fix(fresh_dio): add assert to prevent infinite refresh loop (#98)

Co-authored-by: Felix Angelov <[email protected]>

packages/fresh_dio/lib/src/fresh.dart

@@ -80,6 +80,29 @@ class Fresh<T> extends QueuedInterceptor with FreshMixin<T> {
     RequestOptions options,
     RequestInterceptorHandler handler,
   ) async {
+    assert(
+      _httpClient.interceptors.every((interceptor) => interceptor != this),
+      '''
+Cycle Detected!
+
+The Fresh instance was created using an http client 
+which already contains the Fresh instance as an interceptor.
+
+This will cause an infinite loop on token refresh.
+  
+Example:
+
+  ```
+  final httpClient = Dio();
+  final fresh = Fresh.oAuth2(
+    httpClient: httpClient,
+    ...
+  );
+  httpClient.interceptors.add(fresh); // <-- BAD
+  ```
+''',
+    );
+
     final currentToken = await token;
     final headers = currentToken != null
         ? _tokenHeader(currentToken)

packages/fresh_dio/test/fresh_test.dart

@@ -112,6 +112,30 @@ void main() {
 
     group('onRequest', () {
       const oAuth2Token = OAuth2Token(accessToken: 'accessToken');
+
+      test(
+        'throws AssertionError when httpClient.interceptors '
+        'contains the Fresh instance as an interceptor',
+        () {
+          when(() => tokenStorage.read()).thenAnswer((_) async => oAuth2Token);
+
+          final httpClient = Dio();
+
+          final fresh = Fresh.oAuth2(
+            tokenStorage: tokenStorage,
+            refreshToken: emptyRefreshToken,
+            httpClient: httpClient,
+          );
+
+          httpClient.interceptors.add(fresh);
+
+          expect(
+            fresh.onRequest(RequestOptions(), RequestInterceptorHandler()),
+            throwsA(isA<AssertionError>()),
+          );
+        },
+      );
+
       test(
           'appends token header when token is OAuth2Token '
           'and tokenHeader is not provided', () async {