Command Design Pattern in C#
Table Of Content
Could you give an example where we actually need Invoker? First, it isn’t that easy to save an application’s state because some of it can be private. Some operations, such as copying/pasting text, would need to be invoked from multiple places. For example, a user could click a small “Copy” button on the toolbar, or copy something via the context menu, or just hit Ctrl+C on the keyboard. Let’s look into the Invoker class now which will invoke the received commands on the receiver class.
Real-world code in C#
It may help you in taking the right decision about using command pattern. Just one thing, @Override annotations is not allowed for methods that implement an interface method. Although there are many ways to implement undo/redo, the Command pattern is perhaps the most popular of all.
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The client program create the receiver object and then attach it to the Command. Then it creates the invoker object and attach the command object to perform an action. Now when client program executes the action, it’s processed based on the command and receiver object. The Command Interface is like a rulebook that all command classes follow. It declares a common method, execute(), ensuring that every concrete command knows how to perform its specific action. It sets the standard for all commands, making it easier for the remote control to manage and execute diverse operations without needing to know the details of each command.
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We have concrete command classes BuyStock and SellStock implementing Order interface which will do actual command processing. A class Broker is created which acts as an invoker object. Undo/Redo OperationsCommand design pattern provides the ability to undo and redo a command. Each concrete command object can have a unexecute() method that can undo an operation. A separate stack can be maintained to keep track of the executed command so that it can be used to undo/redo an operation. Before the Invoker processes an undoable command, it stores it in the undo queue.
Implementation
While this is enough for most use cases, this approach has some drawbacks. Once executed, we cannot track how many of these requests we made or what parameters they received during the call. We simply cannot do the same with them as we could do with objects. Create a class file named SaveCommand.cs and copy and paste the following code. First, it has a reference to the Receiver Object, i.e., the Document object. Third, it has the Execute method, which calls the Save method of the Receiver Object, i.e., the Document object Save Method.
After we apply the Command pattern, we no longer need all those button subclasses to implement various click behaviors. It’s enough to put a single field into the base Button class that stores a reference to a command object and make the button execute that command on a click. The Device interface declares methods related to device functionality, such as turnOn() and turnOff(). This interface sets a contract for device classes, defining common operations that concrete devices should support. Imagine you are tasked with designing a remote control system for various electronic devices in a smart home. The devices include a TV, a stereo, and potentially other appliances.
Concrete Command Classes
It is up to the application team to decide what action will be executed when a button is clicked. With command design pattern, the executor of the command (button component) does not need to know anything about the command being executed. For example, execute add operation when + button is clicked and execute subtract operation when- butting is clicked. We implement command design pattern by creating separate command classes. Command pattern is a behavioral design pattern which is useful to abstract business logic into discrete actions which we call commands. This command object helps in loose coupling between two classes where one class (invoker) shall call a method on other class (receiver) to perform a business operation.
Receiver
This pattern is particularly useful in scenarios where you want to parameterize objects with operations and queue, specify, and execute requests at different times. We will look at a real life scenario where we can implement Command pattern. Let’s say we want to provide a File System utility with methods to open, write and close file.
Classes and Methods
This is possible because objects are a central OOP concept, and languages provide many different ways to interact with them. Use this pattern whenever you want to create actions that can be executed on receivers at a later point in time. For example, you can create and store commands to be performed by a computer AI, and then execute these over time. We need to create three command classes by implementing the above ICommand interface. Yes, the Command Design Pattern can support undoable operations.
The Invoker Object (i.e., Menu Options) does not know how to handle the requests. So, what the Invoker Object will do is he will call the Execute method of the command object. As you can see, the menus have Open, Save, and Close options. The Execute Method of the command object will be called the Open Method of the Document Object. Similarly, the Save Command has the request to save a document.
What the Invoker will do is it will call the Execute method of the Command Object. The Execute method of the command object will be called the Receiver Object Method. The Receiver Object Method will perform the necessary action to handle the request. For a better understanding, please have a look at the following diagram. As with any other object, a command can be serialized, which means converting it to a string that can be easily written to a file or a database.
This approach is, for instance, used in the Windows Presentation Foundation (WPF). The client code (GUI elements, command history, etc.) isn’t coupled to concrete command classes because it works with commands via the command interface. This approach lets you introduce new commands into the app without breaking any existing code. In command design patterns, we create command objects for the above-mentioned On, Off, Up, and Down commands. The Command Design Pattern is a Behavioral Design pattern that turns a request into a stand-alone object that contains all information about the request.
The Command interface declares a method, often named execute(). This method is meant to encapsulate a specific operation. The interface sets a contract for concrete command classes, defining the execute() method that encapsulates the operation to be performed. Redux's Actions are conceptually similar to Commands, as they represent the operations users can execute to change the application's state. However, since they are simple JSON objects consisting only of an id and a payload - without any implementation logic - they are much lighter. The implementation moves entirely to the receivers, called reducers in Redux.
A visualization of the classes and objects participating in this pattern. Command objects can be thought of as "tokens" that are created byone client that knows what need to be done, and passed to anotherclient that has the resources for doing it. In the above class, we created the Light instance and use it to create all the command instances, which are then configured through the Invoker. Assume a front-end team responsible for creating UI (User Interface) components such as button, dropdown, checkbox, etc. These components can then be used to build UI for any application. For example, the button component can be used in a calculator application.
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