What is copyWith and how can I use it in Flutter and what are it's use cases?

Issue

//File: email_sign_in_model.dart

class EmailSignInModel {
  EmailSignInModel({
    this.email='',
    this.formType=EmailSignInFormType.signIn,
    this.isLoading=false,
    this.password='',
    this.submitted=false,
  });

  final String email;
  final String password;
  final EmailSignInFormType formType;
  final bool isLoading;
  final bool submitted;

  EmailSignInModel copyWith({
    String email,
    String password,
    EmailSignInFormType formType,
    bool isLoading,
    bool submitted,

  }) {
    return EmailSignInModel(
    email: email ?? this.email,
    password: password?? this.password,
    formType: formType?? this.formType,
    isLoading: isLoading?? this.isLoading,
    submitted: submitted?? this.submitted

    );
  }
}



//File: email_sign_in_bloc.dart

import 'dart:async';
import 'package:timetrackerapp/app/sign_in/email_sign_in_model.dart';

class EmailSignInBloc {
 final StreamController<EmailSignInModel> _modelController = StreamController<EmailSignInModel>();
 Stream<EmailSignInModel> get modelStream => _modelController.stream;
 EmailSignInModel _model = EmailSignInModel();

 void dispose() {
   _modelController.close();
 }

void updateWith({
  String email,
  String password,
  EmailSignInFormType formType,
  bool isLoading,
  bool submitted

}) {
  //update model
  _model = _model.copyWith(
    email:email,
    password: password,
    formType: formType,
    isLoading: isLoading,
    submitted: submitted


  );
  //add updated model _tomodelController
  _modelController.add(_model);
}

}

Hi, I am new to Flutter and dart and trying to learn bloc in Flutter, I am trying to use BLOC and the also created a model class. My question is What is that copyWith({}) and what it is doing for the email_sign_in_model and for that email_sign_in_bloc? and what is that updateWith doing in the code? Thank you!

Solution

Let’s say you have an object in which you want to change some properties. One way to do is by changing each property at a time like object.prop1 = x object.prop2 = y and so on. This will go cumbersome if you have more than few properties to change. Then copyWith method comes handy. This method takes all the properties(which need to change) and their corresponding values and returns new object with your desired properties.

updateWith method is doing same thing by again calling copyWith method and at the end it is pushing returned object to stream.

Answered By – SriDatta Yalla

Answer Checked By – Marilyn (FlutterFixes Volunteer)

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