ServiceHelper: load EventService more aggressively

Whenever we have @eventhandler annotated methods, we need an
EventService to be loaded as a dependency. Otherwise, even if the
EventService is loaded later, those event handlers will not be properly
registered with it, since at the time registration was attempted, the
EventService was not yet loaded and hence null.

This change obviates the need for DefaultEventService being set to an
absurdly high priority, as explained at:

imagej/ImageJ@b9abe17

Author: ctrueden

src/main/java/org/scijava/service/ServiceHelper.java

@@ -41,6 +41,7 @@
 import org.scijava.AbstractContextual;
 import org.scijava.Context;
 import org.scijava.Optional;
+import org.scijava.event.EventHandler;
 import org.scijava.event.EventService;
 import org.scijava.log.LogService;
 import org.scijava.log.StderrLogService;
@@ -305,6 +306,11 @@ private <S extends Service> S createServiceRecursively(final Class<S> c)
 		final Double priority = classPoolMap.get(c);
 		if (priority != null) service.setPriority(priority);
 
+		// NB: If there are any @EventHandler annotated methods, we treat the
+		// EventService as a required dependency, _unless_ there is also an
+		// EventService field annotated with @Parameter(required = false).
+		boolean eventServiceRequired = true;
+
 		// populate service parameters
 		final List<Field> fields =
 			ClassUtils.getAnnotatedFields(c, Parameter.class);
@@ -332,10 +338,18 @@ private <S extends Service> S createServiceRecursively(final Class<S> c)
 				// recursively obtain needed service
 				final boolean required = f.getAnnotation(Parameter.class).required();
 				s = loadService(serviceType, required);
+				// NB: Remember when there is an optional EventService parameter.
+				if (s instanceof EventService) eventServiceRequired = required;
 			}
 			ClassUtils.setValue(f, service, s);
 		}
 
+		// check for event handlers
+		if (!ClassUtils.getAnnotatedMethods(c, EventHandler.class).isEmpty()) {
+			// NB: There are @EventHandler methods; we need an EventService.
+			loadService(EventService.class, eventServiceRequired);
+		}
+
 		service.initialize();
 		service.registerEventHandlers();
 		return service;