add entity store class (#70)

src/core/utils/createSubject.ts

@@ -0,0 +1,23 @@
+import {createAdapter} from '@most/adapter';
+import {Stream} from '@most/types';
+import {hold} from '@most/hold';
+
+export type Subject<T> = {
+    stream$: Stream<T>;
+    next: (val: T) => void;
+    getValue: () => T;
+}
+export const createSubject = <T>(data: T): Subject<T> => {
+    let cache = data;
+
+    const [handler, stream$] = createAdapter<T>();
+
+    return {
+        next: (val: T) => {
+            cache = val;
+            handler(val);
+        },
+        stream$: hold(stream$),
+        getValue: () => cache
+    };
+};