Fix item channel links not properly initialized

Fixes openhab#1596

Signed-off-by: Wouter Born <github@maindrain.net>

bundles/org.openhab.core.thing/src/main/java/org/openhab/core/thing/internal/ChannelLinkNotifier.java

@@ -0,0 +1,99 @@
+/**
+ * Copyright (c) 2010-2020 Contributors to the openHAB project
+ *
+ * See the NOTICE file(s) distributed with this work for additional
+ * information.
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License 2.0 which is available at
+ * http://www.eclipse.org/legal/epl-2.0
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ */
+package org.openhab.core.thing.internal;
+
+import java.util.function.Consumer;
+
+import org.eclipse.jdt.annotation.NonNullByDefault;
+import org.openhab.core.common.registry.RegistryChangeListener;
+import org.openhab.core.thing.ChannelUID;
+import org.openhab.core.thing.Thing;
+import org.openhab.core.thing.ThingRegistry;
+import org.openhab.core.thing.ThingUID;
+import org.openhab.core.thing.binding.ThingHandler;
+import org.openhab.core.thing.link.ItemChannelLink;
+import org.openhab.core.thing.link.ItemChannelLinkRegistry;
+import org.openhab.core.thing.util.ThingHandlerHelper;
+import org.osgi.service.component.annotations.Activate;
+import org.osgi.service.component.annotations.Component;
+import org.osgi.service.component.annotations.Deactivate;
+import org.osgi.service.component.annotations.Reference;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * The {@link ChannelLinkNotifier} notifies initialized thing handlers of channels being linked or unlinked.
+ *
+ * @author Łukasz Dywicki - Initial contribution
+ * @author Wouter Born - Initial contribution
+ */
+@Component(immediate = true)
+@NonNullByDefault
+public class ChannelLinkNotifier implements RegistryChangeListener<ItemChannelLink> {
+
+    private final Logger logger = LoggerFactory.getLogger(ChannelLinkNotifier.class);
+
+    private final ItemChannelLinkRegistry itemChannelLinkRegistry;
+    private final ThingRegistry thingRegistry;
+
+    @Activate
+    public ChannelLinkNotifier(@Reference ItemChannelLinkRegistry itemChannelLinkRegistry,
+            @Reference ThingRegistry thingRegistry) {
+        this.itemChannelLinkRegistry = itemChannelLinkRegistry;
+        this.thingRegistry = thingRegistry;
+
+        itemChannelLinkRegistry.addRegistryChangeListener(this);
+        // registry does not dispatch notifications about existing links to listeners
+        itemChannelLinkRegistry.getAll().forEach(this::added);
+    }
+
+    @Deactivate
+    public void deactivate() {
+        itemChannelLinkRegistry.removeRegistryChangeListener(this);
+    }
+
+    @Override
+    public void added(ItemChannelLink element) {
+        ChannelUID channelUID = element.getLinkedUID();
+        ThingUID thingUID = channelUID.getThingUID();
+
+        call(thingUID, handler -> handler.channelLinked(channelUID), "channelLinked");
+    }
+
+    @Override
+    public void removed(ItemChannelLink element) {
+        ChannelUID channelUID = element.getLinkedUID();
+        ThingUID thingUID = channelUID.getThingUID();
+
+        call(thingUID, handler -> handler.channelUnlinked(channelUID), "channelUnlinked");
+    }
+
+    @Override
+    public void updated(ItemChannelLink oldElement, ItemChannelLink element) {
+        removed(oldElement);
+        added(element);
+    }
+
+    private void call(ThingUID thingUID, Consumer<ThingHandler> consumer, String method) {
+        Thing thing = thingRegistry.get(thingUID);
+        if (thing != null) {
+            ThingHandler handler = thing.getHandler();
+            if (handler != null && ThingHandlerHelper.isHandlerInitialized(handler)) {
+                consumer.accept(handler);
+            } else {
+                logger.debug("Skipping notification to thing {} handler '{}' method call, as it is not initialized",
+                        thingUID, method);
+            }
+        }
+    }
+}