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2.9.2 Compound Document Framework

   In the next release of IBM Open Class library there is to be a high-level 
   framework delivered using the programming model developed by Taligent.  A 
   framework is much more than a class library.  While a class library is a 
   collection of related classes, a framework is a class library that also 
   embraces design knowledge and provides a running subsystem for a special 
   problem domain.  This problem domain may be business related as in 
   accounting, or may be an infrastructure domain, as in compound documents 
   or communication. 

   The programming paradigm is changing with this new technology as shown in 
   Figure 55.  The evolution started with the introduction of graphical user 
   interfaces in the 80s.  Before the graphical user interfaces, applications 
   were totally in control of the application flow.  Reusable code was put in 
   procedural libraries and called by the application. 

   With the introduction of graphical user interfaces like Mac, OS/2 PM or 
   Windows, the user started to have the control over the program flow by 
   having multiple ways to interact with the applications. The programs 
   consist of an event loop waiting for user interactions. 


 


                                              



 
   Figure 55. Evolution Towards Frameworks 

   An abstract class is a design for a single object. A framework is the 
   design of a set of objects that collaborate to carry out a set of 
   responsibilities. Thus frameworks are larger scale designs than abstract 
   classes. Frameworks are a way to reuse high-level design. Ralph Johnson, 
   Vincent Russo 

   With a framework, a developer is no longer writing a program that is in 
   control of the application.  The framework is the application, which is 
   customized by the developer writing small puzzle pieces of code where he 
   wants to modify the default behavior of the system.  It is sometimes 
   called the Hollywood Principle: Don't call us, we will call you. 
   Frameworks can be specialized and modified by subclassing.  A framework 
   has the following two interfaces as shown in Figure 56: 

   °   A using interface, which is small and simple to use 

   °   A subclassing interface used to customize the behavior of the 
       framework 



 


                                              



 
   Figure 56. Frameworks 

   The Workplace Shell is an example of an implemented framework.  The 
   Workplace Shell controls and owns the process.  Other Workplace 
   Shell-based classes written by third parties using WPS objects and classes 
   can be registered and plugged in during run time, inheriting all the 
   functions provided by this framework. 

   The Compound Document Framework is based on four base classes, a model 
   class, a view class, selection class  and a command class as shown in 
   Figure 57.  These four classes have to be modified to create a component 
   either for OLE or OpenDoc with this framework.  All the other functions 
   such as embedding, linking, drag and drop, undo, scriptability, and 
   interoperability are provided by the framework.  The model class is used 
   to define the content of a component the data part. View classes are used 
   to define the user interface.  The views are separated from the model very 
   strictly.  There may be several views used in parallel by different 
   applications as shown in Figure 58.  The Invoice, Accounts receivable and 
   Rolodex application are using different views on the same model Account 
   Info.  Account Info has the following three attributes in this example: 

   °   Name/Address 

   °   Phone No. 

   °   Credit Info 


   If the model, for example the address, changes, all views on this model 
   are notified by the framework, so they can update themselves.  The 
   presenter class handles all user interactions. 


 


                                              



 
   Figure 57. Compound Document Framework 

   With a view, the user can also make a selection, which specifies a certain 
   range of the model data, for example the address.  A selection is very 
   independent from the way it is created. It may be defined by a mouse, 
   keyboard actions, a menu item or a script.  These selections can be used 
   to manipulate the model data.  A selection is associated with a set of 
   commands specified for this selection to manipulate the specified data. 
   Commands can also be launched in very different ways.  It may be done by a 
   menu action, a command line, a mouse event or a script. 


 


                                              



 
   Figure 58. Compound Document Framework 

   But commands are always acting on selections and never directly on the 
   model data.  The actions performed as command - selection pairs are stored 
   in a history log, thus providing a multi-level undo, implemented by the 
   framework. Because these command - selection classes are independent from 
   the way they were created,  the parts get scriptability for free, as shown 
   in Figure 59.  The Compound Document Framework, first delivered with 
   Visual Age C++ for Windows  and later with the next release for OS/2, can 
   generate OpenDoc components as well as OLE components. 


 


                                              



 
   Figure 59. Compound Document Framework 

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