How to Parse JSON in Dart/Flutter | Shalini Chauhan
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How to Parse JSON in Dart/Flutter
Parsing JSON is a very common task for apps that need to fetch data from the Internet.
And depending on how much JSON data you need to process, you have two options:
write all the JSON parsing code manually
automate the process with code generation
Encoding and Decoding JSON
When a JSON response is sent over the network, the entire payload is encoded as a string.
But inside our Flutter apps, we don't want to extract the data from a string manually:
Instead, we can decode the JSON first to obtain a map of key-value pairs that can be more easily parsed. To send JSON data over the network, it first needs to be encoded or serialized. Encoding is the process of turning a data structure into a string. The opposite process is called decoding or deserialization. When you receive a JSON payload as a string, you need to decode or deserialize it before you can use it.
Decoding JSON with dart:convert
For simplicity, let's consider this small JSON payload:
// this represents some response data we get from the network, for example:// ```// final response = await http.get(uri);// final jsonData = response.body// ```final jsonData = '{ "name": "Pizza da Mario", "cuisine": "Italian" }';To read the keys and values inside it, we first need to decode it using the dart:convert package:
// 1. import dart:convertimport 'dart:convert';// this represents some response data we get from the networkfinal jsonData = '{ "name": "Pizza da Mario", "cuisine": "Italian" }';// 2. decode the jsonfinal parsedJson = jsonDecode(jsonData);// 3. print the type and valueprint('${parsedJson.runtimeType} : $parsedJson');the result type is the same as Map<String, dynamic>.
Note how the values are of type dynamic. This makes sense because each JSON value could be a primitive type (boolean/number/string), or a collection (list or map).
In fact, jsonDecode is a generic method that works on any valid JSON payload, regardless of what's inside it. All it does is decode it and return a dynamic value.
But if we work with dynamic values in Dart, we lose all the benefits of strong type-safety. A much better approach is to define some custom model classes that represent our response data.
Parsing JSON to a Dart model class
Given this simple map representing a JSON payload:
{
"name": "Pizza da Mario",
"cuisine": "Italian"
}
We can write a Restaurant class to represent it:
class Restaurant {
Restaurant({required this.name, required this.cuisine});
final String name;
final String cuisine;
}
As a result, rather than reading the data like this:
parsedJson['name']; // dynamic
parsedJson['cuisine']; // dynamic
We can read it like this:
restaurant.name; // guaranteed to be a non-nullable, immutable String
restaurant.cuisine; // guaranteed to be a non-nullable, immutable String
This way, we can leverage the type system to get compile-time safety and avoid typos and other mistakes.
However, we haven't specified how to convert our parsedJson to a Restaurant object yet!
JSON to Dart: Adding a factory constructor
Let's define a factory constructor to take care of this:
factory Restaurant.fromJson(Map<String, dynamic> data) { // ! there's a problem with this code (see below) final name = data['name']; final cuisine = data['cuisine']; return Restaurant(name: name, cuisine: cuisine);}A factory constructor is a good choice for JSON parsing as it lets us do some work (create variables, perform some validation) before returning the result. This is not possible with regular (generative) constructors.
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