packages/core: add common Observable type based on TC39

packages/core/src/api/index.ts

@@ -18,3 +18,4 @@ export * from './apis';
 export * from './app';
 export * from './navTargets';
 export * from './plugin';
+export * from './types';

packages/core/src/api/types.ts

@@ -0,0 +1,63 @@
+/*
+ * Copyright 2020 Spotify AB
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * This file contains non-react related core types used throught Backstage.
+ */
+
+/**
+ * Observer interface for consuming an Observer, see TC39.
+ */
+export type Observer<T> = {
+  next?(value: T): void;
+  error?(error: Error): void;
+  complete?(): void;
+};
+
+/**
+ * Subscription returned when subscribing to an Observable, see TC39.
+ */
+export type Subscription = {
+  /**
+   * CAncels the subscription
+   */
+  unsubscribe(): void;
+
+  /**
+   * Value indicating whether the subscription is closed.
+   */
+  readonly closed: Boolean;
+};
+
+/**
+ * Observable sequence of values and errors, see TC39.
+ *
+ * https://github.com/tc39/proposal-observable
+ *
+ * This is used as a common return type for observable values and can be created
+ * using many different observable implementations, such as zen-observable or RxJS 5.
+ */
+export type Observable<T> = {
+  /**
+   * Subscribes to this observable to start receiving new values.
+   */
+  subscribe(observer: Observer<T>): Subscription;
+  subscribe(
+    onNext: (value: T) => void,
+    onError?: (error: Error) => void,
+    onComplete?: () => void,
+  ): Subscription;
+};