How to Edit(Update) data in JSON file flutter

Question

I am trying to update specific value to a JSON File in external Storage.

Though I am able to write to the file but It is replacing the whole JSON File with the single data.

// This one is Replacing the whole document with single value

 Future setBookmark(int questionId, String isBookmark) async {
   Map<String, dynamic> content = {questionId.toString(): isBookmark};
   var dir = await getExternalStorageDirectory();

   var testdir = new Io.Directory('${dir.path}/BCS/bcs.json');
   File jsonFile = File(dir.path + "/BCS/" + "bcs.json");

   Map<String, dynamic> jsonFileContent = 
     json.decode(jsonFile.readAsStringSync());
   jsonFileContent.addAll(content);
   jsonFile.writeAsStringSync(json.encode(_listQuestions
     .firstWhere((question) => question.id == questionId)
     .bookmark = isBookmark));
 }

//this is changing the value temporarily but not writing to the file

 Future setBookmark(int questionId, String isBookmark) async {
    _listQuestions
        .firstWhere((question) => question.id == questionId)
        .bookmark = isBookmark;
 }

if I may, I'd like to clarify something here quickly: "JSON" itself is literally "JavaScript Object Notation". so a "JSON file", is quite simply "JavaScript Object Notation in a file".. in this scenario, you (1) read the JSON String from a file (jsonFile.readAsStringSync()), (2) parse the resulting String into a Map (json.decode(jsonFileReadAsString)), (3) modify whatever you need to, (4) convert that object back to a String (json.encode(objectAfterEdits)), and then (5) write that String back to the file (jsonFile.writeAsStringSync(newDataString)). — Andre Greeff
– Andre Greeff, Commented Mar 31, 2022 at 11:59

Richard Heap · Accepted Answer · 2019-06-29 11:27:58Z

2

You need to write the whole question list. Break it into two statements

 Future setBookmark(int questionId, String isBookmark) async {
    // update the list
    _listQuestions
        .firstWhere((question) => question.id == questionId)
        .bookmark = isBookmark;
    // and write it
    jsonFile.writeAsStringSync(json.encode(_listQuestions));
 }

answered Jun 29, 2019 at 11:27

Richard Heap

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2 Comments

MEHEDI HASAN Over a year ago

this worked but i have other objects which are being empty. is there anyway to just edit the specific data without re-writing the whole json?

Richard Heap Over a year ago

No. You have to write the whole JSON every time.

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