Practicing the Chain of Responsibility Pattern
page 3 of 8
by Xianzhong Zhu
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The First Design: Simple yet Available

Actually, according to the requirements introduced a moment ago, we can naturally conceive a very simple while practical solution. We can define a ServiceManager class which will be responsible for maintaining the common manipulations of the service objects (start or stop). Listing 2 indicates the definition for class ServiceManager.

Listing 2: The definition for class ServiceManager

public class ServiceManager
{
    private List<ServiceObject> m_serviceList = new List<ServiceObject>();
    static ServiceManager()
    {
        Initialize();
    }
    public List<ServiceObject> ServiceList
    {
        get { return m_serviceList; }
    }
    private static void Initialize()
    {
        //omit the concrete implementation...
      //initializing the service objects and 
       //add them into variable m_serviceList
    }
    public static void StartService(ServiceObject obj)
    {
        if (obj.CurrentState.Equals(ServiceState.Stop))
        {
            Switch (obj.ActiveMode)
            {
                  case ActiveMode.Singleton:
                        RemotingConfiguration.RegisterWellKnownServiceType(
                            obj.ObjectType,obj.ServiceName, 
                            WellKnownObjectMode.Singleton);
                        obj.CurrentState=ServiceState.Start;
                        break;
                  case ActiveMode.SingleCall:
                        RemotingConfiguration.RegisterWellKnownServiceType(
                            obj.ObjectType,obj.ServiceName, 
                            WellKnownObjectMode.SingleCall);
                        obj.CurrentState=ServiceState.Start;
                        break;
                  case ActiveMode.Activation:
                        RemotingServices.Marshal(
                            (MarshalByRefObject)obj.ServiceInstance, 
                            obj.ServiceName);
                        obj.CurrentState=ServiceState.Start;
                        break;
            }
        }
    }
 
    public static void StopService(ServiceObject obj)
    {
        if (obj.CurrentState.Equals(ServiceState.Start))
        {
            Switch (obj.ActiveMode)
            {
                  case ActiveMode.Singleton:
                        //no implementation
                        break;
                  case ActiveMode.SingleCall:
                        //no implementation
                        break;
                  case ActiveMode.Activation:
                        RemotingServices.Disconnect
 (MarshalByRefObject)obj.ServiceInstance);
                        obj.CurrentState=ServiceState.Stop;
                        break;
            }
        }
    }
}

Herein, the Initialize method is used to initialize the service object. Concretely, it takes the responsibility of reading out information about the service object from inside the configure file, establishing the service object, and then adding it into the m_seviceList object.

In addition to what is listed above, SeviceManager has to provide methods to start/stop all the service objects. Because the stop related methods are similar to start, we are still to abridge the stop service related code while only give the start related ones below.

Listing 3

public static void StartAllServices()
{
    foreach (ServiceObject obj in m_serviceList)
    {
        StartService(obj);
    }
}
public static void StartAllServices(ActiveMode activeMode)
{
    foreach (ServiceObject obj in m_serviceList)
    {
        if (obj.ActiveMode.Equals(activeMode))
        {
            StartService(obj);
        }
    }
}

Because the shortcut menu PopupMenu is associated with the ListView control, we can obtain the service object information through the currently-selected item on the ListView control when right-clicking the ListView control. For example, the Name of the service can be obtained through the following code.

Listing 4

String serviceName=lvService.SelectedItems[0].
 SubItems[0].Text;

In the ServiceManager definition, method StartService and StopService receivesthe object of the ServiceObject type. To invoke these methods only knowing of the service name is not enough, and we have to also provide the ServiceObject object. Therefore, we still need to define another method-- LookUpServiceObject, which seeks the related ServiceObject object in the m_serviceList object according to the given argument ServiceName.

Listing 5

protected static ServiceObject LookUpServiceObject(string serviceName)
{
    foreach (ServiceObject obj in m_serviceList)
    {
        if (obj.ServiceName.Equals(serviceName))
        {
            return obj;
        }
    }
    return null;
 
}

The menu item "Start--Start By Service Name" in the shortcut menu related Click event handler can be implemented below:

Listing 6

private void startByServiceNameToolStripMenuItem_Click(object sender, EventArgs e)
{
    string serviceName = lvService.SelectedItems[0].SubItems[0].Text;
    ServiceObject obj=ServiceManager.LookUpServiceObject(serviceName);
    if(obj!=null)
    {
        ServiceManager.StartService(obj);
    }
}

As for the click handler for the menu item "Start All", it is much simpler, as follows.

private void munStartAll_Click(object sender, EventArgs e)
{
    ServiceManager.StartAllServices();
}

For now, we have nearly accomplished the design of the service explorer application. Surveying the whole design, it is simple yet practical, which can meet the requirements brought out at the beginning. In the above programming, we have used OOP as much as possible. Although the design of class ServiceManager is not very elegant, while from the angle of availability, such a design is enough.

By digging deeper, we find out that the above design can still be improved, which can be done by introducing the Template Method pattern.


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