To apply the repository pattern in a Flutter app that uses the jsonplaceholder API, you can follow these steps:

1. Define the model: Create a model class that represents the data you’ll be working with. For example, if you’re using the jsonplaceholder API to retrieve posts, you could create a Post class with properties like id, title, and body.

class Post {
  final int id;
  final String title;
  final String body;

  Post({required this.id, required this.title, required this.body});
}

2. Create the repository: Create a repository class that encapsulates the logic for retrieving data from the API. The repository should have a method for each operation you need to perform (e.g. getPosts, getPostById, etc.). These methods should return Future objects that resolve to the appropriate data types.

import 'package:http/http.dart' as http;
import 'dart:convert';

class PostRepository {
  final String apiUrl = "https://jsonplaceholder.typicode.com/posts";

  Future<List<Post>> getPosts() async {
    var response = await http.get(Uri.parse(apiUrl));
    if (response.statusCode == 200) {
      List<dynamic> body = json.decode(response.body);
      List<Post> posts = body.map((post) => Post.fromJson(post)).toList();
      return posts;
    } else {
      throw "Failed to get posts";
    }
  }

  Future<Post> getPostById(int id) async {
    var response = await http.get(Uri.parse('$apiUrl/$id'));
    if (response.statusCode == 200) {
      dynamic body = json.decode(response.body);
      Post post = Post.fromJson(body);
      return post;
    } else {
      throw "Failed to get post";
    }
  }
}

Note that the PostRepository class makes use of the http package to make HTTP requests to the jsonplaceholder API.

3. Use the repository in your app: Finally, you can use the repository in your app to retrieve data from the API. For example, you could create a PostList widget that displays a list of posts:

class PostList extends StatefulWidget {
  @override
  _PostListState createState() => _PostListState();
}

class _PostListState extends State<PostList> {
  late Future<List<Post>> _postsFuture;

  @override
  void initState() {
    super.initState();
    _postsFuture = PostRepository().getPosts();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Posts'),
      ),
      body: FutureBuilder<List<Post>>(
        future: _postsFuture,
        builder: (BuildContext context, AsyncSnapshot<List<Post>> snapshot) {
          if (snapshot.connectionState == ConnectionState.done) {
            if (snapshot.hasData) {
              return ListView.builder(
                itemCount: snapshot.data!.length,
                itemBuilder: (BuildContext context, int index) {
                  Post post = snapshot.data![index];
                  return ListTile(
                    title: Text(post.title),
                    subtitle: Text(post.body),
                  );
                },
              );
            } else if (snapshot.hasError) {
              return Text("${snapshot.error}");
            }
          }
          return Center(child: CircularProgressIndicator());
        },
      ),
    );
  }
}

In this example, the PostList widget uses the PostRepository to retrieve a list of posts, and displays them in a ListView. The FutureBuilder widget is used to handle the asynchronous loading of data from the API.