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+ The MIT License
+ Copyright (c) 2017 Rodolfo Forte
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+<!DOCTYPE html>
+<html>
+ <head>
+ <title>Design Patterns - Command Presentation</title>
+ <meta charset="utf-8">
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+ <textarea id="source">
+
+class: center, middle
+
+# Command
+
+---
+
+# Also known as
+
+* Action, Transaction
+
+---
+
+# Intent
+
+* Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.
+
+---
+
+# 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;
+
+---
+
+# 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;
+
+---
+
+# 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]
+ }
+```
+
+---
+
+# Diagram
+
+.center[]
+
+
+---
+
+# 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;
+
+---
+
+# 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;
+
+---
+
+# 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;
+
+---
+
+# 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
+
+---
+
+# Use Cases
+
+* To keep a history of requests;
+
+* Implement callback functionality;
+
+* Implement the undo functionality;
+
+---
+
+# 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)
+
+---
+
+# Credits
+
+* [Design Patterns: Elements of Reusable Object-Oriented Software](http://www.amazon.com/Design-Patterns-Elements-Reusable-Object-Oriented/dp/0201633612)
+
+---
+
+# Tutorials
+
+* Source code http://java-design-patterns.com/patterns/command/
+
+ </textarea>
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+</html>
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