add State pattern

Author: tk-codes

BehavioralPatterns/State/README.md

@@ -0,0 +1,54 @@
+# State Pattern
+
+Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.
+
+## Problem
+
+- An object should change its behavior when its internal state changes.
+- State-specific behavior should be defined independently.
+  - New states should be added
+  - the behavior of existing states should be changed independently
+
+## Solution
+
+- Define separate State objects that encapsulate state-specific behavior for each state.
+- A class delegates state-specific behavior to its current state object instead of implementing state-specific behavior directly.
+
+## Common Structure
+
+![Common structure of State pattern](img/structure.jpg)
+
+* Context
+  * maintains an instance of ConcreteState that defines the current state.
+* State
+  * defines an interface for encapsulating the behavior associated with a particular state of the Context.
+* ConcreteState
+  * each subclass implements a behavior associated with a state of the Context.
+
+## Collaboration
+
+* Context delegates state-specific requests to the current ConcreteState object.
+* A context may pass itself as an argument to the State object handling the request. This lets the State object access the context if necessary.
+* Context is the primary interface for clients. Clients can configure a context with State objects. Once a context is configured, its clients don't have to deal with the State objects directly.
+* Either Context or the ConcreteState subclasses can decide which state succeeds another and under what circumstances.
+
+## Benefits
+
+* Organizes the code related to particular states into separate classes.
+* It makes state transitions explicit by using separate objects for different states.
+* Simplifies the code of the context
+* State objects can be shared, if they have no instance variables (cf. Flyweight with no intrinsic state but only behavior)
+
+## Drawbacks
+
+* Can be overkill if a state machine has only a few states or rarely changes.
+
+## Example
+
+**Definition**
+
+**Usage**
+
+## Comparison with other patterns
+
+* **Flyweight** pattern explains when and how State objects can be shared.

BehavioralPatterns/State/img/structure.jpg

@@ -40,7 +40,7 @@ Design Patterns in C# / .NET
 |:heavy_check_mark:| [Memento](/BehavioralPatterns/Memento)|
 | :heavy_check_mark: | [Null Object](/BehavioralPatterns/NullObject) |
 | :heavy_check_mark:| [Observer](/BehavioralPatterns/Observer/) |
-| | State | :warning:
+| | [State](/BehavioralPatterns/State)
 |:heavy_check_mark: | [Strategy](/BehavioralPatterns/Strategy/) |
 |:heavy_check_mark: | [Template Method](/BehavioralPatterns/TemplateMethod) |
 | | Visitor