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regarding copyright ownership.  The ASF licenses this file
to you under the Apache License, Version 2.0 (the
"License"); you may not use this file except in compliance
with the License.  You may obtain a copy of the License at

  http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing,
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"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
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specific language governing permissions and limitations
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*/

package org.apache.edgent.apps.iot;

import static org.apache.edgent.topology.services.ApplicationService.SYSTEM_APP_PREFIX;

import org.apache.edgent.connectors.iot.Events;
import org.apache.edgent.connectors.iot.IotDevice;
import org.apache.edgent.connectors.iot.QoS;
import org.apache.edgent.connectors.pubsub.PublishSubscribe;
import org.apache.edgent.topology.TStream;
import org.apache.edgent.topology.TopologyElement;

import com.google.gson.JsonObject;

/**
 * Application sharing an {@code IotDevice} through publish-subscribe. <BR>
 * This application shares an {@link IotDevice} across multiple running
 * jobs. This allows a single connection to a back-end message hub to be shared
 * across multiple independent applications, without having to build a single
 * topology.
 * <P>
 * Applications coded to {@link IotDevice} obtain a topology specific
 * {@code IotDevice} using {@link #addIotDevice(TopologyElement)}. This returned
 * device will route events and commands to/from the actual message hub
 * {@code IotDevice} through publish-subscribe.
 * <P>
 * An instance of this application is created by first creating a new topology and
 * then creating a {@link IotDevice} specific to the desired message hub. Then
 * the application is created by calling {@link #createApplication(IotDevice)}
 * passing the {@code IotDevice}. <BR>
 * Then additional independent applications (topologies) can be created and they
 * create a proxy {@code IotDevice} for their topology using
 * {@link #addIotDevice(TopologyElement)}. This proxy {@code IotDevice} is then
 * used to send device events and receive device commands in that topology. <BR>
 * Once all the topologies have been declared they can be submitted.
 * </P>
 * <P>
 * At startup this application sends a single device event with
 * identifier {@link Events#IOT_START}. This performs two functions:
 * </P>
 * <UL>
 * <LI>Initiates a connection to the message hub.</LI>
 * <LI>Allows external applications to be notified (by subscribing to device events)
 * when an Edgent provider starts.</LI>
 * </UL>
 * 
 * @see PublishSubscribe
 */
public class IotDevicePubSub {
    
    /**
     * IotDevicePubSub application name.
     */
    public static final String APP_NAME = SYSTEM_APP_PREFIX + "IotDevicePubSub";
    
    /**
     * Events published to topic {@value} are sent as device events using the
     * actual message hub {@link IotDevice}. <BR>
     * it is recommended applications use the {@code IotDevice} returned by
     * {@link #addIotDevice(TopologyElement)} to send events rather than
     * publishing streams to this topic.
     */
    public static final String EVENTS_TOPIC = PublishSubscribe.RESERVED_TOPIC_PREFIX + "iot/events";
    
    /**
     * Device commands are published to {@value} by
     * this application. <BR>
     * it is recommended applications use the {@code IotDevice} returned by
     * {@link #addIotDevice(TopologyElement)} to receive commands rather than
     * subscribing to streams with this topic prefix.
     */
    public static final String COMMANDS_TOPIC = PublishSubscribe.RESERVED_TOPIC_PREFIX + "iot/commands";

    /**
     * Create an instance of this application using {@code device} as the device
     * connection to a message hub.
     * 
     * @param device the IotDevice
     */
    public static void createApplication(IotDevice device) {

        TStream<JsonObject> events = PublishSubscribe.subscribe(device, EVENTS_TOPIC, JsonObject.class);

        device.events(events, ew -> ew.getAsJsonPrimitive("eventId").getAsString(), ew -> ew.getAsJsonObject("event"),
                ew -> ew.getAsJsonPrimitive("qos").getAsInt());
        
        PublishSubscribe.publish(device.commands(), COMMANDS_TOPIC, JsonObject.class);
        
        // Publish a single event at startup
        TStream<JsonObject> start = device.topology().of(new JsonObject());
        device.events(start, Events.IOT_START, QoS.AT_MOST_ONCE);
    }

    /**
     * Add a proxy {@code IotDevice} for the topology containing {@code te}.
     * <P>
     * Any events sent through the returned device are sent onto the message hub
     * device through publish-subscribe. <BR>
     * Subscribing to commands using the returned device will subscribe to
     * commands received by the message hub device.
     * </P>
     * 
     * @param te
     *            Topology the returned device is contained in.
     * @return Proxy device.
     */
    public static IotDevice addIotDevice(TopologyElement te) {
        return new PubSubIotDevice(te.topology());
    }
}