Model–View–ViewModel (MVVM) is a very established architectural pattern when it's come to software development. Here I have tried to put MVVM in simple example which will help you to implement in any project.

https://uploads.toptal.io/blog/image/127608/toptal-blog-image-1543413671794-80993a19fea97477524763c908b50a7a.png

Introduction

This sample demonstrates how one can

Setup base architecture of Flutter app using Stacked plugin
Use dependency injection for layers separation
Code generator to generate boilerplate code for DI, routes and JSON parsing
Make api calls using Retrofit plugin (mostly Android developers might be familier with it).

Apart from the basic architecture setup, this sample also demonstrates

Project folder structure
Navigation using ViewModel (without context)
Easy data sharing between the screens
And few more...

dependencies

dio : http client
retrofit : apis
result_monad : Result Monad (e.g wrapping result for success and failure)
pretty_dio_logger : logs network calls in a pretty, easy to read format
stacked : mvvm plugins (viewmodels)
auto_route : navigation
get_it: dependency injection
mockito: unit testing

Module Structure

https://camo.githubusercontent.com/a5485a38e6af7aa1055807a47e1833fc9a35eb7b997940b26936dcffae760623/68747470733a2f2f6d69726f2e6d656469756d2e636f6d2f6d61782f3737322f302a73664344456235373157442d374566502e6a7067

There are 3 main modules to help separate the code. They are Data, Domain, and Application.

Data contains Local Storage, APIs, Data objects (Request/Response object, DB objects), and the repository implementation.
Domain contains UseCases, Domain Objects/Models, and Repository Interfaces
Application contains UI, View Objects, Widgets, etc. Can be split into separate modules itself if needed. For example, we could have a module called Device handling things like camera, location, etc.

Package structure

using modular architecture to architect the app per feature to be easier and more readable and isolate the feature from each other

https://user-images.githubusercontent.com/19567934/183311532-5efde826-f8ef-48a7-8df5-a0ed1731a2a0.png

Repository

Bridge between Data layer and Domain layer
Connects to data sources and returns mapped data
Data sources include DB and Api

UseCase

Responsible for connecting to repository to retrieve necessary data. returns a Stream that will emit each update.
This is where the business logic takes place.
Returns data downstream.
Single use.
Lives in Domain (No Platform dependencies. Very testable).

ViewModel

Organizes data and holds View state.
Talks to use cases.
Does not know about the View.

View

Updates UI
Knows about the ViewModel
Observes changes to ViewModel.

Router

I leave this open ended to suit each projects needs. The main point here is that it is important to consolidate navigation logic to one place. This helps with maintenance and unit testing.

Setup

One can download the code in zip format or take checkout from the git repository.

Note

You need to run build_runner command for code generation everytime when you make changes in your original file.

Todo

to add more unit tests to increase code coverage
add widget & integration testing