Command Design Pattern

The command pattern is best used when you have a command that must be executed by one object, while another object has the responsibility of deciding WHEN that command will be executed.

The three entities associated with the command pattern are:

client: the object that CREATES the command object and assigns it's receiver receiver: the object that HAS the method that will be run invoker: the object that DECIDES WHEN the command will be run
Now let's look at an example. You have an IDE, and the 'undo' command is extremely important. Every action that is performed on the IDE has a method called undo() on it. But the actions have to be stored somewhere after they perform their task though, so that it's possible to call the undo() methods later, right? The solution is to push each action onto a stack when it's run, and register a listener for the 'undo' keyboard-combination/menu-item. The listener will pop() the stack, and invoke the popped action's undo method...voila!

So what are the entities? The module in the IDE that pushes each action onto the stack is the client, each action on the stack is the receiver, and the listener is the invoker. The actions can also be thought of as command objects.


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