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I'm looking to implement the command pattern in a web application (asp.net c#)... Since the commands come in text format from the client, what is the best way to translate the string to a command object? Should I use reflection? Currently I just assume the command that comes in matches the file name of a user control. This is a bit of a hack.

Rather than have a select case statement that says if string = "Dashboard" then call Dashboard.Execute(), is there a pattern for working with commands that originate as strings?

5

You could use a hash table to map from input strings to Execute methods.

  • Bonus points for making it a IDictionary<string, Action> so you don't even need to have methods to call. – Wyatt Barnett Apr 4 '12 at 17:58
  • I think I will use a combination of the two. Using reflection seems a bit goofy although it would prevent me from having to register the commands. I'm not using Unity or an IOC container for this particular application. – KingOfHypocrites Apr 4 '12 at 18:07
  • 1
    Action isn't reflection at all, it is a language feature. – Wyatt Barnett Apr 4 '12 at 19:48
  • I wasn't saying action was reflection. I was talking writing my own TryGetInstance and passing a string. There are various solutions that help you get around having to register all your instances, but I'd rather just register then explicitly using a dictionary. – KingOfHypocrites Apr 5 '12 at 19:52
5

The best way to do this in just about any app is to use an IoC container. Then your command pattern looks something like:

public static class CommandLineActionFactory
{
    public static ICommandLineAction GetAction(string actionName)
    {
        if (actionName == null) throw new ArgumentNullException(nameof(actionName));
        ICommandLineAction ret = ObjectFactory.TryGetInstance<ICommandLineAction>(actionName);
        if (ret == null)
        {
            throw new ArgumentOutOfRangeException(nameof(actionName), $"Action '{actionName}' does not exist.");
        }
        return ret;
    }
}

[code is from a command line app but the pattern works for the web]

The huge advantage here is that you can take advantage of the IoC container to instantiate the handlers, including patching in their dependencies. Also opens the door to stunts like runtime addition of plugins.

  • Awesome... Exactly what I need... How do you resolve the action name in trygetinstance... Are you using reflection I'm assuming? I don't see that method in Unity, so I was wondering if this is something you wrote? – KingOfHypocrites Apr 4 '12 at 17:16
  • If i use activator.getinstance, i have to include the assembly name and prepend the namespace to the actionname. This seems a bit goofy. Is this the only way? – KingOfHypocrites Apr 4 '12 at 17:44
  • Sorry, the IoC container in this example is structuremap, not unity nor the activator, which isn't an IoC container. I'll add that the example is a bit dated, you would probably call things in a slightly different way with current versions of structuremap. – Wyatt Barnett Apr 4 '12 at 17:57
0

This is a command pattern written for django 1.4

It can be used the following way:

    p = DesignPattern()
    imps = Implementation.objects.filter(project='Example')
    DesignPatterImplementationsM2MBind(p, imps) 

Django application models.py file:

    class AbstractM2MBindCommand(AbstractCommand):
        """
        Realizes behaviour design pattern `Command'
        """
        model = None
        model_m2m_field = None

        MAX_COMMA_SEPARATED_LENGTH = 1024

        bound_objects = models.CommaSeparatedIntegerField(max_length=MAX_COMMA_SEPARATED_LENGTH, null=True, blank=True, default=None)
        bound_objects_long = models.TextField(null=True, blank=True, default=None)

        class Meta:
            abstract = True

        @classmethod
        def do(cls, model_instance, queryset_to_link):
            #assert isinstance(queryset_to_link, QuerySet)
            setattr(model_instance, cls.model_m2m_field, queryset_to_link)

            ids =  [u'%d' % item.pk for item in queryset_to_link]

            model_instance.save()

            fk_field = camel_to_underscore_convert(cls.model.__name__)

            cmd = cls()
            cmd.direction = cls.FORWARD
            joined_ids = u','.join(ids)
            if len(joined_ids) > cls.MAX_COMMA_SEPARATED_LENGTH:
                cmd.bound_objects_long = joined_ids
                cmd.bound_objects = None
            else:
                cmd.bound_objects_long = None
                cmd.bound_objects = u','.join(ids)
            setattr(cmd, fk_field, model_instance)
            cmd.save()


        @classmethod
        def undo(cls, command_id):
            pass

         @classmethod
         def redo(cls, command_id):
             pass 


    class DesignPatterImplementationsBindCommand(AbstractM2MBindCommand):
        model = DesignPattern
        model_m2m_field = 'implementations'

        design_pattern = models.ForeignKey(DesignPattern)

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