# Pub sub

## Overview
Publish–subscribe pattern is a messaging pattern that is one of the fundamentals patterns used in reusable software components.

In short, services that implements this pattern, can have listeners subscribed to their broadcasted events. After the event is fired, the corresponding listener will execute the function that is registered.

You can read more about this design pattern [here](https://cloud.google.com/pubsub/docs/overview).


## Example: Default Initialization
In `Mode.jsx` we have a default initialization that demonstrates
a series of subscriptions to various events.


```js
async function defaultRouteInit({
  servicesManager,
  studyInstanceUIDs,
  dataSource,
}) {
  const {
    DisplaySetService,
    HangingProtocolService,
  } = servicesManager.services;

  const unsubscriptions = [];

  const {
    unsubscribe: instanceAddedUnsubscribe,
  } = DicomMetadataStore.subscribe(
    DicomMetadataStore.EVENTS.INSTANCES_ADDED,
    ({ StudyInstanceUID, SeriesInstanceUID, madeInClient = false }) => {
      const seriesMetadata = DicomMetadataStore.getSeries(
        StudyInstanceUID,
        SeriesInstanceUID
      );

      DisplaySetService.makeDisplaySets(seriesMetadata.instances, madeInClient);
    }
  );

  unsubscriptions.push(instanceAddedUnsubscribe);

  studyInstanceUIDs.forEach(StudyInstanceUID => {
    dataSource.retrieveSeriesMetadata({ StudyInstanceUID });
  });

  const { unsubscribe: seriesAddedUnsubscribe } = DicomMetadataStore.subscribe(
    DicomMetadataStore.EVENTS.SERIES_ADDED,
    ({ StudyInstanceUID }) => {
      const studyMetadata = DicomMetadataStore.getStudy(StudyInstanceUID);
      HangingProtocolService.run(studyMetadata);
    }
  );
  unsubscriptions.push(seriesAddedUnsubscribe);

  return unsubscriptions;
}
```

## Unsubscription
You need to be careful if you are adding custom subscriptions to the app. Each subscription will return a unsubscription function that needs to be executed on component destruction to avoid adding multiple subscriptions to the same observer.

Below, we can see `simplified` `Mode.jsx` and the corresponding `useEffect` where the unsubscription functions are executed upon destruction.

```js
// platform/viewer/src/routes/Mode/Mode.jsx
export default function ModeRoute(/**..**/) {
  /**...**/
  useEffect(
    () => {
      /**...**/

      DisplaySetService.init(extensionManager, sopClassHandlers)

      extensionManager.onModeEnter()
      mode?.onModeEnter({ servicesManager, extensionManager })

      hangingProtocols.forEach((extentionProtocols) => {
        const { protocols } =
          extensionManager.getModuleEntry(extentionProtocols)
        HangingProtocolService.addProtocols(protocols)
      })

      const setupRouteInit = async () => {
        if (route.init) {
          return await route.init(/**...**/)
        }

        return await defaultRouteInit(/**...**/)
      }

      let unsubscriptions
      setupRouteInit().then((unsubs) => {
        unsubscriptions = unsubs
      })

      return () => {
        extensionManager.onModeExit()
        mode?.onModeExit({ servicesManager, extensionManager })
        unsubscriptions.forEach((unsub) => {
          unsub()
        })
      }
    },
  )
  return <> /**...**/ </>
}
```