GEF 命令模式介绍

命令模式的结构如图：

在我们流程设计器中，实现复制粘贴功能，就是采用了命令模式，以剪切操作为例，当在编辑器中选中活动，在菜单中选择“剪切”，就会产生一个剪切命令（CutCommand对象）。

剪切操作所用到的类如下：

名称	说明
org.eclipse.gef.commands.Command	命令的抽象类
org.eclipse.gef.commands.Command.CompoundCommand	“结合多个命令的命令”的类
com.example.workflow.commands.CutCommand	“剪切命令”的类
com.example.workflow.model.WorkflowProcess	流程类
com.example.workflow.action.CutAction	剪切动作类

Command抽象类，这个类中有个execute()方法。

CompoundCommand表示“结合多个命令的命令”的类，此类继承了Command类，这个类的commandList属性是java.util.ArrayList类型，用来存放多个继承Command的对象，这个类在执行execute()方法时，会把commandList中所用命令的execute()方法按ArrayList中顺序执行一边，在执行undo()方法时，会把commandList中所用命令的undo ()方法按ArrayList中反序执行一边。

CutCommand类表示“剪切活动命令”，该类继承了Command类，该类有parent和child两个属性，parent是WorkflowProcess对象，表示流程对象，child是AbstractActivity对象，表示要剪切的活动对象，在构造函数中，给这两个属性赋值，指明要从哪个流程模型中删除哪个活动，在execute()方法中调用parent的removeChild()方法，把活动从流程模型中删除。

CutAction类是调用命令的类，这个类继承了org.eclipse.gef.ui.actions.SelectionAction,在这个类的run()方法中新建了CutCommand和CompoundCommand对象，并且把CutCommand对象加入到CompoundCommand对象的commandList中，这样如果剪切多个活动，执行了多个CutCommand命令，但这些命令都放在commandList中，这样在用户开来，就是执行一个命令。

对照命令模式的结构图，CompoundCommand和CutCommand就相当于ConcreteCommand，WorkflowProcess相当于Reciever,而removeChild()方法就是Reciever的action()方法，而CutAction则同时扮演了Client和Invoker角色。

CutCommand类代码：

package com.example.workflow.commands;

import org.eclipse.gef.commands.Command;

import com.example.workflow.model.AbstractActivity;

import com.example.workflow.model.WorkflowProcess;

public class CutCommand extends Command{

private WorkflowProcess parent;

private AbstractActivity child;

public CutCommand(WorkflowProcess parent,AbstractActivity child){

super();

if (parent == null || child == null) {

throw new IllegalArgumentException();

}

this.parent = parent;

this.child = child;

setLabel("cut activity");

}

public boolean canExecute() {

return parent != null && child != null;

}

public void execute() {

redo();

}

public void redo() {

parent.removeChild(child);

}

public void undo() {

parent.addChild(child);

}

}

CutAction类代码：

package com.example.workflow.action;

import java.util.List;

import org.eclipse.gef.EditPart;

import org.eclipse.gef.commands.Command;

import org.eclipse.gef.commands.CompoundCommand;

import org.eclipse.gef.ui.actions.Clipboard;

import org.eclipse.gef.ui.actions.SelectionAction;

import org.eclipse.jface.resource.ImageDescriptor;

import org.eclipse.ui.IWorkbenchPart;

import org.eclipse.ui.actions.ActionFactory;

import com.example.workflow.Activator;

import com.example.workflow.commands.CutCommand;

import com.example.workflow.model.AbstractActivity;

import com.example.workflow.model.WorkflowProcess;

import com.example.workflow.parts.WorkflowProcessEditPart;

public class CutAction extends SelectionAction{

private List selectionList = null;

private Clipboard clipboard = null;

private WorkflowProcess workflowProcess;

public CutAction(IWorkbenchPart part) {

super(part);

setId(ActionFactory.CUT.getId());

setText("Cut");

setImageDescriptor(ImageDescriptor.createFromFile(Activator.class, "icons/cut.gif"));

}

public Clipboard getClipboard() {

return clipboard;

}

public void setClipboard(Clipboard clipboard) {

this.clipboard = clipboard;

}

public WorkflowProcess getWorkflowProcess() {

return workflowProcess;

}

public void setWorkflowProcess(WorkflowProcess workflowProcess) {

this.workflowProcess = workflowProcess;

}

protected boolean calculateEnabled() {

selectionList = super.getSelectedObjects();

if(selectionList.isEmpty() ||

(selectionList.size() == 1 && selectionList.get(0) instanceof WorkflowProcessEditPart)){

return false;

}

return true;

}

public void run() {

getClipboard().setContents(selectionList);

execute(createCutCommand());

}

private Command createCutCommand(){

CompoundCommand compoundCmd = new CompoundCommand();

for(int i=0;i<selectionList.size();i++){

EditPart part = (EditPart)selectionList.get(i);

CutCommand cmd = new CutCommand(getWorkflowProcess(),(AbstractActivity)part.getModel());

compoundCmd.add(cmd);

}

return compoundCmd;

}

}

WorkflowProcess类代码：

package com.example.workflow.model;

import java.util.ArrayList;

import java.util.List;

/**

* 流程模型，可以包含多个活动和转移模型

* @author Administrator

*

*/

public class WorkflowProcess extends ModelElement{

private static final long serialVersionUID = -5478693636480758659L;

/** Property ID to use when a child is added to this WorkflowProcess. */

public static final String CHILD_ADDED_PROP = "WorkflowProcess.ChildAdded";

/** Property ID to use when a child is removed from this WorkflowProcess. */

public static final String CHILD_REMOVED_PROP = "WorkflowProcess.ChildRemoved";

private List activities = new ArrayList();

/**

* Add a Activity to this WorkflowProcess.

* @param s a non-null Activity instance

* @return true, if the Activity was added, false otherwise

*/

public boolean addChild(AbstractActivity a) {

if (a != null && activities.add(a)) {

firePropertyChange(CHILD_ADDED_PROP, null, a);

return true;

}

return false;

}

/** Return a List of Activities in this WorkflowProcess. The returned List should not be modified. */

public List getChildren() {

return activities;

}

/**

* Remove a Activity from this WorkflowProcess.

* @param s a non-null Activity instance;

* @return true, if the Activity was removed, false otherwise

*/

public boolean removeChild(AbstractActivity a) {

if (a != null && activities.remove(a)) {

firePropertyChange(CHILD_REMOVED_PROP, null, a);

return true;

}

return false;

}

}