JavaScript Behavioral Design Patterns Command
Example
The command pattern encapsulates parameters to a method, current object state, and which method to call. It is useful to compartmentalize everything needed to call a method at a later time. It can be used to issue a "command" and decide later which piece of code to use to execute the command.
There are three components in this pattern:
- Command Message - the command itself, including the method name, parameters, and state
- Invoker - the part which instructs the command to execute its instructions. It can be a timed event, user interaction, a step in a process, callback, or any way needed to execute the command.
- Reciever - the target of the command execution.
Command Message as an Array
var aCommand = new Array();
aCommand.push(new Instructions().DoThis); //Method to execute
aCommand.push("String Argument"); //string argument
aCommand.push(777); //integer argument
aCommand.push(new Object {} ); //object argument
aCommand.push(new Array() ); //array argument
Constructor for command class
class DoThis {
constructor( stringArg, numArg, objectArg, arrayArg ) {
this._stringArg = stringArg;
this._numArg = numArg;
this._objectArg = objectArg;
this._arrayArg = arrayArg;
}
Execute() {
var receiver = new Instructions();
receiver.DoThis(this._stringArg, this._numArg, this._objectArg, this._arrayArg );
}
}
Invoker
aCommand.Execute();
Can invoke:
- immediately
- in response to an event
- in a sequence of execution
- as a callback response or in a promise
- at the end of an event loop
- in any other needed way to invoke a method
Receiver
class Instructions {
DoThis( stringArg, numArg, objectArg, arrayArg ) {
console.log( `${stringArg}, ${numArg}, ${objectArg}, ${arrayArg}` );
}
}
A client generates a command, passes it to an invoker that either executes it immediately or delays the command, and then the command acts upon a receiver. The command pattern is very useful when used with companion patterns to create messaging patterns.
