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App components are the essential building blocks of an Android app. Each component is an entry point through which the system or a user can enter your app. There are four different types of app components :

• Activities - An activity is the entry point for interacting with the user. It represents a single screen with a user interface.
• Services - A service is a general-purpose entry point for keeping an app running in the background for all kinds of reasons. It is a component that runs in the background to perform long-running operations or to perform work for remote processes.
• Broadcast receivers - A broadcast receiver is a component that enables the system to deliver events to the app outside of a regular user flow, allowing the app to respond to system-wide broadcast announcements.
• Content providers - A content provider manages a shared set of app data that you can store in the file system, in a SQLite database, on the web, or on any other persistent storage location that your app can access.

An activity provides the window in which the app draws its UI. This window typically fills the screen, but may be smaller than the screen and float on top of other windows. Generally, one activity implements one screen in an app. For instance, one of an app’s activities may implement a Preferences screen, while another activity implements a Select Photo screen.

*Note - The onCreate() and onDestroy() methods are called only once throughout the activity lifecycle.

onCreate()

onStart()

onResume()

Here A == first activity and B == second activity

onPause() - (A)

onCreate() - (B)

onStart() - (B)

onResume() - (B)

onStop() - (A)

onPause() - (B)

onRestart() - (A)

onStart() - (A)

onResume() - (A)

onStop() - (B)

onDestroy() - (B)

onStart() -> called when the activity becomes visible, but might not be in the foreground (e.g. an AlertFragment is on top or any other possible use case).

onResume() -> called when the activity is in the foreground, or the user can interact with the Activity.

onPause() -> If you can still see any part of it (Activity coming to foreground either doesn't occupy the whole screen, or it is somewhat transparent).

onStop() -> If you cannot see any part of it

A dialog, for example, may not cover the entire previous Activity, and this would be a time for onPause() to be called

• Tell me about Dependency Injection
• Tell me about Jetpack Libraries
• Why shoud we use an app architecture and what are the best practices? read
• What are Services, Broadcast receivers and Content providers? read
• Tell me about the Manifest file, what is it's role? read