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diff --git a/bundles/org.simantics.browsing.ui/src/org/simantics/browsing/ui/content/Viewpoint.java b/bundles/org.simantics.browsing.ui/src/org/simantics/browsing/ui/content/Viewpoint.java
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+/*******************************************************************************
+ * Copyright (c) 2007, 2010 Association for Decentralized Information Management
+ * in Industry THTH ry.
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License v1.0
+ * which accompanies this distribution, and is available at
+ * http://www.eclipse.org/legal/epl-v10.html
+ *
+ * Contributors:
+ *     VTT Technical Research Centre of Finland - initial API and implementation
+ *******************************************************************************/
+package org.simantics.browsing.ui.content;
+
+import org.simantics.browsing.ui.GraphExplorer;
+import org.simantics.browsing.ui.NodeContext;
+import org.simantics.browsing.ui.PrimitiveQueryUpdater;
+
+/**
+ * A Viewpoint is used for describing the child node generation of a single UI
+ * (e.g. tree) node. Viewpoints are created on a per UI node basis, including
+ * the invisible root input of provided to
+ * {@link GraphExplorer#setRoot(Object)}. Viewpoints are created by
+ * {@link ViewpointFactory}s.
+ * 
+ * <p>
+ * A Viewpoint is responsible for informing of any changes happening in the set
+ * of children provided by it. Updates are performed using the
+ * {@link PrimitiveQueryUpdater} received by the {@link ViewpointFactory} that
+ * created the Viewpoint. To signal that the set of children has (possibly)
+ * changed, invoke
+ * {@link PrimitiveQueryUpdater#scheduleReplace(NodeContext, org.simantics.browsing.ui.NodeContext.PrimitiveQueryKey, Object)}
+ * and eventually the query system will re-request the viewpoint for the new
+ * children.
+ * </p>
+ * 
+ * @author Antti Villberg
+ * 
+ * @see ViewpointStub
+ */
+public interface Viewpoint {
+
+    static final NodeContext[] PENDING_CHILDREN     = {};
+    static final Boolean        PENDING_HAS_CHILDREN = new Boolean(false);
+
+    /**
+     * @return an array of child contexts
+     */
+    NodeContext[] getChildren();
+
+    /**
+     * @return <code>true</code> if the viewpoint <em>may</em> produce > 0,
+     *         <code>false</code> otherwise.
+     */
+    Boolean getHasChildren();
+
+}