Design Mode Study Notes 20-memento Mode

Motivation : Capture the internal state of an object without compromising encapsulation, and save the state outside the object. In this way, the object can be restored to the previously saved state.

Scenario: The Graphic System is used as an example. The circle is the primary machine. The system processes the circle and stores the recoverable sequence.

Structure chart

Code

Namespace Designpattern. Memento
{
/**/ /// <Summary>
///Primary machine
/// </Summary>
Public   Class Round
{
Private Point centerpoint;
Private   Int Radius;

Public Round (point centerpoint, Int Radius)
{
This. Centerpoint=Centerpoint;
This. Radius=Radius;
}

Public Roundmemento creatememento ()
{
Roundmemento memento= NewRoundmemento ();
Memento. setstate (This. Centerpoint,This. Radius );
ReturnMemento;
}

Public   Void Setmemento (roundmemento memento)
{
This. Centerpoint=Memento. centerpoint;
This. Radius=Memento. radius;
}

Public   Void Move (point)
{
}

Public   Void Setlength ( Int Radius)
{
}
}

/**/ /// <Summary>
///Memorandum
/// </Summary>
Public   Class Roundmemento
{
Internal Point centerpoint;
Internal   Int Radius;

Public Roundmemento ()
{
}

Internal   Void Setstate (point centerpoint, Int Radius)
{
This. Centerpoint=Centerpoint;
This. Radius=Radius;
}
}
}

Namespace Designpattern. Memento
{
Public   Class Graphicssystem
{
Round round =   New Round ( New Point (), 10 );
Ilist < Roundmemento > Mementos;

Public Graphicssystem ()
{
Mementos= NewList<Roundmemento>();
}

Public   Void Process ()
{
Mementos. Add (round. creatememento ());
//Operation circle
}

Public   Void Undo ( Int Index)
{
Round. setmemento (mementos [Index]);
}
}
}

Key Points :
1. Memento stores the internal status of the originator object and restores the status of the original object as needed. This mode applies to "information that is managed by the primary but must be stored outside the primary ".
2. When implementing this mode, you must prevent objects other than the primary object from accessing the memorandum object. The memorandum object has two interfaces: one is the wide interface used by the primary object and the other is the narrow interface used by other objects.
3. When implementing this mode, you must consider the efficiency of copying object States. If the object overhead is large, you can use an incremental change to improve this mode.