Handling HTTP and JSON

Contents

• Core Guidelines
• Workflow: Executing HTTP Operations
• Workflow: Implementing JSON Serialization
• Workflow: Parsing Large JSON in the Background
• Examples

Core Guidelines

• Enforce HTTPS: iOS and Android disable cleartext (HTTP) connections by default. Always use HTTPS endpoints. If HTTP is strictly required for debugging, configure network_security_config.xml (Android) and NSAppTransportSecurity (iOS).
• Construct URIs Safely: Always use Uri.https(authority, unencodedPath, [queryParameters]) to safely build URLs. This handles encoding and formatting reliably, preventing string concatenation errors.
• Handle Status Codes: Always validate the http.Response.statusCode. Treat 200 (OK) and 201 (Created) as success. Throw explicit exceptions for other codes (do not return null).
• Prevent UI Jank: Move expensive JSON parsing operations (taking >16ms) to a background isolate using the compute() function.
• Structured AI Output: When integrating LLMs, enforce reliable JSON output by specifying a strict JSON schema in the system prompt and setting the response MIME type to application/json.

Workflow: Executing HTTP Operations

Use this workflow to implement network requests using the http package.

Task Progress:

• Add the http package to pubspec.yaml.
• Configure platform permissions (Internet permission in AndroidManifest.xml and macOS .entitlements).
• Construct the target Uri.
• Execute the HTTP method.
• Validate the response and parse the JSON payload.

Conditional Implementation:

• If fetching data (GET): Use http.get(uri).
• If sending new data (POST): Use http.post(uri, headers: {...}, body: jsonEncode(data)). Ensure Content-Type is application/json; charset=UTF-8.
• If updating data (PUT): Use http.put(uri, headers: {...}, body: jsonEncode(data)).
• If deleting data (DELETE): Use http.delete(uri, headers: {...}).

Feedback Loop: Validation & Error Handling

1. Run the HTTP request.
2. Check response.statusCode.
3. If 200 or 201, call jsonDecode(response.body) and map to a Dart object.
4. If any other code, throw an Exception('Failed to load/update/delete resource').
5. Review errors -> fix endpoint, headers, or payload structure.

Workflow: Implementing JSON Serialization

Choose the serialization strategy based on project complexity.

Conditional Implementation:

• If building a small prototype or simple models: Use manual serialization with dart:convert.
• If building a production app with complex/nested models: Use code generation with json_serializable.

Manual Serialization Setup

Task Progress:

• Import dart:convert.
• Define the Model class with final properties.
• Implement a factory Model.fromJson(Map<String, dynamic> json) constructor.
• Implement a Map<String, dynamic> toJson() method.

Code Generation Setup (json_serializable)

Task Progress:

• Add dependencies: flutter pub add json_annotation and flutter pub add -d build_runner json_serializable.
• Import json_annotation.dart in the model file.
• Add the part 'model_name.g.dart'; directive.
• Annotate the class with @JsonSerializable(). Use explicitToJson: true if the class contains nested models.
• Define the fromJson factory and toJson method delegating to the generated functions.
• Run the generator: dart run build_runner build --delete-conflicting-outputs.

Workflow: Parsing Large JSON in the Background

Use this workflow to prevent frame drops when parsing large JSON payloads (e.g., lists of 1000+ items).

Task Progress:

• Create a top-level or static function that takes a String (the response body) and returns the parsed Dart object (e.g., List<Model>).
• Inside the function, call jsonDecode and map the results to the Model class.
• In the HTTP fetch method, pass the top-level parsing function and the response.body to Flutter's compute() function.

Examples

Example 1: HTTP GET with Manual Serialization

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

class Album {
  final int id;
  final String title;

  const Album({required this.id, required this.title});

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

Future<Album> fetchAlbum() async {
  final uri = Uri.https('jsonplaceholder.typicode.com', '/albums/1');
  final response = await http.get(uri);

  if (response.statusCode == 200) {
    return Album.fromJson(jsonDecode(response.body) as Map<String, dynamic>);
  } else {
    throw Exception('Failed to load album');
  }
}

Example 2: HTTP POST Request

Future<Album> createAlbum(String title) async {
  final uri = Uri.https('jsonplaceholder.typicode.com', '/albums');
  final response = await http.post(
    uri,
    headers: <String, String>{
      'Content-Type': 'application/json; charset=UTF-8',
    },
    body: jsonEncode(<String, String>{'title': title}),
  );

  if (response.statusCode == 201) {
    return Album.fromJson(jsonDecode(response.body) as Map<String, dynamic>);
  } else {
    throw Exception('Failed to create album.');
  }
}

Example 3: Background Parsing with compute

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

// 1. Top-level function for parsing
List<Photo> parsePhotos(String responseBody) {
  final parsed = (jsonDecode(responseBody) as List<Object?>)
      .cast<Map<String, Object?>>();
  return parsed.map<Photo>(Photo.fromJson).toList();
}

// 2. Fetch function using compute
Future<List<Photo>> fetchPhotos(http.Client client) async {
  final uri = Uri.https('jsonplaceholder.typicode.com', '/photos');
  final response = await client.get(uri);

  if (response.statusCode == 200) {
    // Run parsePhotos in a separate isolate
    return compute(parsePhotos, response.body);
  } else {
    throw Exception('Failed to load photos');
  }
}

Example 4: Code Generation (json_serializable)

import 'package:json_annotation/json_annotation.dart';

part 'user.g.dart';

@JsonSerializable(explicitToJson: true)
class User {
  final String name;
  
  @JsonKey(name: 'registration_date_millis')
  final int registrationDateMillis;

  User(this.name, this.registrationDateMillis);

  factory User.fromJson(Map<String, dynamic> json) => _$UserFromJson(json);
  Map<String, dynamic> toJson() => _$UserToJson(this);
}