Add system events section to README

This commit is contained in:
Kai S. K. Engelbart 2021-02-21 13:50:12 +01:00
parent 6a2cad4ae5
commit 7c3cd017de
Signed by: kske
GPG Key ID: 8BEB13EC5DF7EF13
2 changed files with 33 additions and 1 deletions

View File

@ -123,6 +123,38 @@ This applies to all event handlers that would have been executed after the one c
Avoid cancelling events while using multiple event handlers with the same priority. 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. As event handlers are ordered by priority, it is not defined which of them will be executed after the event has been consumed.
## System Events
To accommodate for special circumstances in an event distribution, system events have been introduced.
At the moment, there are two system events, which are explained in this section.
### Detecting Unhandled Events
When an event is dispatched but not delivered to any handler, a dead event is dispatched that wraps the original event.
You can declare a dead event handler to respond to this situation:
```java
private void onDeadEvent(DeadEvent deadEvent) { ... }
```
### Detecting Exceptions Thrown by Event Handlers
When an event handler throws an exception, an exception event is dispatched that wraps the original event.
A exception handler is declared as follows:
```java
private void onExceptionEvent(ExceptionEvent ExceptionEvent) { ... }
```
Both system events reference the event bus that caused them and a warning is logged if they are unhandled.
### What About Endless Recursion Caused By Dead Events and Exception Events?
As one might imagine, an unhandled dead event would theoretically lead to an endless recursion.
The same applies when an exception event handler throws an exception.
To avoid this, system events never cause system events and instead just issue a warning to the logger.
## Installation ## Installation
Event Bus is available in Maven Central. Event Bus is available in Maven Central.

View File

@ -89,7 +89,7 @@ public final class EventBus {
if (event instanceof DeadEvent || event instanceof ExceptionEvent) if (event instanceof DeadEvent || event instanceof ExceptionEvent)
// Warn about system event not being handled // Warn about system event not being handled
logger.log(Level.WARNING, event + " not handled", e); logger.log(Level.WARNING, event + " not handled due to exception", e);
else else
// Dispatch exception event // Dispatch exception event