Bug 250133 [Contexts] ContextAction's leak when hiding actions sets
bundles/org.eclipse.core.commands/src/org/eclipse/core/commands/contexts/ContextManager.java
}
		
		if (activeContextIds.contains(contextId)) {
			return;
		}
		activeContextsChange = true;
		if (caching) {
			activeContextIds.add(contextId);
		} else {
			final Set previouslyActiveContextIds = new HashSet(activeContextIds);
			activeContextIds.add(contextId);
			fireContextManagerChanged(new ContextManagerEvent(this, null,
					false, true, previouslyActiveContextIds));
		}
		if (DEBUG) {
			Tracing.printTrace("CONTEXTS", activeContextIds.toString()); 
		}
	}
	/**
	 * Adds a listener to this context manager. The listener will be notified
	 * when the set of defined contexts changes. This can be used to track the
	 * global appearance and disappearance of contexts.
	 * 
	 * @param listener
	 *            The listener to attach; must not be <code>null</code>.
	 */
	public final void addContextManagerListener(
			final IContextManagerListener listener) {
		addListenerObject(listener);
	}