Sommerville-Software-Engineering-10ed

15.2 ■ Application frameworks 445
GUI
Event
loop
Callbacks
Application-specific classes
Callbacks
Callbacks
Figure 15.6 Inversion of
control in frameworks
Database
Event
loop
Platform
Event
loop
While each framework includes slightly different functionality, web application
frameworks usually provide components and classes that support:
1. Security WAFs may include classes to help implement user authentication
(login) and access control to ensure that users can only access permitted functionality
in the system.
2. Dynamic web pages Classes are provided to help you define web page templates
and to populate these dynamically with specific data from the system database.
3. Database integration Frameworks don’t usually include a database but assume
that a separate database, such as MySQL, will be used. The framework may
include classes that provide an abstract interface to different databases.
4. Session management Classes to create and manage sessions (a number of interactions
with the system by a user) are usually part of a WAF.
5. User interaction Web frameworks provide AJAX (Holdener 2008) and/or
HTML5 support (Sarris 2013), which allows interactive web pages to be created.
They may include classes that allow device-independent interfaces to be
created, which adapt automatically to mobile phones and tablets.
To implement a system using a framework, you add concrete classes that inherit
operations from abstract classes in the framework. In addition, you define
“callbacks” —methods that are called in response to events recognized by the framework.
The framework objects, rather than the application-specific objects, are
responsible for control in the system. Schmidt et al. (Schmidt, Gokhale, and
Natarajan 2004) call this “inversion of control.”
In response to events from the user interface and database framework objects
invoke “hook methods” that are then linked to user-provided functionality. The userprovided
functionality defines how the application should respond to the event
(Figure 15.6). For example, a framework will have a method that handles a mouse
click from the environment. This method is called the hook method, which you must
configure to call the appropriate application methods to handle the mouse click.