Add ConnectionStatusListener to RestClient

Clients can add a connection status listener to the RestClient to be
notified when the connection goes up or down. The connection status is
detected based on whether any HTTP request can be completed or not, or
based on the returned HTTP status code.

When a listener is added it will immediately receive the current status
through the listener interface.

Change-Id: I5f7fcbf4b94380b2ea349755a400bd61bbadc7d2
Signed-off-by: Patrick Tasse <patrick.tasse@ericsson.com>

Author: PatrickTasse

tsp-typescript-client/src/protocol/rest-client.ts

@@ -21,15 +21,27 @@ export interface HttpResponse {
     headers?: Headers
 }
 
+/**
+ * A listener that is called when the connection status is changed.
+ * The status 'true' indicates that the server was last reached successfully.
+ * The status 'false' indicates that the server could not be reached or
+ * returned a status code indicating that it cannot service requests (>= 502).
+ */
+export type ConnectionStatusListener = ((status: boolean) => void);
+
 /**
  * Rest client helper to make request.
  * The request response status code indicates if the request is successful.
  * The json object in the response may be undefined when an error occurs.
  */
 export class RestClient {
 
+    static status = false;
+    static connectionStatusListeners: ConnectionStatusListener[] = [];
+
     static get<T>(url: string, parameters?: Map<string, string>): Promise<TspClientResponse<Deserialized<T>>>;
     static get<T>(url: string, parameters: Map<string, string> | undefined, normalizer: Normalizer<T>): Promise<TspClientResponse<T>>;
+
     /**
      * Perform GET
      * @template T is the expected type of the json object returned by this request
@@ -103,7 +115,10 @@ export class RestClient {
                 },
                 body: this.jsonStringify(body)
             });
+            const status = response.status <= 501;
+            this.updateConnectionStatus(status);
         } catch (err) {
+            this.updateConnectionStatus(false);
             return new TspClientResponse(err.toString(), 503, 'Service Unavailable');
         }
 
@@ -148,7 +163,7 @@ export class RestClient {
     /**
      * Stringify JS objects. Can stringify `BigInt` values.
      */
-     protected static jsonStringify(data: any): string {
+    protected static jsonStringify(data: any): string {
         return JSONBig.stringify(data);
     }
 
@@ -159,4 +174,36 @@ export class RestClient {
     protected static jsonParse(text: string): any {
         return JSONBig.parse(text);
     }
+
+    /**
+     * Adds a connection status listener.
+     * The listener will immediately be called with the current status.
+     *
+     * @param listener The listener to add
+     */
+    public static addConnectionStatusListener(listener: ConnectionStatusListener): void {
+        if (!this.connectionStatusListeners.includes(listener)) {
+            this.connectionStatusListeners.push(listener);
+        }
+        listener(this.status);
+    }
+
+    /**
+     * Removes a connection status listener.
+     *
+     * @param listener The listener to remove
+     */
+    public static removeConnectionStatusListener(listener: ConnectionStatusListener): void {
+        this.connectionStatusListeners = this.connectionStatusListeners.filter(element => element !== listener);
+    }
+
+    private static updateConnectionStatus(status: boolean) {
+        if (this.status === status) {
+            return;
+        }
+        for (const listener of this.connectionStatusListeners) {
+            listener(status);
+        }
+        this.status = status;
+    }
 }