Design Patterns - Command Presentation

class: center, middle

# Command

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# Also known as

* Action, Transaction

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# Intent

* Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.

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# Explanation

* [Wikipedia](https://en.wikipedia.org/wiki/Command_pattern) says:
  > In object-oriented programming, the command pattern is a behavioral design pattern in which an object is used to encapsulate all information needed to perform an action or trigger an event at a later time. This information includes the method name, the object that owns the method and values for the method parameters.

<br />

* In plain words:

* Enable the construction of components that are able to delegate, sequence or execute method calls at a time of their choosing;
  
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# Explanation

Four terms always associated with the pattern:

* Command
	* Object that knows about the receiver and invokes a method of the receiver;
  
  * Receiver
	* Object that does the work;
  
  * Invoker
	* Knows how to execute a command, and optionally does the bookkeeping about the command execution;
  
  * Client 
	* Decides which commands to execute at which points, passing the command object to the invoker object;
  
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# Example

In our example, a Wizard can cast spells on targets.

* Spell will be the command (implements the Command interface);
  * Goblin (a Target object) will be the receiver;
  * Wizard will be the invoker;
  * App will be the client;

```

//App.java
	public static void main(String[] args) {
		Wizard wizard = new Wizard();
		Goblin goblin = new Goblin();
		goblin.printStatus(); 		//Goblin, [size=normal] [visibility=visible]

wizard.castSpell(new ShrinkSpell(), goblin);
		wizard.castSpell(new InvisibilitySpell(), goblin);

goblin.printStatus(); 		//Goblin, [size=small] [visibility=invisible]

wizard.undoLastSpell();

goblin.printStatus(); 		//Goblin, [size=small] [visibility=visible]

wizard.redoLastSpell();
		goblin.printStatus(); 		//Goblin, [size=small] [visibility=invisible]
	}
```

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# Diagram

.center[![Diagram](diagram.png)]

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# Applicability

Use the Command pattern when you want to:

* Parameterize objects by an action to perform; 
  * You can express such parameterization in a procedural language with a callback function, that is, a function that's registered somewhere to be called at a later point;
  
  * Commands are an object-oriented replacement for callbacks;

<br />
  
* Specify, queue, and execute requests at different times; 
  * A Command object can have a lifetime independent of the original request;
  
  * If the receiver of a request can be represented in an address space-independent way, then you can transfer a command object for the request to a different process and fulfill the request there;

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# Applicability

Use the Command pattern when you want to:

* Support undo; 
  * The Command's execute() operation can store state for reversing its effects in the command itself;
  
  * The Command interface must have an added unexecute() operation that reverses the effects of a previous call to execute;
  
  * Executed commands are stored in a history list;
  
  * Unlimited-level undo and redo is achieved by traversing this list backwards and forwards calling unexecute() and execute(), respectively;

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# Applicability
  
Use the Command pattern when you want to:
  
* Support logging changes so that they can be reapplied in case of a system crash;
  * By augmenting the Command interface with load() and store() operations, you can keep a persistent log of changes;
  
  * Recovering from a crash involves reloading logged commands from disk and re-executing them with the execute operation;

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# Applicability

Use the Command pattern when you want to:

* Structure a system around high-level operations build on primitive operations;
  * Such a structure is common in information systems that support transactions;
  
  * A transaction encapsulates a set of changes to data;
  
  * The Command pattern offers a way to model transactions; 
  
  * Commands have a common interface, letting you invoke all transactions the same way;
  
  * The pattern also makes it easy to extend the system with new transactions

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# Use Cases

* To keep a history of requests;

* Implement callback functionality;

* Implement the undo functionality;

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# Real world examples

[java.lang.Runnable](http://docs.oracle.com/javase/8/docs/api/java/lang/Runnable.html)

[Netflix Hystrix](https://github.com/Netflix/Hystrix/wiki)

[javax.swing.Action](http://docs.oracle.com/javase/8/docs/api/javax/swing/Action.html)

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# Credits

* [Design Patterns: Elements of Reusable Object-Oriented Software](http://www.amazon.com/Design-Patterns-Elements-Reusable-Object-Oriented/dp/0201633612)

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# Tutorials

* Source code http://java-design-patterns.com/patterns/command/