event-bus/README.md

# Event Bus

## Introduction

This library allows passing events between different objects without them having a direct reference to each other.
Any object can serve as an event.

Using an instance of the `EventBus` class, an instant of the event class can be dispatched.
This means that it will be forwarded to all listeners registered for it at the event bus.

In addition, a singleton instance of the event bus is provided by the `EventBus#getInstance()` method.

To listen to events, register event handling methods using the `Event` annotation.
For this to work, the method must have a return type of `void` and declare a single parameter of the desired event type.
Alternatively, a parameter-less event handler can be declared as shown [below](#parameter-less-event-handlers).

## A Simple Example

Lets look at a simple example: we declare the empty class `SimpleEvent` whose objects can be used as events.

```java
public class SimpleEvent {}
```

Next, an event listener for the `SimpleEvent` is declared:

```java
import dev.kske.eventbus.core.*;

public class SimpleEventListener {

    public SimpleEventListener() {

        // Register this listener at the event bus
        EventBus.getInstance().register(this);

        // Dispatch a SimpleEvent
        EventBus.getInstance().dispatch(new SimpleEvent());
    }

    @Event
    private void onSimpleEvent(SimpleEvent event) {
        System.out.println("SimpleEvent received!");
    }
}
```

In this case, an event bus is created and used locally.
In a more sophisticated example the class would acquire an external event bus that is used by multiple classes.

Note that creating static event handlers like this

```java
@Event
private static void onSimpleEvent(SimpleEvent event) { ... }
```

is technically possible, however you would still have to create an instance of the event listener to register it at an event bus.

## Polymorphic Event Handlers

On certain occasions it's practical for an event handler to accept both events of the specified type, as well as subclasses of that event.
To include subtypes for an event handler, use the `@Polymorphic` annotation in addition to `@Event`:

```java
@Event
@Polymorphic
private void onSimpleEvent(SimpleEvent event) { ... }
```

## Event Handler Execution Order

Sometimes when using multiple handlers for one event, it might be useful to define in which order they will be executed.
Event Bus assigns a priority to every handler, which is `100` by default, but can be explicitly set using the `@Priority` annotation in addition to `@Event`:

```java
@Event
@Priority(250)
private void onSimpleEvent(SimpleEvent event) { ... }
```

**Important:**
Events are dispatched to handlers in descending order of their priority.
The execution order is undefined for handlers with the same priority.

## Parameter-Less Event Handlers

In some cases an event handler is not interested in the dispatched event instance.
To avoid declaring a useless parameter just to specify the event type of the handler, there is an alternative:

```java
@Event(SimpleEvent.class)
private void onSimpleEvent() {
	System.out.println("SimpleEvent received!");
}
```

Make sure that you **do not** both declare a parameter and specify the event type in the annotation, as this would be ambiguous.

## Event Consumption

In some cases it might be useful to stop the propagation of an event.
Event Bus makes this possible with event consumption:

```java
@Event(SimpleEvent.class)
@Priority(100)
private void onSimpleEvent() {
	EventBus.getInstance().cancel();
}

@Event(SimpleEvent.class)
@Priority(50)
private void onSimpleEvent2() {
	System.out.println("Will not be printed!");
}
```

In this example, the second method will not be executed as it has a lower priority and the event will not be propagated after consumption.
This applies to all event handlers that would have been executed after the one consuming the event.

**Important:**
Avoid cancelling events while using multiple event handlers with the same priority.
As event handlers are ordered by priority, it is not defined which of them will be executed after the event has been consumed.

## Installation

Event Bus is currently hosted at [kske.dev](https://kske.dev/maven-repo/dev/kske/event-bus/).
To include it inside your project, just add the Maven repository and the dependency to your `pom.xml`:

```xml
<repositories>
	<repository>
		<id>kske-repo</id>
		<url>https://kske.dev/maven-repo</url>
	</repository>
</repositories>

<dependencies>
    <dependency>
        <groupId>dev.kske</groupId>
        <artifactId>event-bus</artifactId>
        <version>1.0.0</version>
    </dependency>
</dependencies>
```

Then, require the Event Bus Core module in your `module-info.java`:

```java
requires dev.kske.eventbus.core;
```

If you intend to use event handlers that are inaccessible to Event Bus by means of Java language access control, make sure to allow reflective access from your module:

```java
opens my.module to dev.kske.eventbus.core;
```

## Compile-Time Error Checking with Event Bus AP

To assist you with writing event listeners, the Event Bus AP (Annotation Processor) module enforces correct usage of the `@Event` annotation during compile time.
This reduces difficult-to-debug bugs that occur during runtime to compile-time errors which can be easily fixed.

The event annotation processor detects invalid event handlers and event type issues with more to come in future versions.

When using Maven, it can be registered using the Maven Compiler Plugin:

```xml
<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>3.8.1</version>
    <configuration>
        <annotationProcessorPaths>
            <annotationProcessorPath>
                <groupId>dev.kske</groupId>
                <artifactId>event-bus-ap</artifactId>
                <version>1.0.0</version>
            </annotationProcessorPath>
        </annotationProcessorPaths>
    </configuration>
</plugin>
```

Alternatively, a JAR file containing the processor is offered with each release for the use within IDEs and environments without Maven support.