How to update the local json field in flutter

问题:

I am new to flutter and dart I read a lot of web article and documentation but didn't able to figure out how to update JSON field I have a local JSON file like

 {
"category": "Happiness",
"quotes":[
  {
  "quote":"I hope you will find a reason to smile",
  "favorite":false
  },
  {
  "quote":"Sometimes your joy is the source of your smile, but sometimes your smile can be the source of your joy.",
  "favorite":false
  }]}

I want to update the filed favorite to true in JSON file Is there any way to do this

回答1:

You can use a forEach loop on the quotes list in the JSON and set favourite to true inside it. I'm assuming you have a parsed your String data into JSON using jsonDecode or json.decode.

If not, here's a link on how to parse JSON data in Dart.

Consider the below code -

Map<String, dynamic> jsonData =  {
    "category": "Happiness",
    "quotes":[
     {
          "quote":"I hope you will find a reason to smile",
          "favorite":false
     },
     {
          "quote":"Sometimes your joy is the source of your smile, but sometimes your smile can be the source of your joy.",
          "favorite":false
     }
]};

(jsonData["quotes"] as List<dynamic>).forEach((item) {
    item["favorite"] = true;
});

You can also alternatively use shorthand syntax for the forEach loop function like below -

(jsonData["quotes"] as List<dynamic>).forEach((item) => item["favorite"] = true);

回答2:

first take this json array to any variable and then understand the hierarchy .

e.g "jsonArr" is the array that hold that data then ..

jsonArr['quotes'][0]['favorite'] = true;

or you can put this in a for loop and traverse this loop to the length of the json array.

回答3:

You Can parse the JSON like this

Map<String, dynamic> data = jsonDecode(jsonString);

And if you want to make each favorite true then You can use looping like this

(data["quotes"] as List<dynamic>).forEach((item) => item["favorite"] = true);

if you want to set a single object value true then you need to pass the position like this

(data["quotes"] as List<dynamic>)[position]['favorite'] = true;

Addition

Here is the link to encode and decode the JSON

回答4:

You can use the following method. The field favorite is changed to true when the button is pressed.

FlatButton(
    child: Text('Add Favourite'),
    onPressed: (){
         myJson['quotes'][myIndex]['favorite'] = true;
})

回答5:

If you use JSON models and declare them as mutable, then you will be able to perform this modification directly on the source data.

import 'dart:convert';

import 'json_objects.dart';

void main() {
  var json = jsonDecode(_source) as Map<String, dynamic>;
  var response = Response1.fromJson(json);
  for (var quote in response.quotes) {
    quote.favorite = false;
  }

  json = response.toJson();
  print(json);
}

final _source = r'''
{
    "category": "Happiness",
    "quotes": [
        {
            "quote": "I hope you will find a reason to smile",
            "favorite": false
        },
        {
            "quote": "Sometimes your joy is the source of your smile, but sometimes your smile can be the source of your joy.",
            "favorite": false
        }
    ]
}''';

Reuult:

{category: Happiness, quotes: [{favorite: false, quote: I hope you will find a reason to smile}, {favorite: false, quote: Sometimes your joy is the source of your smile, but sometimes your smile can be the source of your joy.}]}

Used JSON models.

class Response1 {
  String category;
  List<Response1Quotes> quotes;

  Response1({this.category, this.quotes});

  factory Response1.fromJson(Map<String, dynamic> json) {
    return Response1(
      category: json['category'] as String,
      quotes: _toObjectList(json['quotes'], (e) => Response1Quotes.fromJson(e)),
    );
  }

  Map<String, dynamic> toJson() {
    return {
      'category': category,
      'quotes': _fromList(quotes, (e) => e.toJson()),
    };
  }
}

class Response1Quotes {
  bool favorite;
  String quote;

  Response1Quotes({this.favorite, this.quote});

  factory Response1Quotes.fromJson(Map<String, dynamic> json) {
    return Response1Quotes(
      favorite: json['favorite'] as bool,
      quote: json['quote'] as String,
    );
  }

  Map<String, dynamic> toJson() {
    return {
      'favorite': favorite,
      'quote': quote,
    };
  }
}

List _fromList(data, Function(dynamic) toJson) {
  if (data == null) {
    return null;
  }
  var result = [];
  for (var element in data) {
    var value;
    if (element != null) {
      value = toJson(element);
    }
    result.add(value);
  }
  return result;
}

List<T> _toObjectList<T>(data, T Function(Map<String, dynamic>) fromJson) {
  if (data == null) {
    return null;
  }
  var result = <T>[];
  for (var element in data) {
    T value;
    if (element != null) {
      value = fromJson(element as Map<String, dynamic>);
    }
    result.add(value);
  }
  return result;
}

/*
Response1:
  "category": String
  "quotes": List<Response1Quotes>

Response1Quotes:
  "quote": String
  "favorite": bool
*/