Hi Peter,

This looks great. I thought about white board pattern but problem is this, 
there is no single mqttProtocolProvider! they are also created through factory 
configuration and bind to device through configuration. Please see the actual 
configuration used in application project here.

https://github.com/daghanacay/com.easyiot.application/blob/master/com.easyiot.heatmap.application/configuration/configuration.json

Correct me if i am wrong but Whiteboard pattern assumes there is only one 
mqttprotocolprovider. Peter, I still can work with the code that you have 
explained but still won't be as clean. I am thinking of using service tracker 
may be in the device but then it is not DS. I think it is an interesting case 
that might be worthy of you experts time :)

PS: please excuse me if i misunderstand you or i am making this something more 
complicated then it should be.

Cheers
Daghan

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-------- Original Message --------
From: Peter Kriens <peter.kri...@aqute.biz>
Sent: Friday, October 14, 2016 05:58 PM
To: OSGi Developer Mail List <osgi-dev@mail.osgi.org>
Subject: Re: [osgi-dev] DS component life cycle.

@Christian: The MQTT client is optional and dynamic. So the activate method 
cannot be used. You need to subscribe/unsubscribe based on the availability of 
the mqtt server.

@Daghan:

Divide and conquer! You’re trying to do multiple responsibilities in one 
component and that is the antithesis of modularity, also called lack of 
cohesion. This is a perfect example of how things could get simpler by choosing 
the right decomposition.

The best solution imho is to introduce a second,component. Let the Device 
component just be a Device, it should not have to worry about Mqtt. After all, 
you could be connected to other event queues. (It also makes it easier to 
test.) You made mqtt optional to reflect this. This is exactly the reason the 
Whiteboard Pattern was invented!

This would look like:


        @Component( property = “subscriptionChannel=” )
        public class LT100HDeviceImpl implements Device, TtnMqttMessageListener 
{
                // remove the mqtt subscription code but
                // implement the TtnMqttMessageListener
       }

And the whiteboard component. Notice the _ in the _mqtt field. This ensures it 
is set before the event method that has a name that will appear in a sorted 
list later. (References are sorted by name.) (I vaguely recall field references 
are done before bind methods but this makes it certain.)

        @Component(immediate=true)
        public class MQTTWhiteboard {

                @Reference
                TtnMqttProtocol _mqtt;

                @Reference( cardinality = ReferenceCardinality.OPTIONAL, policy 
= ReferencePolicy.DYNAMIC )
                void addMqttListener( TtnMqttMessageListener l, 
Map<String,Object> props ) {
                        String channel = props.get( “subscriptionChannel” );
                        if ( channel != null && !channel.isEmpty() ) {
                                _mqtt. subscribe( channel, l );
                }

                void removeMqttListener( TtnMqttMessageListener l, 
Map<String,Object> props ) {
                        String channel = props.get( “subscriptionChannel” );
                        if ( channel != null && !channel.isEmpty() ) {
                                _mqtt.unsubscribe( channel );

                }
        }

You now created a whiteboard service that can also be used by other Device 
implementations while significantly reducing the complexity of your 
implementation.

This is why after all those years I still love OSGi … you can only do this when 
you have dynamic components. As your struggle showed, trying to manage this is 
quickly becoming quite complex. Whenever you enter in such a struggle, think, 
lack of cohesion is often your problem.

Kind regards,

        Peter Kriens


>
> On 14 okt. 2016, at 08:09, Christian Schneider <ch...@die-schneider.net> 
> wrote:
>
> In your case simply inject the ttnMqttClient in the @Reference and do the 
> subscribe in @Activate when you get the config and the unsubscribe in 
> @Deactivate.
>
> Christian
>
> 2016-10-13 23:00 GMT+02:00 Daghan ACAY <daghana...@hotmail.com>:
> Hi all,
>
> I am trying to create a component that is instantiated by ConfigAdmin and 
> uses multiple references to operate. Basically the component should 
> instantiate through a factory configuration and use that configuration to set 
> up its own @Reference s. You can see the code here:
>
> https://github.com/daghanacay/com.easyiot.device/blob/master/com.easyiot.LT100H.device.provider/src/com/easyiot/LT100H/device/provider/LT100HDeviceImpl.java
>
> All the mentioned @Reference ed components are instantiated by configuration 
> as well, so at a given time the @Reference might not be available but my own 
> component should still work. yet should the Reference available then it 
> should be injected, basic 0-1 strategy.
>
> Problem I am facing with the current form of the code is that, the @Reference 
> injection is happening before the @Activate method is called. This leads to 
> NPE in the @Reference method due to null configuration. Is it possible to 
> make this code work such that config is provided to the component before the 
> dependency injection?
>
> I have tried annotating the class fields and set them "volatile". I even make 
> them a list and use the class fields in the activate method this time the 
> class fields were null due to 0-1 strategy. so I end up with annotating the 
> methods.
>
> I might have designed this all wrong, so any help simple or fundamental is 
> appreciated.
>
> Regards
>
> -Daghan
>
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>
>
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>
>
> --
> --
> Christian Schneider
> http://www.liquid-reality.de
>
> Open Source Architect
> http://www.talend.com
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