Observer pattern

When communicating between modules, there are some cases which are lower modules deliver message to higher modules. For example, when user acts to connect blocks. Canvas module, which is a lower module of the GUI module, needs to send message to GUI module. At this moment, the module that the user actually interacts with is Canvas. But an arrow, which is drawn for showing how the blocks will be connected, is on the GUI module. So although user interacts with Canvas, Canvas must request message to the GUI module to generate an arrow. Sadly, however, Canvas is a lower module of the GUI module and does not know the interface of the GUI. To resolve this issue, Dluid use Observer pattern for reverse communication between modules.

So shortly from the above example

1. GUI module registers an observer for drawing an arrow to observable of Canvas module.
2. if the Canvas module wants to deliver the message to the Gui module, the Canvas module calls setChanged() and notifyObserver() to Observable.

Custom usage

As mentioned earlier, each module has one singleton and one facade. And there is one state machine in a singleton.

State machine

The state machine is the only one observable that exists in each module. The reason why Dluid limits availability of observable to one per module is simply because of ease of management. When notification occurs, observers registered in the state machine decide whether they should act or not by looking at the action type. At this point, some concepts like action, payload, dispatch, reducer are used in the Dluid. These concepts were inspired from the web front-end library, Redux.

Action

Action is a parameter passed by notifyObserver method. There are two types of information, type and payload. Observers check the action type for determining whether message came to me or not. Action type is a predefined enumerator.

public class Action {
    private ActionType type;
    private Object payload;
}

payload

Payload is additional data that you want to send to the Observer through action.

dispatch

It is the act of making an action through a state machine.

reducer

Abstract class that inherits the observer . This class has some method like support and update. support method is method that determines what type of action it is and an update method that determines how it works. Observers using state machine must inherit Reducer.

public abstract class Reducer implements Observer {
    // ...
    public abstract boolean support(Action action);
    public abstract void update(Observable o, Action action);
}