Command Pattern - Purpose?

After reading this: http://sourcemaking.com/design_patterns/command

I sti开发者_如何学Cll don't quite understand why we need this.

The idea is that if commands are encapsulated as objects then those commands can be captured, stored, queued, replayed etc.

It also makes it easier for the command to know how to undo themselves (ie perform the reverse operation) so that then if a command is processed it can be stored in a list and then 'undone' in the reverse order to restore the state before the commands were done.

Also it decouples the sender of the command from the receiver. This can allow multiple things to generate the same command (a menu item and a button for example) and they will be handled in the same way.

It's a good way to encapsulate asynchronous operations and keep their parameters and context in one place.

E.g. a HTTP request: You send a request over a socket, and wait for the response to arrive. If your application is e.g. a web browser, you don't want to block until the request is done but move on. If the response arrives, you have to continue in the context were you stopped, e.g. reading the data and putting it into the right place (e.g. put the downloaded image data somewhere for later rendering). To match the response to the context it belongs to can become tricky if you have one big client class firing off multiple asynchronous operations. Responses might arrive in arbitrary order. Which response belongs to what? What again should be done with the response? How to handle possible errors? If you have those requests encapsulated in commands and let the commands only receive their own response, they'll know better how to continue from there and handle the response. If you have sequences of requests/responses, it's also much easier to keep track of the sequence's state. One can group commands to composite commands (composite pattern). The client passes everything needed to the command, and waits until the command finishes, reporting back either success or error.

Another big advantage is when using multi-threading: if all data needed for the operation is encapsulated in the command object, it's easy to move the command to another thread and have it executed there, without the usual locking headaches you get when sharing objects among threads. Create command, pass everything it needs to it (copy, not by reference), pass to other thread, synchronize only when receiving the result, done.

The command pattern separates the code that knows how to do some work from the code that knows when it needs to be done, and with what parameters.

The most obvious case is a button that knows when you click it, but doesn't know what work to do at that moment. The command pattern lets you pass a do-some-work object to the button, which invokes the object when it is clicked.

Basically, the Command pattern is a way to partially achieve "Function as object" in Java (or C#).

Since you can't just create a function (or method) and do whatever you want with it like pass it as a parameter to some other function or keep it in a variable for later execution, this is the workaround to do that:

1. You wrap some code in a class (this is your execute method).
2. Instantiate the class. Now, this object you have is "a function as an object".
3. You can pass the object as a parameter, keep it around or whatever.
4. Eventually, you'll want to call the execute method.

It describes a solution to a problem. Mainly, that we want to issue commands and don't want to define 30 methods over 8 classes to achieve this. By using the pattern mention, we issue a Command object and the object is free to ignore it, or act on it in someway. The complexity of the Command object is implementation-defined, but this is a nice way to tell objects "hey, do this".

Additionally, because we have encapsulated this in an object we can go further and queue commands, dispatch them at intervals we wish and also revert them (provided of course, that the object you send the command to can 'undo' a Command as well as 'do it').

So, imagine a drawing package that allows you to add shapes to a canvas. Each time the user does this, a command can be issued:

m_Canvas.push_back(new Line(1.0f, 2.0f));
m_Canvas.push_back(new Line(3.5f, 3.1f));
m_Canvas.push_back(new Circle(2.0f, 3.0f, 1.5f));

and so on. This assumed Line and Circle are derived from a common Command base class.

Our renderer can use this canvas collection as a way of rendering and un-doing is simply a case of removing the last performed command. By tracking what the user un-does in a separate collection, we can also redo.