MVVM

1. Model

• What it is

  • Business logic

  • Data layer

  • Rules of the app

• In Android terms

  • Repositories

  • Data sources (Room, Retrofit, DataStore)

  • Domain models

  • Use cases (if you’re being disciplined)

• What it must NOT know

  • Activities

  • Fragments

  • Views

2. View

• What it is

  • UI only

  • Displays data

  • Sends user actions upward

• Android examples

  • Activity

  • Fragment

  • Jetpack Compose UI

  • XML layouts

• Responsibilities

  • Observe state

  • Render UI

  • Forward user events

• What it must NOT do

  • Business logic

  • Network calls

  • Database operations

  • Decision making beyond basic UI stuff

3. ViewModel

• What it is

  • Holds UI-related state

  • Handles user actions

  • Talks to Model

  • Survives configuration changes

Key idea: ViewModel does NOT know the View exists.

No references to Activity, Fragment, Context (unless Application is explicitly justified).

• Android ViewModel responsibilities

  • Expose state via LiveData, StateFlow, or Compose State

  • Process events from View

  • Call repositories / use cases

  • Transform data into UI-friendly form

Data flow

• User interacts with View

• View calls a method on ViewModel

• ViewModel updates state

• View observes state change

• UI re-renders

LiveData

LiveData is an observable data holder class.

Unlike a regular observable, LiveData is lifecycle-aware, meaning it respects the lifecycle of other app components, such as activities, fragments, or services.

This awareness ensures LiveData only updates app component observers that are in an active lifecycle state.

• Follow these steps to work with LiveData objects:

  • Create an instance of LiveData to hold a certain type of data. This is usually done within your ViewModel class.

  • Create an Observer object that defines the onChanged() method, which controls what happens when the LiveData object's held data changes. You usually create an Observer object in a UI controller, such as an activity or fragment.

  • Attach the Observer object to the LiveData object using the observe() method. The observe() method takes a LifecycleOwner object. This subscribes the Observer object to the LiveData object so that it is notified of changes.

Why MVVM works well on Android

1. Configuration changes

   • ViewModel survives rotation

2. Lifecycle awareness

   • LiveData / Flow respect lifecycle

   • Fewer memory leaks

3. Testability

   • ViewModel can be unit tested without Android framework

4. Jetpack support