Flutter – Map String Dynamic Code Example

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In this post, we will understand the Flutter concepts related to Map<String, Dynamic> with some code example / sample. For greater details, visit Flutter website.

Map<String, dynamic> maps a String key with the dynamic value. Since the key is always a String and the value can be of any type, it is kept as dynamic to be on the safer side. It is very useful in reading a JSON object as the JSON object represents a set of key-value pairs where key is of type String while value can be of any type including String, List<String>, List<Object> or List<Map>. Take a look at the following example:

{
    "name": "Vitalflux.com",
    "domain": ["Data Science", "Mobile", "Web"],
    "noOfArticles": [
        {"type": "data science", "count": 50},
        {"type": "web", "count": 75}
    ]
}

Above is an example of JSON object comprising of three keys such as name with value of type String, “domain” with value of type List<String> and “noOfArticles” with value of type List<Object> (List<Map>)

Such JSON objects can easily be read with Map<String, dynamic> object and displayed on screen appropriately.

There are various ways in which one could work with Map<String, dynamic> object. They are some of the following:

  • Process it as a Map object
  • Parse it and use it as a Dart object

The following will be required to be done in case the Map<String, dynamic> is parsed and read as a Dart object.

  • A parse method which will parse the Map
  • A Dart object which is created as a result of parsing

The following code represents the parse method which is used to create a Dart object.

class PortalInfoWidget extends StatelessWidget {

  Map<String, dynamic> _portaInfoMap = {
    "name": "Vitalflux.com",
    "domains": ["Data Science", "Mobile", "Web"],
    "noOfArticles": [
      {"type": "data science", "count": 50},
      {"type": "web", "count": 75}
    ]
  };

  @override
  Widget build(BuildContext context) {
    PortalInfo portalInfo =  PortalInfo.fromJson(_portaInfoMap);
    return ListView(
        children: List.generate(portalInfo.domains.length,(index){
          return Container(
              padding: const EdgeInsets.fromLTRB(20.0, 10.0, 10.0, 10.0),
              child: Text(portalInfo.domains[index].toString())
          );
        }),
      );
  }
}

Note the fromJson factory method called on the PortalInfo (Dart object) which takes the Map<String, dynamic> object and returns the parsed PortalInfo object. A ListView widget is then created on portalInfo.domains object which is List<String> as represented in Map<String, dynamic> JSON representation.

Here is the code for PortalInfo object.

class PortalInfo {
  final String name;
  final List<String> domains;
  final List<Object> noOfArtcles;

  PortalInfo({
    this.name,
    this.domains,
    this.noOfArtcles
  });

  factory PortalInfo.fromJson(Map<String, dynamic> parsedJson){
    return PortalInfo(
        name: parsedJson['name'],
        domains : parsedJson['domains'],
        noOfArtcles : parsedJson ['noOfArticles']
    );
  }
}

Running the app will create a view like the following:

Flutter - Map String Dynamic Concepts and code examples
Fig 1. Printing List<String> value of Map<String, dynamic> object
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