feat: add state$ and getState

affects: @userlike/messenger-types

packages/messenger-types/src/Observable.ts

@@ -0,0 +1,36 @@
+/**
+ * This is the interface for observables as defined in
+ * https://github.com/tc39/proposal-observable. It's a proposal, but lots of
+ * libraries implement this interface. Most prominent one is rxjs, which is also
+ * used by Angular.
+ */
+export interface Observable<T> {
+  // Subscribes to the sequence with an observer
+  subscribe(observer: Observer<T>): Subscription;
+
+  // Subscribes to the sequence with callbacks
+  subscribe(
+    onNext: (v: T) => void,
+    onError?: (err: unknown) => void,
+    onComplete?: () => void
+  ): Subscription;
+}
+
+export interface Subscription {
+  // Cancels the subscription
+  unsubscribe(): void;
+
+  // A boolean value indicating whether the subscription is closed
+  closed: boolean;
+}
+
+export interface Observer<T> {
+  // Receives the next value in the sequence
+  next(value: T): void;
+
+  // Receives the sequence error
+  error(errorValue: unknown): void;
+
+  // Receives a completion notification
+  complete(): void;
+}

packages/messenger-types/src/versions/v1.ts

@@ -1,9 +1,25 @@
 import { ActionResult } from "../ActionResult";
+import { Observable } from "../Observable";
 
-export interface Api extends ApiActions {
+export interface Api extends ApiActions, ApiState {
   version: 1;
 }
 
+export enum UI {
+  Hidden = "hidden",
+  Minimized = "minimized",
+  Maximized = "maximized",
+}
+
+export interface State {
+  ui: UI;
+}
+
+export interface ApiState {
+  state$: Observable<State>;
+  getState(): State;
+}
+
 export interface ApiActions {
   /**
    * Render the messenger.