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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.
Implementation Details
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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.
Receiver
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.
Complete code for the above example
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".
Invoker
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.
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.
Now we need to create implementations for all the different types of action performed by the receiver. Since we have three actions we will create three Command implementations. Each Command implementation will forward the request to the appropriate method of receiver. You’ll implement a bunch of command classes for every possible operation and link them with particular buttons, depending on the buttons’ intended behavior.
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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).
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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.
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.
Usually it has just a single execution method that takes no parameters. This interface lets you use various commands with the same request sender, without coupling it to concrete classes of commands. As a bonus, now you can switch command objects linked to the sender, effectively changing the sender’s behavior at runtime. Command is a behavioral design pattern that turns a request into a stand-alone object that contains all information about the request. This transformation lets you pass requests as a method arguments, delay or queue a request’s execution, and support undoable operations.
