Everyone makes mistakes sometimes , I hope there is a kind of “ Regret ” Make up for your own mistakes , Let yourself start over , But the reality is brutal . In computer applications , Customers also make mistakes often , Can you provide “ Regret ” Give it to them ？ Of course you can , And it's necessary . This function consists of “ Memo mode ” To achieve . In fact, many applications provide this function , Such as Word、 Notepad 、Photoshop、Eclipse Wait for the software to press when editing Ctrl+Z Key combination can undo the current operation , Restore the document to its previous state ; And in IE Back key in 、 Rollback operation in database transaction management 、 Save intermediate results when playing games 、 Database and operating system backup operation 、 The repentance function in chess games belongs to this kind of game . Memo mode can record the internal state of an object , When the user regrets, the current operation can be undone , Restore data to its original state . Memorandum （Memento） Definition of pattern ： Without breaking encapsulation , Capture the internal state of an object , And save the state outside the object , So that the object can be restored to the original saved state when needed later , This mode is also called snapshot mode .

Catalog

• Definition and characteristics
• Structure and Implementation
  • The structure of the pattern
  • The realization of pattern
• Application scenarios
• Expand ： Mixed with prototype patterns

Everyone makes mistakes sometimes , I hope there is a kind of “ Regret ” Make up for your own mistakes , Let yourself start over , But the reality is brutal . In computer applications , Customers also make mistakes often , Can you provide “ Regret ” Give it to them ？ Of course you can , And it's necessary . This function consists of “ Memo mode ” To achieve .

In fact, many applications provide this function , Such as Word、 Notepad 、Photoshop、Eclipse Wait for the software to press when editing Ctrl+Z Key combination can undo the current operation , Restore the document to its previous state ; And in IE Back key in 、 Rollback operation in database transaction management 、 Save intermediate results when playing games 、 Database and operating system backup operation 、 The repentance function in chess games belongs to this kind of game .

Memo mode can record the internal state of an object , When the user regrets, the current operation can be undone , Restore data to its original state .

Definition and characteristics

Memorandum （Memento） Pattern The definition of ： Without breaking encapsulation , Capture the internal state of an object , And save the state outside the object , So that the object can be restored to the original saved state when needed later , This mode is also called snapshot mode .

Memo mode is an object behavior pattern , Its main advantages are as follows ：

• Provides a mechanism for restoring state ： When the user needs to be able to more easily restore the data to a historical state .
• Realize the encapsulation of internal state ： In addition to the founders who created it , No other object can access this state information .
• Simplified the originator ： Initiators do not need to manage and save individual backups of their internal state , All status information is stored in the memo , And managed by the manager , This is in line with the principle of single responsibility .

Its main drawback is ： High resource consumption , If the internal state information to be saved is too much or too frequent , It will occupy a large amount of memory resources .

Structure and Implementation

The core of the memo pattern is to design the memo class and the manager class for managing the memo , Now let's learn about its structure and implementation .

The structure of the pattern

The main roles of the memo model are as follows ：

• Originator （Originator） role ： Record the internal status information of the current time , Provides the ability to create and recover memo data , Implement other business functions , It has access to all the information in the memo .
• Memorandum （Memento） role ： Responsible for storing the internal state of the initiator , Provide these internal states to the originator when needed .
• managers （Caretaker） role ： Manage memos , Provides the ability to save and retrieve memos , But it cannot access and modify the contents of the memorandum .

The structure of the memo mode is shown in the figure ：

The realization of pattern

The implementation code of memo mode is as follows ：

class Program{ static void Main(string[] args) {  Originator or = new Originator();  Caretaker cr = new Caretaker();  or.SetState("S0");  Console.WriteLine(" The initial state :" + or.GetState());  cr.SetMemento(or.CreateMemento()); // Save state     or.SetState("S1");    Console.WriteLine(" New status :" + or.GetState());  or.RestoreMemento(cr.GetMemento()); // Restore the state   Console.WriteLine(" Restore the state :" + or.GetState());    Console.Read();   }}// Memorandum public class Memento{  private String state;  public Memento(String state) {   this.state=s.........