File Handling & REST API Networking

Modern applications rarely run in complete isolation. They need to persist data to the local disk and communicate with remote servers over the internet.

In this lesson, we will cover file input/output (I/O) operations using dart:io and HTTP communication using the popular http package.

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1. File Handling in Dart

The dart:io library provides classes to interact with the local filesystem, including files and directories.

Reading Files

You can read a file entirely as a string or line-by-line (which is safer for large files).

dart
import 'dart:io';

void main() async {
  final file = File('data.txt');

  if (await file.exists()) {
    // Read entire content
    String content = await file.readAsString();
    print("Content: $content");

    // Read line-by-line
    List<String> lines = await file.readAsLines();
    for (var line in lines) {
      print("Line: $line");
    }
  } else {
    print("File not found!");
  }
}

Writing Files

Writing to a file will create the file if it doesn't exist, and overwrite it if it does. To append content instead of overwriting, use the FileMode.append flag.

dart
import 'dart:io';

void main() async {
  final file = File('output.txt');

  // Overwrite content
  await file.writeAsString('Hello, Dart Files!');

  // Append content
  await file.writeAsString('\nAppended text.', mode: FileMode.append);
}

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2. REST API Networking with HTTP

To make HTTP requests, you should add the http package to your dependencies in your pubspec.yaml file:

yaml
dependencies:
  http: ^1.1.0

Performing a GET Request

Let's fetch a list of posts from a public mock REST API (JSONPlaceholder) and parse it.

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

// Data model representing the response object
class Post {
  final int id;
  final String title;
  final String body;

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

  factory Post.fromJson(Map<String, dynamic> json) {
    return Post(
      id: json['id'],
      title: json['title'],
      body: json['body'],
    );
  }
}

Future<void> fetchPosts() async {
  final url = Uri.parse('https://jsonplaceholder.typicode.com/posts/1');

  try {
    final response = await http.get(url);

    if (response.statusCode == 200) {
      // Decode the raw JSON string
      final Map<String, dynamic> data = jsonDecode(response.body);
      
      // Parse to our strongly-typed Post object
      final post = Post.fromJson(data);
      print("Post Title: ${post.title}");
    } else {
      print("Request failed with status: ${response.statusCode}");
    }
  } catch (e) {
    print("An error occurred during request: $e");
  }
}

void main() async {
  await fetchPosts();
}

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3. Performing a POST Request

To send data to a server, perform a POST request and encode your request body as a JSON string.

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

Future<void> createPost() async {
  final url = Uri.parse('https://jsonplaceholder.typicode.com/posts');

  final response = await http.post(
    url,
    headers: {
      'Content-Type': 'application/json; charset=UTF-8',
    },
    body: jsonEncode({
      'title': 'New Dart Post',
      'body': 'This was created using http package.',
      'userId': 1,
    }),
  );

  if (response.statusCode == 201) {
    print("Post created successfully!");
    print("Response: ${response.body}");
  } else {
    print("Failed to create post. Status code: ${response.statusCode}");
  }
}

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Summary

• Use the dart:io library to perform file operations like readAsString and writeAsString.
• Use the http package to interact with REST APIs.
• Always check the response.statusCode (e.g., 200 for OK, 201 for Created) before parsing.
• Use jsonDecode() from dart:convert to parse JSON response strings into Dart Maps.
• Handle networking errors safely with try-catch blocks.