Mastering the Command Design Pattern with Practical Examples by Nikolay Nikolov
Table Of Content
Initially, when our app only had the toolbar, it was okay to place the implementation of various operations into the button subclasses. In other words, having the code for copying text inside the CopyButton subclass was fine. This structural code demonstrates the Command pattern which stores requests as objects allowing clients to execute or playback the requests.
Creational Software Design Patterns in C++
Need to issue requests to objects without knowing anything about theoperation being requested or the receiver of the request. Commands with sophisticated logic that need to orchestrate many receivers are definitely heavy-weight. They have many dependencies, and there is a high chance that a change in any of them will propagate to your command implementation. You lose type safety when casting the parameter to the desired type. This can be problematic when you try to reuse your command in a different context. While your code will compile without errors, your cast may fail during runtime if the new context is incompatible with the old one.
Mastering the Command Design Pattern with Practical Examples
Order, i.e., what kind of food the customer wants, is the Command. So, the Waiter passes the request to the Receiver, i.e., the Cook, who will prepare the food and give it back to the Waiter, and the Waiter gives it back to the customer. Ignatius Teo is a freelance PHP programmer and has been developing n-tier web applications since 1998. In his spare time he enjoys dabbling with Ruby, martial arts, and playing Urban Terror.
Difference between Command Pattern and Strategy Pattern
This transformation allows you to parameterize methods with different requests, delay or queue a request’s execution, and support undoable operations. It’s useful in scenarios where you need to issue requests without knowing anything about the operation being requested or the receiver of the request. Furthermore, it supports undoable operations, as every command is an object with a specific method. The next step is to make your commands implement the same interface.
Mastering Design Patterns with Examples — Command Pattern
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Before long, you realize that this approach is deeply flawed. First, you have an enormous number of subclasses, and that would be okay if you weren’t risking breaking the code in these subclasses each time you modify the base Button class. Put simply, your GUI code has become awkwardly dependent on the volatile code of the business logic. While all of these buttons look similar, they’re all supposed to do different things. Where would you put the code for the various click handlers of these buttons?
Command Pattern Implementation
Finally, we create the client class RemoteApplication responsible for creating the concrete command objects and assigning them to the Invoker. Our file system utility implementation is ready and we can move to write a simple command pattern client program. But before that I will provide a utility method to create the appropriate FileSystemReceiver object.
Heavy or Lightweight Commands
Let us go ahead and implement the above example step by step using the Command Design Pattern in C#. The Command Design Pattern is a great example of encapsulation – one of the fundamental principles of object-oriented design. It encapsulates a request as an object, thereby letting you parameterize clients with different requests. Let’s solve above home automation problem with command design pattern and design each component one at a time. Suppose we need to build a remote control for home automation system which shall control different lights/electrical units of the home. A single button in remote may be able to perform same operation on similar devices e.g. a TV ON/OFF button can be used to turn ON/OFF different TV set in different rooms.
These classes act as executable instructions that the remote control can trigger without worrying about the nitty-gritty details of how each command accomplishes its task. The invoker is responsible for triggering the execution of commands. It holds references to the commands and can execute them. It acts as an intermediary between the sender (client) and the receiver, ensuring that the sender remains decoupled from the receiver.
This file system utility should support multiple operating systems such as Windows and Unix. To implement our File System utility, first of all we need to create the receiver classes that will actually do all the work. Since we code in terms of interface in java, we can have FileSystemReceiver interface and it’s implementation classes for different operating system flavors such as Windows, Unix, Solaris etc.
Command decouples the object that invokes the operation from the onethat knows how to perform it. To achieve this separation, the designercreates an abstract base class that maps a receiver (an object) with anaction (a pointer to a member function). The base class contains anexecute() method that simply calls the action on the receiver.
The invoker class holds a reference to a Command object and triggers its execution through the execute() method. The invoker doesn’t know the specific details of the command or the devices. It simply calls the execute() method on the current command, allowing for flexible and dynamic control over different devices. As per the Command Design Pattern, the command object has three things. The second one is the Receiver object reference, and the third is the Execute method, which will call the receiver object method to handle the request. This information includes the method name, the object that owns the method and values for the method parameters.
The Command pattern allows requests to be encapsulated as objects,thereby allowing clients to be parametrized with different requests.The "check" at a diner is an example of a Command pattern. The waiteror waitress takes an order or command from a customer and encapsulatesthat order by writing it on the check. Note that the pad of "checks" used by each waiter isnot dependent on the menu, and therefore they can support commands tocook many different items. This is an interface or abstract class that declares an execute method, defining the contract for concrete commands. It ensures that all commands have a method to perform their actions.
The class that hosted our method implementation before is not gone. It will still have to interact with our new Method class. It is often called the Receiver, as it's the actual target of our action. In Object-Oriented-Programming, we have objects representing all kinds of concepts, like database entities, graphical elements, or concrete things modeled after the real world around us. And we have methods we can call on these objects to mimic their behavior and alter their state. Both concepts, objects and methods, are first-class citizens in most OOP languages - essential aspects made explicit by the corresponding language.
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