IMPLEMENTING COMPONENT GROUPINGS in VB.NET

Encoder QR Code in VB.NET IMPLEMENTING COMPONENT GROUPINGS

class Invoker< Receiver> { private List<ICommand< Receiver>> _commands = new List<ICommand<Receiver>>(); public void Add( ICommand<Receiver> command) { _commands.Add(command); } public void Run( Receiver receiver) { foreach(ICommand<Receiver> command in _commands) { command.Execute(receiver); } } } The Invoker class has two methods: Add and Run. The Add method adds additional ICommand implementations to a list. The Run method iterates the ICommand implementations and executes each one of them. The Run method has a single parameter, which is the Receiver type that will be passed to each ICommand implementation. The Invoker class in this example doesn t implement any serialization routines, because I want to hold off discussing serialization in detail until 8, which is where the Invoker class will be extended to include serialization. Putting it all together, the client source code would be as follows. Invoker< CommandReceiver> macro = new Invoker< CommandReceiver>(); macro.Add( new Operation( "First")); macro.Add( new Operation("Second")); macro.Run( new CommandReceiver()); The variable macro references an Invoker instance. Using the Add method, operations are added, and then the commands are executed using the Run statement. The client is responsible for providing the initial state of the command object and providing a receiver. Once the state has been assigned, then it shouldn t be modified or adapted. If a modified command chain is required, then clone the original and modify the cloned elements. Any other type of interaction isn t allowed and would break the intent of the Command pattern. The Command pattern can be implemented using delegates, but only for transient lists that don t need serialization, initialization, or destruction. The problem with using delegates is that only the Run method can be implemented. For most Command pattern implementations, you need the Initialize and Destroy methods. When using delegates, a typecast is necessary to perform serialization, initialization, or destruction. And if a typecast is necessary, there is no advantage to delegates, and an interface can be used. The Command pattern has the following distinguishing attributes: The ICommand implementations have an assigned state that isn t manipulated by external classes. Once assigned, the state of the ICommand implementation is essentially read-only and typically not altered. An external type that is called an Invoker manages the ICommand implementations as a single collection. It s possible to create a proxy that converts an ICommand interface to reference another Invoker that executes another list of ICommand implementations.

The ICommand implementations interact with the external environment using the Receiver type. Any other interaction should be avoided, as it could lead to a referencing nightmare. The ICommand implementations are read only once they have been assigned, and if the state needs to be modified, an ICommand implementation is cloned. The Invoker class is responsible for managing the collection of ICommand implementations, including serialization.

The purpose of the Composite pattern9 is to define a structure of objects in a hierarchy, where some objects are containers, and others are leaves. The UML diagram of the Composite pattern is illustrated in Figure 5-7.

Figure 5-7. UML implementation of the Composite pattern The UML diagram for the Composite pattern looks similar to that of the Chain of Responsibility or Command pattern, and from a high-level perspective it is. There are two main features that compromise a Composite pattern implementation: dynamic storage of data occurs in a hierarchical structure, and the implementations don t process requests. The Composite pattern is implemented using either interfaces or abstract base classes. Using delegates isn t recommended because a typical Composite pattern base type has multiple methods. The simplest way to illustrate a Composite pattern is to implement a storage system. For example, on the UNIX operating system, the storage system is defined by a hierarchy of nodes, where individual nodes can be directories or files. Following is the definition of the base INode abstract class: