Add feature to retrieve subscribed topics

Author: chinh-tran

Sources/FCM/Helpers/FCM+GetTopics.swift

@@ -0,0 +1,40 @@
+import Foundation
+import Vapor
+
+extension FCM {
+    public func getTopics(token: String, on eventLoop: EventLoop) -> EventLoopFuture<[String]> {
+        return _getTopics(token: token, on: eventLoop)
+    }
+
+    public func _getTopics(token: String, on eventLoop: EventLoop) -> EventLoopFuture<[String]> {
+        guard let configuration = self.configuration else {
+            fatalError("FCM not configured. Use app.fcm.configuration = ...")
+        }
+        guard let serverKey = configuration.serverKey else {
+            fatalError("FCM: GetTopics: Server Key is missing.")
+        }
+        let url = self.iidURL + "info/\(token)?details=true"
+        return getAccessToken().flatMap { accessToken -> EventLoopFuture<ClientResponse> in
+            var headers = HTTPHeaders()
+            headers.add(name: .authorization, value: "key=\(serverKey)")
+
+            return self.client.get(URI(string: url), headers: headers)
+        }
+        .validate()
+        .flatMapThrowing { response in
+            struct Result: Codable {
+                let rel: Relations
+
+                struct Relations: Codable {
+                    let topics: [String: TopicMetadata]
+                }
+
+                struct TopicMetadata: Codable {
+                    let addDate: String
+                }
+            }
+            let result = try response.content.decode(Result.self, using: JSONDecoder())
+            return Array(result.rel.topics.keys)
+        }
+    }
+}