Kiichigo

Most instances of IAction also implement IGuarded interface, that allows programmer to control when certain actions will be executed. It acts a lot like guards from declarative languages, such as Prolog, Erlang or Scala. Essentially speaking: if IGuard.when evaluates into true IAction will be executed successfully, otherwise it won’t.

Predicate for guard ( value of IGuard.when ) can be in a form of: Boolean, function ():Boolean or function ( percept:Object ):Boolean. Here is small example with fluent dsl:

var router:IRouter = new Router().
	route( Handle, { event: LoginEvent.LOGIN } ).
		run( Run, { closure: Alert.show, arguments: ["Logging in "] } ).
			when( check ).
end;

protected function check( percept:LoginEvent ):Boolean
{
	if( percept.username == null || percept.username.length == 0 ||
	    percept.password == null || percept.password.length == 0 )
		return false;
	return true;
}

Action Run will be executed only if incoming LoginEvent has defined username and password. Of course, you will prefer sophisticated validation than simple length check.

Following example shows how to use multiple IGuarded.when predicates with some Functional Programming mojo:

var router:IRouter = new Router().
	route( Handle, { event: LoginEvent.LOGIN } ).
		run( Run, { closure: Alert.show, arguments: ["Logging in "] } ).
			when( check( "username" ) ).
			when( check( "password" ) ).
end;

/**
 * Checks if <code>String</code> that's evaluated via <code>percept[property]</code> is not empty and is not <code>null</code>.
 *
 * @param	property	<code>String</code> name of property on <code>percept</code> argument.
 * @param	percept		<code>percept</code> received by <code>IRoute</code> for testing.
 *
 * @returns	<code>Boolean</code>. <code>true</code> if string is not <code>null</code> and contains at least 1 symbol, <code>false</code>otherwise</code>.
 */
protected function checkString( property:String, percept:LoginEvent ):Boolean
{
	var string:String = percept[property];
	if( string == null || string.length == 0 )
		return false;
	return true;
}

/**
 * Higher order function. Simplistic Curry function.
 */
protected function check( property:String ):Function
{
	return function( percept:LoginEvent ):Boolean
	{
		return checkString( property, percept );
	}
}

I’ve used Higher Order Function, to construct actual predicates. If you have some experience working with Functional Programming, you will find this methods more elegant and logical.

MXML syntax also allows us to use Data Bindings in this example:

<route:Router id="router">
	<route:Handle event="{ LoginEvent.LOGIN }">
		<route:Run closure="{ Alert.show }" arguments="{ ['Logging in'] }"
			   when="{ router.percept.username != null && router.percept.username.length > 0 &&
					   router.percept.password != null && router.percept.password.length > 0 }" />
	</route:Handle>
</route:Router>

No additional functions are requited.

This is rather simple, yet very powerful mechanism of controlling execution cycle of application, without need to create special actions that will check data before execution that can be achieved by simple injection of a guard.

Example and Sources

Futher Reading

About Guards on Wikipedia.