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Alpine

A lightweight event system for Java 8+

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Setup

The following setup assumes the usage of the Impact Development Maven repo.
Alternatively, the GitHub package may be used.

Gradle

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dependencies {
    compile 'com.github.ZeroMemes:Alpine:3.1.0'
}

Maven

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<dependency>
    <groupId>com.github.ZeroMemes</groupId>
    <artifactId>Alpine</artifactId>
    <version>3.1.0</version>
</dependency>

Tutorial

For starters, we must create an EventBus to handle events and their respective listeners.
Alpine provides a default implementation of EventBus that is configurable through a builder, so we'll be using that:

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public class MyApplication {

    public static final EventBus EVENT_BUS = EventManager.builder()
        .setName("my_application/root") // Descriptive name for the bus
        .setSuperListeners()            // Enable Listeners to receive subtypes of their target
        .build();
}

Specifying a name for the bus is required; although, there is no hard restriction on its uniqueness. Additional settings
such as setSuperListeners can be seen in the documentation of EventBusBuilder.

Now to actually receive events that are posted to the event bus, we'll need to create a Listener object and supply its
generic argument with the type of event we'd like to receive. One of the ways that this can be done by creating a
Listener member variable in a class implementing Subscriber, and annotating it with @Subscribe. Let's do that
in our existing class:

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public class MyApplication implements Subscriber {

    public static final EventBus EVENT_BUS = ...;

    @Subscribe
    private Listener<String> stringListener = new Listener<>(str -> {
        System.out.println(str);
    });
}

In order to use our Listener, we need to create a new instance of the Subscriber implementation and subscribe
it to the event bus.

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public class MyApplication implements Subscriber {

    public static final EventBus EVENT_BUS = ...;

    public static void main(String[] args) {
        MyApplication app = new MyApplication();
        EVENT_BUS.subscribe(app);
    }

    @Subscribe
    private Listener<String> stringListener = new Listener<>(str -> {
        System.out.println(str);
    });
}

An alternative to creating a Subscriber implementation and using annotated Listener fields to receive events
is creating an independent Listener instance and subscribing it directly:

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public class MyApplication {

    public static final EventBus EVENT_BUS = ...;
    
    public static void main(String[] args) {
        EVENT_BUS.subscribe(new Listener<String>(str -> {
            System.out.println(str);
        }));
        // or, alternatively... (println has a String implementation which will get bound to here)
        EVENT_BUS.subscribe(new Listener<String>(System.out::println));
    }
}

In cases where a method reference (::) is used for a Listener body and the underlying method's argument isn't the
Listener target, a ClassCastException can occur during runtime. This is due to incorrect target resolution, and can
be fixed by explicitly specifying the target type:

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public class MyApplication {

    public static final EventBus EVENT_BUS = ...;

    public static void main(String[] args) {
        // Incorrect, can cause ClassCastException upon runtime depending on EventBus configuration
        EVENT_BUS.subscribe(new Listener<String>(Main::supertypeAcceptor));

        // Correct, explicitly defines the target and no runtime exception or unintended behavior will occur
        EVENT_BUS.subscribe(new Listener<>(String.class, Main::supertypeAcceptor));
    }

    // Note the 'Object' argument type, this is what causes the need for an explicit target
    private static void supertypeAcceptor(Object o) {
        System.out.println(o);
    }
}

Providing our Listener with an event, in this case, any String, is straight-forward:

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public class MyApplication {

    public static final EventBus EVENT_BUS = ...;

    public static void main(String[] args) {
        MyApplication app = new MyApplication();
        EVENT_BUS.subscribe(app);
        EVENT_BUS.post("Test");
    }

    @Subscribe
    private Listener<String> stringListener = new Listener<>(str -> {
        // Prints "Test"
        System.out.println(str);
    });
}

Listeners may have filters applied which must pass in order for the body to receive a given event. A listener can have as
many filters as is needed, which are added as the last arguments in the Listener constructor.

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public class MyApplication {

    ...

    @Subscribe
    private Listener<String> stringListener = new Listener<>(str -> {
        // No output, "Test".length() != 3
        System.out.println(str);
    }, new LengthOf3Filter()); // <-- Predicate<? super String>... as last argument to Listener

    // Create nested class implementation of our filter
    public static class LengthOf3Filter implements Predicate<CharSequence> {

        @Override
        public boolean test(CharSequence t) {
            return t.length() == 3;
        }
    }
}

The complete example class can be found in Java and Kotlin.