Flutter: Send JSON body for Http GET request

uri.replace... returns a new Uri, so you have to assign it into a new variable or use directly into the get function.

final newURI = uri.replace(queryParameters: params);

var response = await http.get(newURI, headers: {
  "Authorization": Constants.APPOINTMENT_TEST_AUTHORIZATION_KEY,
  HttpHeaders.contentTypeHeader: "application/json",
  "callMethod" : "DOCTOR_AVAILABILITY"
});

using post:

      var params = {
        "doctor_id": "DOC000506",
        "date_range": "25/03/2019-25/03/2019" ,
        "clinic_id":"LAD000404"
      };

      var response = await http.post("http://theapiiamcalling:8000", 
      body: json.encode(params)
      ,headers: {
        "Authorization": Constants.APPOINTMENT_TEST_AUTHORIZATION_KEY,
        HttpHeaders.contentTypeHeader: "application/json",
        "callMethod" : "DOCTOR_AVAILABILITY"
      });

GET

GET requests are not intended for sending data to the server (but see this). That's why the http.dart get method doesn't have a body parameter. However, when you want to specify what you are getting from the server, sometimes you need to include query parameters, which is a form of data. The query parameters are key-value pairs, so you can include them as a map like this:

final queryParameters = {
  'name': 'Bob',
  'age': '87',
};
final uri = Uri.http('www.example.com', '/path', queryParameters);
final headers = {HttpHeaders.contentTypeHeader: 'application/json'};
final response = await http.get(uri, headers: headers);

POST

Unlike GET requests, POST requests are intended for sending data in the body. You can do it like this:

final body = {
  'name': 'Bob',
  'age': '87',
};
final jsonString = json.encode(body);
final uri = Uri.http('www.example.com', '/path');
final headers = {HttpHeaders.contentTypeHeader: 'application/json'};
final response = await http.post(uri, headers: headers, body: jsonString);

Note that the parameters were a Map on the Dart side. Then they were converted to a JSON string by the json.encode() function from the dart:convert library. That string is the POST body.

So if the server is asking you to pass it data in a GET request body, check again. While it is possible to design a server in this way, it isn't standard.