Command
In object-oriented programming, the Command pattern is a design pattern in which an object is used to represent and encapsulate all the information needed to call a method at a later time. This information includes the method name, the object that owns the method and values for the method parameters.
Three terms always associated with the Command pattern are client, invoker and receiver. The client instantiates the command object and provides the information required to call the method at a later time. The invoker decides when the method should be called. The receiver is an instance of the class that contains the method's code.
Using command objects makes it easier to construct general components that need to delegate, sequence or execute method calls at a time of their choosing without the need to know the owner of the method or the method parameters.
When two objects communicate, often one object is sending a command to the other object to perform a particular function. The most common way to accomplish this is for the first object (the "issuer") to hold a reference to the second (the "recipient"). The issuer executes a specific method on the recipient to send the command.
But what if the issuer is not aware of, or does not care who the recipient is? That is, the issuer simply wants to abstractly issue the command?
The Command design pattern encapsulates the concept of the command into an object. The issuer holds a reference to the command object rather than to the recipient. The issuer sends the command to the command object by executing a specific method on it. The command object is then responsible for dispatching the command to a specific recipient to get the job done.
In the above diagram, the invoker holds an abstract command and issues a command by calling the abstract execute() method. This command is translated into a specific action on a specific receiver by the various concrete command objects. It is also possible for a command to be a collection of commands, called a "macro" command. Calling the execute method of a macro command will invoke a collection of commands.
