Table Of Content
Create a class file called CloseCommand.cs and copy and paste the following code. The following class also does the same thing as the previous two classes. Second, a constructor initializes the Receiver Object, i.e., the document. It also has the Execute method, which calls the Close method of the Receiver Object, i.e., the Document object Close Method. These benefits contribute to a more robust and adaptable design. Let’s move now to create our command pattern example client program that will consume our file system utility.
Command Pattern Examples in C++
Use the Command pattern when you want to queue operations, schedule their execution, or execute them remotely. Your current task is to create a toolbar with a bunch of buttons for various operations of the editor. You created a very neat Button class that can be used for buttons on the toolbar, as well as for generic buttons in various dialogs. Generic model class diagram of Command design pattern is as shown in image. All clients of Command objects treat each object as a "black box" bysimply invoking the object's virtual execute() method whenever theclient requires the object's "service".
Difference between Command Pattern and Strategy Pattern
In that way, separate command classes can implement different action logic and be invoked in a standardized way through the caller. The Command Design Pattern is primarily used to decouple the sender and receiver of a request. This means that the sender does not need to know the details of the operation being performed or the receiver of the request. Instead, the sender knows how to issue a command, and the command knows how to execute a request.
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Classes and Methods
We create the Command interface which will be implemented by the concrete command classes. The text editor in this example creates new command objects each time a user interacts with it. After executing its actions, a command is pushed to the history stack. So, modify the Main method of the Program class as shown below. First, we create an instance of the Receiver Object, i.e., Document. Then, we create three command objects by passing the Receiver Object as a parameter, i.e., the Document object.
Java
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The simplest solution is to create tons of subclasses for each place where the button is used. These subclasses would contain the code that would have to be executed on a button click. Finally let’s look into the client code which will actually generate random commands towards the receiver class “FMRadio”. The receiver is responsible for performing the actual work when a command is executed. For example, in a home automation system, the receiver could be a “Light” object that understands how to turn on and off. If we want to configure the remote for the speaker, we can just pass the Speaker instance to concrete commands while configuring the Invoker like below.
Receiver
You might have noticed one missing piece of the puzzle, which is the request parameters. A GUI object might have supplied the business-layer object with some parameters. Since the command execution method doesn’t have any parameters, how would we pass the request details to the receiver? It turns out the command should be either pre-configured with this data, or capable of getting it on its own. Command objects serve as links between various GUI and business logic objects. From now on, the GUI object doesn’t need to know what business logic object will receive the request and how it’ll be processed.
Command pattern
Choosing the proper implementation may depend on your use case but also on your preferences regarding some of the following aspects. Instead of a method inside a class, we now have a dedicated Method class. Our new class has everything we implemented in our method before, including information about the caller and our request's arguments. Since we still need a way to invoke our new "Method", our class also needs an execute function. While the Command Design Pattern has many benefits, it’s not without its drawbacks.
When to Use Command Pattern
Command design pattern leverages the Open/Closed principle. A method contains the implementation code that describes a specific behavior of an object during runtime. In contrast to classes and objects, methods don't have a runtime representation. There is not such a concept as a method instance (which would basically be a request).
Another option is to equip the command with all relevant information during construction. In this case, we don't have to pass any parameters when calling - as long as all relevant data can be derived from the constructor parameters. This approach may work better for specific scenarios as it lessens the coupling between the command and its context, allowing for easier reuse. Commands are frequent - almost all applications use them in one way or another.
After that, the command object is put on the redo stack so that it gets executed the next time the user invokes Redo. One central question to answer is how to inject all relevant context information, like receivers and parameters, into your commands. One way would be to have an execute method that accepts a parameter of type Object or any. The concrete command implementations are responsible for casting this parameter to the desired type.
Each reducer has to define which type of action it can handle and how this action would affect the current state. This approach already resembles most of the Command pattern. There is only a slight addition we have to make, which is providing a general command interface so that all types of commands can be handled uniformly.
It holds a reference to a command but doesn’t delve into the specifics of how each command works. It’s like a button that, when pressed, makes things happen. The remote control’s role is to coordinate and execute commands without getting involved in the complexities of individual actions. Command pattern is a data driven design pattern and falls under behavioral pattern category. A request is wrapped under an object as command and passed to invoker object. Invoker object looks for the appropriate object which can handle this command and passes the command to the corresponding object which executes the command.
If you don’t need to perform actions later, you may be better off simply calling the receiver’s methods directly. The Command design pattern allows to encapsulate an action or trigger inside an object which will be used later to trigger an event. Since in this design pattern, commands are encapsulated inside objects, hence we can use additional actions on this commands for example- Queuing of various commands, undo/redo actions etc. This design pattern is very useful in case of GUI actions (button, menu actions etc), transactional behaviour, progress bars etc. For Design patterns basic explanation see (Design Patterns Simplified Version).
This enables one to configure a class with a command object that is used to perform a request. The class is no longer coupled to a particular request and has no knowledge (is independent) of how the request is carried out. Use the Command pattern when you want to parametrize objects with operations. In this example, the Command pattern helps to track the history of executed operations and makes it possible to revert an operation if needed. The GUI objects may access the business logic objects directly. Create concrete classes implementing the Order interface.
Now that our receiver classes are ready, we can move to implement our Command classes. FileSystemReceiver interface defines the contract for the implementation classes. For simplicity, I am creating two flavors of receiver classes to work with Unix and Windows systems.
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