Using Dio Interceptors for Logging and Modifying Requests in Flutter

Ever found yourself debugging API calls in Flutter and wishing you had better visibility into what’s happening under the hood? That’s where Dio interceptors come in – they’re like your personal network traffic controllers, letting you peek into and modify HTTP requests and responses with ease.

Getting Started with Dio Interceptors

Before diving deep, make sure you have Dio in your pubspec.yaml:

dependencies:
  dio: ^5.0.0

Let’s start with a simple logging interceptor. This is super helpful when you’re trying to debug your API calls:

class LoggingInterceptor extends Interceptor {
  @override
  void onRequest(RequestOptions options, RequestInterceptorHandler handler) {
    print('REQUEST[${options.method}] => PATH: ${options.path}');
    return super.onRequest(options, handler);
  }

  @override
  void onResponse(Response response, ResponseInterceptorHandler handler) {
    print('RESPONSE[${response.statusCode}] => PATH: ${response.requestOptions.path}');
    return super.onResponse(response, handler);
  }

  @override
  void onError(DioError err, ErrorInterceptorHandler handler) {
    print('ERROR[${err.response?.statusCode}] => PATH: ${err.requestOptions.path}');
    return super.onError(err, handler);
  }
}

Advanced Request Modification

Sometimes you need to modify every request automatically – like adding authentication tokens. Here’s how you can do that:

class AuthInterceptor extends Interceptor {
  @override
  void onRequest(RequestOptions options, RequestInterceptorHandler handler) {
    // Add auth token to headers
    options.headers['Authorization'] = 'Bearer your-token-here';
    return super.onRequest(options, handler);
  }
}

Putting It All Together

Now, let’s see how to use these interceptors in your Dio instance:

final dio = Dio();
dio.interceptors.add(LoggingInterceptor());
dio.interceptors.add(AuthInterceptor());

You can even chain multiple interceptors – they’ll execute in the order you add them. This is particularly useful when you want to log the modified request after your auth interceptor has done its job.

Best Practices

1. Keep interceptors focused on a single responsibility
2. Use interceptors for cross-cutting concerns like logging and authentication
3. Remember that interceptors run for every request, so keep them lightweight
4. Consider creating different Dio instances for different use cases