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Factory

How would you usually create a Singleton in Android?

class MyModel{
    private static volatile MyModel instance;
    
    public static synchronized MyModel getInstance(){
        if(instance == null){
            instance = new MyModel(); 
        }
        return instance;
    }
}

class A{
    MyModel myModel = MyModel.getInstance();
}
class B{
    MyModel myModel = MyModel.getInstance();
}

How to do it with Life Cycle?

class MyModel implements Singleton{
    
}

class A{
    MyModel myModel = Factory.inject(MyModel.class);
}
class B{
    MyModel myModel = Factory.inject(MyModel.class);
}

Both classes A and B have the same reference to MyModel class.

• In addition to that you can replace singleton with extended version of it in all the places where it been use.
• When exit the application, you can free all the singletons in your system at once and allow GC to collect them.

• To inject class with constructor parameters, call Factory.injectWithParams()

Signals

A method of sharing information between classes, aka events.
How would you usualy create a custom event in Android?

interface SayHi
{
    void onSayHi()
}

class SayHiEvent{
    
    private static volatile SayHiEvent instance;
    ArrayList<SayHi> listeners = new ArrayList<SayHi>();
    
    public static synchronized SayHiEvent getInstance(){
        if(instance == null){
            instance = new SayHiEvent(); 
        }
        return instance;
    }

    public void addListener(SayHi listener){
        listeners.add(listener);
    }
    
    public void removeListener(SayHi listener){
        listeners.remove(listener);
    }
    
    public void dispatch(){
        for(SayHi listener : listeners){
            listener.handleSayHi();
        }
    }

}

class A{
    SayHiEvent event = SayHiEvent.getInstance();

    void run(){
        event.dispatch();
    }
    
}

class B implements SayHi{
    SayHiEvent event = SayHiEvent.getInstance();
 
    void init(){
        event.addListener(this);
    }
 
    void onSayHi(){
        System.out.println("Hello world!");
    }
 
}

How to do it with Life Cycle?

interface SayHiSignal
{
    void onSayHi()
}

class A{
    Signal<SayHiSignal> signal = SignalsBag.inject(SayHiSignal.class);

    void run(){
        signal.dispatcher.onSayHi();
    }
    
}

class B implements SayHiSignal{
    Signal<SayHiSignal> signal = SignalsBag.inject(SayHiSignal.class);
 
    void init(){
        signal.addListener(this);
    }
 
    void onSayHi(){ 
        System.out.println("Hello world!");
    }
 
}

• There could be multiple listeners for each signal
• It is possible to register and unregister from signal.
• Also signal got the method addListenerOnce(), it will automaticaly unregister the listener after the first dispatch of the signal
• SignalsBag got two methods: inject and create, "inject" is to use signal as singleton and "create" to create new instance of the signal.