key part of what makes a framework different to a library. A library is essentially a set of functions that you can call, these days usually organized into classes. Each call does some work and returns control to the client.

A framework embodies some abstract design, with more behavior built in. In order to use it you need to insert your behavior into various places in the framework either by subclassing or by plugging in your own classes. The framework's code then calls your code at these points.