JSON example

This example demonstrates writing a function that accepts and returns JSON.

The basic shape of the function handler looks like this:

@CloudFunction()
GreetingResponse function(Map<String, dynamic> request) {
}

The client will send a JSON document using the HTTP POST method. Here's an example request:

{
  "name": "World"
}

The function will send a JSON document as the response. Here's an example response after receiving the above request:

{
  "salutation": "Hello",
  "name": "World"
}

The function handler actually receives the JSON request parsed as a map by the Functions Framework. To access the expected name field of the request document (and provide a default value in case the name field is empty), the code looks like this:

@CloudFunction()
GreetingResponse function(Map<String, dynamic> request) {
  final name = request['name'] as String ?? 'World';
}

Finally, the function creates an object returns it. The Functions Framework will take this and attempt "do the right" thing. In this case, the function is typed to return a FutureOr<GreetingResponse>, which has a toJson() method, so the framework will invoke this, then set the response body to the stringified result and set the response header (content-type) to application/json).

@CloudFunction()
GreetingResponse function(Map<String, dynamic> request) {
  final name = request['name'] as String ?? 'World';
  final json = GreetingResponse(salutation: 'Hello', name: name);
  return json;
}

The full code is shown below:

lib/functions.dart

import 'dart:async';

import 'package:functions_framework/functions_framework.dart';
import 'package:json_annotation/json_annotation.dart';

part 'functions.g.dart';

@JsonSerializable(nullable: false)
class GreetingResponse {
  final String salutation;
  final String name;

  GreetingResponse({this.salutation, this.name});

  Map<String, dynamic> toJson() => _$GreetingResponseToJson(this);
}

@CloudFunction()
GreetingResponse function(Map<String, dynamic> request) {
  final name = request['name'] as String ?? 'World';
  final json = GreetingResponse(salutation: 'Hello', name: name);
  return json;
}

Generate project files

The Dart build_runner tool generates the following files

lib/functions.g.dart - the part file for GreetingResponse serialization
bin/server.dart - the main entry point for the function server app, which invokes the function

Run the build_runner tool, as shown here:

$ dart run build_runner build
[INFO] Generating build script completed, took 337ms
[INFO] Reading cached asset graph completed, took 48ms
[INFO] Checking for updates since last build completed, took 426ms
[INFO] Running build completed, took 13ms
[INFO] Caching finalized dependency graph completed, took 29ms
[INFO] Succeeded after 51ms with 0 outputs (0 actions)

Test the function

$ dart test
00:02 +1: All tests passed!

Run the function

$ dart run bin/server.dart
Listening on :8080

From another terminal, send a JSON request:

$ curl -X POST -H "content-type: application/json" -d '{ "name": "World" }' -i localhost:8080
HTTP/1.1 200 OK
date: Sat, 19 Dec 2020 02:17:42 GMT
content-length: 37
x-frame-options: SAMEORIGIN
content-type: application/json
x-xss-protection: 1; mode=block
x-content-type-options: nosniff
server: dart:io with Shelf

{"salutation":"Hello","name":"World"}

Tools like curl (and postman) are good for sending HTTP requests. The options used in this example are:

-X POST - send an HTTP POST request
-H "content-type: application/json" - set an HTTP header to indicate that the body is a JSON document
-d '{ "name": "World" }' - set the request body to a JSON document
-i - show the response headers (to confirm the response body content type is also a JSON document)

The last line, separated by a blank line, prints the response body.

For more details on getting started or to see how to run the function locally on Docker or deploy to Cloud Run, see these quick start guides:

Flutterando/dart-backend

JSON example

Generate project files

Test the function

Run the function