Why I think @UIEventTopic / @EventTopic have been a bad idea

I’ve been working on a document since some time who holds my thoughts on the future direction of Eclipse 4 or better the Eclipse 4 Application Platform.

At first I’d like to mention that these as my personal views, not aligned with the e4 committers, so you’ll most likely find others who disagree.

It’s not my intention put e4 or any e4 committer down and if I’d blame someone it would be myself because I was part of the e4 effort since day 1.

The document is not yet ready but inspired by a tweet from Marcel Bruch

I’d like to take one of the points I’ll mention in this big vision/reflection document and explain why I think in retrospect providing those 2 annotations is wrong.


The first problem I have with those annotations is not an architectural one but caused by the bundle they are currently shipped in. We currently find them in “org.eclipse.e4.ui.di” and “org.eclipse.e4.core.di.extensions” who themselves require things like the Eclipse-DI-container, Equinox-container.

In my world of e4 components this is a deal breaker, as I want my code have no dependency on any of those bundles at compile time nor runtime but in general I think this is a minor problem and fixable. The architectural is the real blocker.

Event data is different to any other DI-Information

The real problem I have with @UIEventTopic and @EventTopic is the information they carry with them is totally different to any other data available in the DI-System (starting from OSGi-Services to Preference or IContextFunction derived values) because it’s only temporary. I think this characteristic makes @UIEventTopic/@EventTopic data not suited for DI.

The guess I have for those annotations being so popular is that they free you from the useage of IEventBroker (who leaks OSGi at an even larger scale by showing OSGi-Classes in its API, once more for no good reason) and the verbose code you need to write to subscribe (wild guess once more nobody was really worried about the OSGi-Class-Leakage).

void init(IEventBroker b) {
   b.subscribe( "my/event/Topic", new org.osgi.service.event.EventHandler() {
     public void handle(org.osgi.service.event.Event event) {
       handleEvent( (String)event.getProperty(IEventBroker.DATA) );
   } );

private void handleEvent(String data) {
   // ...

In a Java8 world the verbosity can be reduced with the help of lamdas and method refs

import static my.sample.Util.extractEventData;

void init(IEventBroker b) {
  b.subscribe( "my/event/Topic", extractEventData(this::handleEvent));

private void handleEvent(String data) {
   // ...

// Util.java
public static <T> Consumer<Event> extractEventData( Consumer<T> dataConsumer) {
   return e -> (T)e.getProperty(IEventBroker.DATA);

The extractEventData could be provided by IEventBroker in a Java8 world BTW.

You loose typesafety for no good reason

IEventBroker and @UIEventTopic/@EventTopic trade IMHO typesafety for no good reason.

One can argue that this is the case for DI in general as well but you trade in this case typesafety against loose coupeling freeing your business code from dependencies on large framework (and yes I consider e4 and OSGi large frameworks) and providing you better testing support – so you get something in return.

This is not true for IEventBroker nor is it true for @UIEventTopic/@EventTopic, while IEventBroker could be fixed to provide a certain amount of typesafety (see e(fx)clipse EventBus) this will never be possible for @UIEventTopic/@EventTopic!

One could argue that on the receiver side you’ll have less framework bindings (in case one splits out the annotations from their existing owner bundle) but because the publisher requires the event bus anyways I don’t see any reason the receiver should not and the EventBus-Service is easy enough to be mocked in Unit-Tests.

And because it is not unlikely that many events are sent through the event system it puts pressure on the DI system which could have been avoid if you used the EventBus directly.

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