feat: allow defining signal type

this makes using especially the frontend API much usable as the signal
can be typed instead having plain json object to play around with.

Signed-off-by: Heikki Hellgren <heikki.hellgren@op.fi>

plugins/signals/api-report.md

@@ -8,6 +8,7 @@ import { DiscoveryApi } from '@backstage/core-plugin-api';
 import { IdentityApi } from '@backstage/core-plugin-api';
 import { JsonObject } from '@backstage/types';
 import { SignalApi } from '@backstage/plugin-signals-react';
+import { SignalSubscriber } from '@backstage/plugin-signals-react';
 
 // @public (undocumented)
 export class SignalClient implements SignalApi {
@@ -23,12 +24,10 @@ export class SignalClient implements SignalApi {
   // (undocumented)
   static readonly DEFAULT_RECONNECT_TIMEOUT_MS: number;
   // (undocumented)
-  subscribe(
+  subscribe<SignalType extends JsonObject = JsonObject>(
     channel: string,
-    onMessage: (message: JsonObject) => void,
-  ): {
-    unsubscribe: () => void;
-  };
+    onMessage: (message: SignalType) => void,
+  ): SignalSubscriber;
 }
 
 // @public (undocumented)

plugins/signals/src/api/SignalClient.ts

@@ -13,14 +13,14 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-import { SignalApi } from '@backstage/plugin-signals-react';
+import { SignalApi, SignalSubscriber } from '@backstage/plugin-signals-react';
 import { JsonObject } from '@backstage/types';
 import { DiscoveryApi, IdentityApi } from '@backstage/core-plugin-api';
 import { v4 as uuid } from 'uuid';
 
 type Subscription = {
   channel: string;
-  callback: (message: JsonObject) => void;
+  callback: (message: any) => void;
 };
 
 const WS_CLOSE_NORMAL = 1000;
@@ -62,16 +62,16 @@ export class SignalClient implements SignalApi {
     private reconnectTimeout: number,
   ) {}
 
-  subscribe(
+  subscribe<SignalType extends JsonObject = JsonObject>(
     channel: string,
-    onMessage: (message: JsonObject) => void,
-  ): { unsubscribe: () => void } {
+    onMessage: (message: SignalType) => void,
+  ): SignalSubscriber {
     const subscriptionId = uuid();
     const exists = [...this.subscriptions.values()].find(
       sub => sub.channel === channel,
     );
     this.subscriptions.set(subscriptionId, {
-      channel: channel,
+      channel,
       callback: onMessage,
     });
 
@@ -178,12 +178,14 @@ export class SignalClient implements SignalApi {
 
   private handleMessage(data: MessageEvent) {
     try {
-      const json = JSON.parse(data.data) as JsonObject;
-      if (json.channel) {
-        for (const sub of this.subscriptions.values()) {
-          if (sub.channel === json.channel) {
-            sub.callback(json.message as JsonObject);
-          }
+      const json = JSON.parse(data.data);
+      if (!json.channel) {
+        return;
+      }
+
+      for (const sub of this.subscriptions.values()) {
+        if (sub.channel === json.channel) {
+          sub.callback(json.message);
         }
       }
     } catch (e) {