Chapter 3. Design Patterns

Functional Design Patterns

One of the other bastions of design we’re all familiar with is the idea of design patterns. Patterns document reusable templates that solve common problems in software architecture. If you spot a problem and you’re familiar with an appropriate pattern, then you can take the pattern and apply it to your situation. In a sense, patterns codify what people consider to be a best-practice approach to a given problem.

In this section, we’re instead going to look at how existing design patterns have become better, simpler, or in some cases, implementable in a different way. In all cases, the application of lambda expressions and a more functional approach are the driving factor behind the pattern changing.

The Command Pattern

A command object is an object that encapsulates all the information required to call another method later. The command pattern is a way of using this object in order to write generic code that sequences and executes methods based on runtime decisions. There are four classes that take part in the command pattern, as shown in Figure 3-1:

Receiver
Performs the actual work
Command
Encapsulates all the information required to call the receiver
Invoker
Controls the sequencing and execution of one or more commands
Client
Creates concrete command instances
Figure 3-1. The command pattern

Let’s look at a concrete example ...

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