GOF mode-03-behavioral mode (bottom)

7. Observer mode （Observer）

Refers to when the state of the target object changes , An object that responds to or handles state change events .

interpretation ：
The observer pattern is sometimes called “ Publisher — Subscriber pattern ”（Publisher-Subscriber Pattern）, In the event processing scenario , Also called a listener （Listener） Pattern .
The observer pattern defines the many to one relationship between dependent and dependent objects , When a dependent object （ Or target object ） When the state of , Dependent objects can receive notifications and respond in a timely manner .

7.1 Use scenarios

（1） When the state of the target object changes , State change events need to be notified to other dependent objects , But do not know the specific number or type of dependent objects .
（2） Objects with dependencies in the abstraction layer need to be encapsulated independently , To reuse or extend .

7.2 Class structure

because Subject The aggregate is Observer Objects defined by abstract types , therefore Observer Changes to implementation classes do not affect Subject, Again ,Subject You can also freely extend subclasses .
When Subject When the state changes , Use notifyObserver（） Method to change the new state （ It can be a message or an event ） Inform all Observer object ,Observer After the object receives the notification , Choose whether to respond or not according to the business rules .

7.3 Use the observer

stay COS In the system , Customers can subscribe to the platform discount information , The subscription method is email or SMS , New subscription methods will be added in the future . When COS When there are new offers , Customers who need to be notified of subscriptions in a timely manner .

8. The state pattern （State）

Used to encapsulate the specific state behavior of context objects , This enables the context object to change its own behavior when its internal state changes .

8.1 Use scenarios

（1） The behavior of a context object depends on its internal state , And the state will change during operation .
（2） It is necessary to eliminate the logical branch statements for state judgment in the context object .

8.2 Class structure

• Context Is the context object type , hold State Object reference of abstract type ;
• State Is encapsulating state behavior handle（） Abstract type of , The concrete state behavior is implemented by subclasses ;
• StateA、StateB and StateC Is a concrete state behavior implementation class . When you need to add a new type of state behavior , Just need to achieve State The extension can be completed by the new state subclass of .

because Context Type object behavior behavior（） The concrete representation of is determined by the internal state object ; therefore , Can eliminate behavior（） Branch statement for internal state judgment .

• Sequence diagram ：
  Client call context Context The behavior corresponding to the internal state of a type object behavior（） when , The context object delegates the customer request to State Type of state object processing .
  

8.3 Use state mode

COS The dish order in has an order being placed （Placing）、 Accepted （Accepted）、 Already prepared （Prepared）、 Completed （Finished） Equal state , Order of dishes in each state “ Cancel ” The behavior is different

• When the order status is Placing、Accepted when , If the customer cancels the order ,COS The order status will be set to “ Cancelled （Canceled）”, Customers get orders 100% refund
• When the order status is Prepared when , Customer cancels order ,COS The order status will be set to Canceled, Customers get orders 30% refund ;
• When the order status is Delivering、Delivered、Finished when , Order's “ Cancel ” Behavior not available .

To ensure the stability of the code , Developers do not want to use branching statements to construct the state behavior of the dish order object .

9. The strategy pattern （Strategy）

An object that encapsulates a single algorithm in a set of algorithms , These strategies can be substituted for each other , So that the change of a single algorithm does not affect the client using it .

9.1 Use scenarios

（1） Algorithms need to implement different 、 Replaceable variants .
（2） To the client class that uses the algorithm （ Or context class ） Mask the data structure inside the algorithm .
（3） Customer class （ Or context class ） Defines a set of interchangeable behaviors , You need to eliminate the branch statements that invoke this set of behaviors .
（4） A set of classes differ only in behavior , But developers do not want to achieve behavior polymorphism through subclass extension .

9.2 Class structure

• Context Is the context class of algorithm policy , It is also a customer class that uses policy objects ;
• Strategy The interface defines the behavior of the algorithm behavior（）, Implementation class ConcreteStrategyA、ConcreteStrategyB And so on .
• It is worth noting that , The data needed to execute the policy algorithm is obtained from the context . therefore ,Context Context object when using policy object , You need to transfer all the data required by the policy behavior into the policy object , Or an access interface that passes itself in and provides the data required by the algorithm behavior （ The diagram structure uses the context object as a method parameter to be passed into the policy behavior ）.

Sequence diagram ：
When the client uses the service of the context object , Pass in a specific policy object reference to it ; After the context object receives the client request , Delegate the policy object to complete the business request processing .

6.3 Example

The customer is browsing COS Menu time , You can choose to sort by price or name （ In ascending or descending order ） Browse . stay COS In the upgraded version of , A new sort method is added （ By heat 、 Seasonal order, etc ）.

10. Template method （Template Method）

A framework for defining algorithms , The variable steps in the algorithm are defined as abstract methods , Specify subclasses that implement or override .

interpretation ：
The template method defines the invariant part of the algorithm （ Including the algorithm process 、 Execution order 、 Input / Output, etc ）, The variable part （ Generally, some sub steps in the algorithm process ） Delay implementation into subclasses . advantage ：

• Avoid code redundancy .
  The immutable part of the algorithm is defined by the template method , Reused by all subclasses , You can avoid subclasses defining the same code repeatedly .
• Improve code stability .
  The variable part of the algorithm is unstable , Define unstable sub steps as abstract methods , Template methods depend on abstract methods , It can ensure the stability of the algorithm framework .

10.1 Use scenarios

（1） When the algorithm contains variable steps and immutable steps , Let subclasses determine the concrete implementation of variable steps .
（2） When multiple classes contain common business behaviors , Developers want to avoid redefining redundant code .
（3） Want to control the extension behavior of subclasses , Only subclasses are allowed to implement specific extension points .

10.2 Class structure

abstract class AbstractClass The template method is defined templateMethod（）, Template methods are algorithmic behaviors （ Or business process ） Framework , Algorithm framework contains immutable steps invariantStep（） And variable steps variantStep（）.

• templateMethod（） Used final Modifiers , Subclasses are not allowed to redefine it .
• invariantStep（） The visibility is private— Subclass not visible , Nor can we redefine ;
• variantStep（） Visibility is protected, Allow subclasses to inherit and redefine .
• therefore , Parent class AbstractClass Subclasses are specified ConcreteClass Only extension points can be overridden variantStep（）.

When AbstractClass Objects of type provide... To the client templateMethod（） The service , The client selects or specifies a specific ConcreteClass example .

Sequence diagram ：

10.3 Example

COS The step for the customer to pay the order is to check and confirm the order 、 Payment order 、 Generate receipt . The way to pay the order can be card payment or payroll deduction , The customer is free to choose the payment method , and COS Make sure the payment process is consistent .

11. Visitor mode （Visitor）

Used to encapsulate the operation of aggregating elements applied to the polymer （ Or algorithm ）, So that the operation （ Or algorithm ） Separate from the aggregate object , Realize free extension without affecting the aggregated objects .

When performing element operations on an aggregate object composed of several types of elements , Different operations need to be applied according to the element type . If you encapsulate the operation of an aggregate element in an aggregate object , The code has two major defects ：

• High code complexity
  Because different operations are applied to different types of aggregation elements , Aggregate objects not only implement the management of aggregate elements , You also need to define the operations of different elements , As a result, the code complexity of aggregated objects increases
• Poor stability
  Aggregate objects encapsulate operations that are imposed on different aggregate elements , When the element operation needs to be extended , The business logic of the aggregate object must be modified , To complete the code extension .

11.1 Use scenarios

（1） The target aggregate object contains different aggregate element types , Developers need to implement different business operations or algorithm behaviors for different aggregation element types .
（2） The target aggregate object structure is stable , But operations on aggregated elements require different extensions .
（3） There are multiple single and unrelated operations imposed on the aggregate element , But I don't want to “ Pollution ” The code of the aggregate element class .

11.2 Class structure

• ObjectStructure It is a polymer type , from Element Aggregate elements of type ;
• Element Is an abstract class of aggregate elements , There are different implementation types ElementA、ElementB etc. ;
• Interface Visitor Defines the imposed on Element The operating visit（）, For different Element Implementation types define different visit（） Overloading methods .

because Visitor Encapsulated for Element Implement the operation behavior of the type , therefore , When a new operation needs to be extended , Definition Visitor A new implementation class is all you need .

Sequence diagram
Client Create and use specific Visitor object , adopt ObjectStructure Defined interfaces service（） Apply actions to aggregate elements .

• When ObjectStructure When an object of type receives a client request , Will carry Visitor The instance request passed accept（） Method is distributed to the corresponding aggregate element .
• Aggregate element object received ObjectStructure After distributing the request , adopt visit（） Method again distributes the request to Visitor example , And pass itself as a method parameter ;
• Visitor Object received the... Submitted by the aggregation element visit（） After the request , Apply the corresponding overload operation behavior according to the aggregation element instance carried by the request , And access the interface provided by the aggregation element when necessary operation（）.