AWS Architecture Study Guide

Structure of Android OS

Android operating system consist of several layers, which are staring from bottom to top

Linux Kernel : Takes care of threading, memory management, process management, hardware drivers, networking and security. Includes drivers for camera, audio, keypad, bluetooth, etc.
Hardware Abstraction Layer HAL - Exposes hardware capabilities of hardware components such as Bluetooth, Camera etc to the Kotlin API through standard interfaces. When device is needed, the Kotlin API makes a call to the interface and a corresponding library is loaded for the hardware component.
Android Runtime ART - Environment used to run the applications. Each application runs in its own process. Designed based on Java Virtual Machine to run apps in shared environment, where battery, CPU, memory are limited. Reads .dex files.
Native C/C++ Libraries - Used to build core Android components such as ART and HAL.Their functionalities are exposed by Java Framework API
Java API Framework - Set of APIs written in Java exposing Android features. Key services and components:

View System - to build the UI
Managers- Notification, Activity, Content , Resource (strings, graphics, layers) etc
Content Providers to share data with other apps

System Apps - Email, Calendar, Camera, etc. Android comes preinstalled with them. Your apps can invoked them to reuse functionality. Can be replaced with custom apps.

Designed to be used by multiple Android apps.
Example: multiple apps needing the same components, multiple APK variations of the same app - paid and free needing the same core components.
Has the same structure as Android app module - manifest, resources, source code.

android.view - TextView, ImageView, etc

android.widget - includes components such as Button, Spinner etc.

Activities: each screen extends the activity class. Made up of several views grouped in a layout
Services: invisible, runs in background, no UI, performs continuous processing
Content Providers: to share data between part of the app or different apps. Data can be stored in database, files, etcAndroid uses content providers to expose contacts, call logs etc.
Intents: used to send messages across the Whole Android system. Can start activity, start service, etc. The Android system interprets them and determines the target that will perform the action
Broadcast Recievers: added to an app to receive messages from intents
Views: objects drawn on the screen. Parents of all UI components
Widgets: part of an app, such as Button, AppBar, Bottom Navigation, have rounded corners starting from Android 12
Notifications: Appear on the status bar, popup style messages

onCreate -> onStart -> onResume -> onPause -> onStop -> onDestroy

Android assigns priority to processes and can kill process to free resources based on its priority. Types of processes starting with highest priority

Foreground Process - visible and active, very few, killed as last resort
Visible Process - visible and inactive, not responding to user events, maybe partially obscured, very few, killed in extreme circumstances
Service Process - no visible interface , continuous processing, still considered foreground, not killed unless resources are needed
Background Process - not visible, have pending services, killed in last seen, first killed basis
Empty - app retained in memory to improve startup time, no active components, routinely killed

Consists of

Android SDK Tools - development and debugging tools
SDK Build tools - required to build the app code, continuously runs in the background

- Target new Android OS to bre able to use new features, but not the latest so your app can be used by majority of Android devices.

Build Automation Tool

Build scripts written in Groovy or Kotlin DSL

Build, run, test, package applications