Add push$ to Collection; make Container listen for retroactive blueprint changes

src/util/collection/Collection.ts

@@ -1,4 +1,5 @@
 import {AsyncPipe, Pipeline} from '../support/Pipe'
+import {Unsubscribe, Subscription} from '../support/BehaviorSubject'
 
 type CollectionItem<T> = T
 type MaybeCollectionItem<T> = CollectionItem<T> | undefined
@@ -50,6 +51,8 @@ export {
 class Collection<T> {
     private storedItems: CollectionItem<T>[] = []
 
+    private pushSubscribers: Subscription<T>[] = []
+
     /**
      * Create a new collection from an array of items.
      * @param items
@@ -966,6 +969,7 @@ class Collection<T> {
      */
     prepend(item: CollectionItem<T>): Collection<T> {
         this.storedItems = [item, ...this.storedItems]
+        this.callPushSubscribers(item)
         return this
     }
 
@@ -975,9 +979,33 @@ class Collection<T> {
      */
     push(item: CollectionItem<T>): Collection<T> {
         this.storedItems.push(item)
+        this.callPushSubscribers(item)
         return this
     }
 
+    /**
+     * Subscribe to listen for items being added to the collection.
+     * @param sub
+     */
+    push$(sub: Subscription<T>): Unsubscribe {
+        this.pushSubscribers.push(sub)
+        return {
+            unsubscribe: () => this.pushSubscribers = this.pushSubscribers.filter(x => x !== sub),
+        }
+    }
+
+    /** Helper to notify subscribers that an item has been pushed to the collection. */
+    private callPushSubscribers(item: T): void {
+        this.pushSubscribers
+            .forEach(sub => {
+                if ( typeof sub === 'object' ) {
+                    sub?.next?.(item)
+                } else {
+                    sub(item)
+                }
+            })
+    }
+
     /**
      * Push the given items to the end of this collection.
      * Unlike `merge()`, this mutates the current collection's items.
@@ -987,6 +1015,7 @@ class Collection<T> {
         const concats = items instanceof Collection ? items.all() : items
         for ( const item of concats ) {
             this.storedItems.push(item)
+            this.callPushSubscribers(item)
         }
         return this
     }