Actions

Java has a very complicated event-handling API. In many applications such as web page navigators, we just want to describe what to do when a component is clicked. DJ's commands are a very simple construct for describing this kind of actions. A command has the following syntax:

     command(Component,MethodInvocation)

where Component is a component^7.1 and MethodInvocation is a call to a method. The command means that when Component is clicked, then execute MethodInvocation. Only one command can be executed for each component. Trying to associate two method invocations with a component will cause an error.

Commands can appear anywhere a constraint can occur. A command has to take a component as the first argument even if the command appears in a component declaration.

MethodInvocation is a call to a method defined some where. The following method

       showDocument(String name)

is a built-in where name is a name or path for a document to be shown when the corresponding component is clicked. Document names are all relative to the code base. It is unnecessary to specify full URL addresses.

Figure is a program that associates an action with every component in the AllComponents object ac.

  

Figure: AllComponentsWithActions.