view: recompute subscription on query change

* If a view's query changes (e.g. if its variables change) whilst it is
  running. The view will now be un-subscribed, then re-subscribed with
  the new query.

src/mixins/subscriptionComponent.js

@@ -22,8 +22,8 @@ import subscriptionMixin from '@/mixins/subscription'
  * of such component is the GScan component, which declares a query used to
  * list the workflows of the system in the UI sidebar.
  *
- * Uses Vue component lifecycle methods (e.g. created, beforeDestroy) to coordinate
- * when a subscription is created in the WorkflowService service.
+ * Uses Vue component lifecycle methods (e.g. created, beforeDestroy) to
+ * coordinate when a subscription is created in the WorkflowService service.
  *
  * @see Subscription
  * @see SubscriptionQuery
@@ -38,5 +38,13 @@ export default {
   },
   beforeDestroy () {
     this.$workflowService.unsubscribe(this)
+  },
+  watch: {
+    query (query) {
+      console.error('QUERY CHANGED')
+      // if the query changes, unsubscribe & re-subscribe
+      this.$workflowService.unsubscribe(this)
+      this.$workflowService.subscribe(this)
+    }
   }
 }