关于MVC

 

关于MVC

 

sun的精辟解释（我以为问题阐述最好的）

http://java.sun.com/blueprints/patterns/MVC-detailed.html

Java BluePrints

Model-View-Controller

Context

Application presents content to users in numerous pages containing various data. Also, the engineering team responsible for designing, implementing, and maintaining the application is composed of individuals with different skill sets.

Problem

Now, more than ever, enterprise applications need to support multiple types of users with multiple types of interfaces. For example, an online store may require an HTML front for Web customers, a WML front for wireless customers, a Java^TM (JFC) / Swing interface for administrators, and an XML-based Web service for suppliers

When developing an application to support a single type of client, it is sometimes beneficial to interweave data access and business rules logic with interface-specific logic for presentation and control. Such an approach, however, is inadequate when applied to enterprise systems that need to support multiple types of clients. Different applications need to be developed, one to support each type of client interface. Non-interface-specific code is duplicated in each application, resulting in duplicate efforts in implementation (often of the copy-and-paste variety), as well as testing and maintenance. The task of determining what to duplicate is expensive in itself, since interface-specific and non-interface-specific code are intertwined. The duplication efforts are inevitably imperfect. Slowly, but surely, applications that are supposed to provide the same core functionality evolve into different systems.

Forces

• The same enterprise data needs to be accessed when presented in different views: e.g. HTML, WML, JFC/Swing, XML
• The same enterprise data needs to be updated through different interactions: e.g. link selections on an HTML page or WML card, button clicks on a JFC/Swing GUI, SOAP messages written in XML
• Supporting multiple types of views and interactions should not impact the components that provide the core functionality of the enterprise application

Solution

By applying the Model-View-Controller (MVC) architecture to a Java^TM 2 Platform, Enterprise Edition (J2EE^TM) application, you separate core business model functionality from the presentation and control logic that uses this functionality. Such separation allows multiple views to share the same enterprise data model, which makes supporting multiple clients easier to implement, test, and maintain.

Structure

The following diagram represents the Model-View-Controller pattern:

Participants & Responsibilities

The MVC architecture has its roots in Smalltalk, where it was originally applied to map the traditional input, processing, and output tasks to the graphical user interaction model. However, it is straightforward to map these concepts into the domain of multi-tier enterprise applications.

• Model - The model represents enterprise data and the business rules that govern access to and updates of this data. Often the model serves as a software approximation to a real-world process, so simple real-world modeling techniques apply when defining the model.
• View -The view renders the contents of a model. It accesses enterprise data through the model and specifies how that data should be presented. It is the view's responsibility to maintain consistency in its presentation when the model changes. This can be achieved by using a push model, where the view registers itself with the model for change notifications, or a pull model, where the view is responsible for calling the model when it needs to retrieve the most current data.
• Controller - The controller translates interactions with the view into actions to be performed by the model. In a stand-alone GUI client, user interactions could be button clicks or menu selections, whereas in a Web application, they appear as GET and POST HTTP requests. The actions performed by the model include activating business processes or changing the state of the model. Based on the user interactions and the outcome of the model actions, the controller responds by selecting an appropriate view.

Strategies

·         Web-based clients such as browsers. JavaServer Pages^TM (JSP^TM) pages to render the view, Servlet as the controller, and Enterprise JavaBeans^TM (EJB^TM) components as the model. The Java Pet Store sample application illustrates this strategy.

·         Centralized controller. Instead of having multiple servlets as controllers, a main Servlet is used to make control more manageable. The Front Controller pattern describes this strategy in more detail.

·         Wireless clients such as cell phones. The Smart Ticket sample application illustrates this strategy.

Consequences

·         Re-use of Model components. The separation of model and view allows multiple views to use the same enterprise model. Consequently, an enterprise application's model components are easier to implement, test, and maintain, since all access to the model goes through these components.

·         Easier support for new types of clients. To support a new type of client, you simply write a view and some controller logic and wire them into the existing enterprise application.

·         Increased design complexity. This pattern introduces some extra classes due to the separation of model, view, and controller.

Related Patterns

[BRMSS96]: Presentation-Abstraction-Control defines a structure with a hierarchy or cooperating agents, each with specific duties of the application functionality. It divides the functionality responsibilities into three components: presentation, abstraction, and control.

MVC meets Swing：Explore the underpinnings of the JFC's Swing components

http://www.javaworld.com/javaworld/jw-04-1998/jw-04-howto.html

以Button为例。

 

Swing中将mvc做了如下改变。

model

swing中的model基本有四类行为：

• Query internal state
• Manipulate internal state
• Add and remove event listeners
• Fire events

Button中对应ButtonModel。

 

View和control

Button中对应ButtonUI接口。

该接口负责按钮的可视化表示，以及处理输入。

 

Swing中的UI有三类行为

• Paint
• Return geometric information
• Handle AWT events

 

脚手架（scaffolding）

程序员并不直接与上述三者打交道，java通过java.awt.Component将三者隐藏起来。

或者说，Component将三者组合在一起，是一个wrapper。

 

Swing中的组件都是从Component继承来的。

java.lang.Object

  java.awt.Component

       java.awt.Container

           javax.swing.JComponent

因此，与AWT组件一起使用时，基本没有问题。同时，swing又模仿了AWT。即AWT有Button，而swing有JButton。

（swing是AWT的继承者，AWT是java最初的ui解决方案）

 

 

举例：Button

 Button做为按钮的wrapper，是model和view/control（ui）的脚手架。

 这点正是如下体现的：

 

 public void setModel(ButtonModel buttonmodel)

   {

      if (this.buttonmodel != null)

      {

         this.buttonmodel.removeChangeListener(buttonchangelistener);

         this.buttonmodel.removeActionListener(buttonactionlistener);

         buttonchangelistener = null;

         buttonactionlistener = null;

      }

      this.buttonmodel = buttonmodel;

      if (this.buttonmodel != null)

      {

         buttonchangelistener = new ButtonChangeListener();

         buttonactionlistener = new ButtonActionListener();

         this.buttonmodel.addChangeListener(buttonchangelistener);

         this.buttonmodel.addActionListener(buttonactionlistener);

      }

      updateButton();

   }

   public void setUI(ButtonUI buttonui)

   {

      if (this.buttonui != null)

      {

         this.buttonui.uninstallUI(this);

      }

      this.buttonui = buttonui;

      if (this.buttonui != null)

      {

         this.buttonui.installUI(this);

      }

      updateButton();

   }

   public void updateButton()

   {

      invalidate();

   }

 

模型部分：

ButtonModel

管理三种状态：pressed，armed，selected。

private boolean boolPressed = false;

   public boolean isPressed()

   {

      return boolPressed;

   }

   public void setPressed(boolean boolPressed)

   {

      this.boolPressed = boolPressed;

      fireChangeEvent(new ChangeEvent(button));

   }

管理事件：注册add，注销remove，激发fire

private Vector vectorChangeListeners = new Vector();

   public void addChangeListener(ChangeListener changelistener)

   {

      vectorChangeListeners.addElement(changelistener);

   }

  public void removeChangeListener(ChangeListener changelistener)

   {

      vectorChangeListeners.removeElement(changelistener);

   }

   protected void fireChangeEvent(ChangeEvent changeevent)

   {

      Enumeration enumeration = vectorChangeListeners.elements();

      while (enumeration.hasMoreElements())

      {

         ChangeListener changelistener =

            (ChangeListener)enumeration.nextElement();

         changelistener.stateChanged(changeevent);

      }

   }

 

UI部分：

ButtonUI

简单绘制

public void update(Button button, Graphics graphics)

   {

      ;

   }

   public void paint(Button button, Graphics graphics)

   {

      Dimension dimension = button.getSize();

      Color color = button.getBackground();

      graphics.setColor(color);

      graphics.fillRect(0,

                        0,

                        dimension.width,

                        dimension.height);

   }

 

处理AWT事件：

ButtonUI不自己处理事件，而是通过一个事件监听器来完成处理工作。

view/controller相关类只是一组方法，不包括状态。所以，多个Button实例共用一个ButtonUI实例，通过不同的标识来区别。

private static ButtonUIListener buttonuilistener = null;

   public void installUI(Button button)

   {

      button.addMouseListener(buttonuilistener);

      button.addMouseMotionListener(buttonuilistener);

      button.addChangeListener(buttonuilistener);

   }

   public void uninstallUI(Button button)

   {

      button.removeMouseListener(buttonuilistener);

      button.removeMouseMotionListener(buttonuilistener);

      button.removeChangeListener(buttonuilistener);

   }

ButtonUIListener帮助Button，将底层事件转换为符合Button模型处理要求，即Button模型能处理的语义事件，其实就是Button模型的三种状态。如下是如何从底层事件到语义事件。

   public void mouseDragged(MouseEvent mouseevent)

   {

      Button button = (Button)mouseevent.getSource();

      ButtonModel buttonmodel = button.getModel();

      if (buttonmodel.isPressed())

      {

         if (button.getUI().contains(button, mouseevent.getPoint()))

         {

            buttonmodel.setArmed(true);

         }

         else

         {

            buttonmodel.setArmed(false);

         }

      }

   }

   public void mousePressed(MouseEvent mouseevent)

   {

      Button button = (Button)mouseevent.getSource();

      ButtonModel buttonmodel = button.getModel();

      buttonmodel.setPressed(true);

      buttonmodel.setArmed(true);

   }

   public void mouseReleased(MouseEvent mouseevent)

   {

      Button button = (Button)mouseevent.getSource();

      ButtonModel buttonmodel = button.getModel();

      buttonmodel.setPressed(false);

      buttonmodel.setArmed(false);

   }

   public void stateChanged(ChangeEvent changeevent)

   {

      Button button = (Button)changeevent.getSource();

      button.repaint();

   }