For each app component like activity and service a corresponding XML element must be declared in the manifest file

Activities

TL;DR

An entry point for interacting with the user. It represents a single screen with a user interface

Explanation

An email app can have two different activities: one to read emails, and the other to write an email. The two activities work together to form a cohesive user experience, but each one is independent of the others. Therefore, each activity must be declared separately. You can also add an ativity to access a camera in order to attach a picture or a video.

What to consider when making an activity

1. Keep track of the current screen to ensure the system runs the process that is hosting the current activity
2. Know that stopped activities contain data the user may return to -> store the data
3. Help the app handle kill an activity to allow users to return to activities with their previous state restored
4. Provide a way for apps to implement user flows betweeen each other

Services

TL;DR

A general-purpose entry point for keeping an app running in the background

Explanation

Since a service tag is used to run an Android app in the background, a service tag does not provide a user inteface. For example, a service tag may play some music in the background while the user is in a different app. Another component like an activity can start the service and let it run or bind to it in order to interact with it.

Two semantics services tell can tell tye system about how to manage an app:

1. Started services tell the system to keep them running until their work is completed. Could be used to sync some data in the background or play music even after the user leaves the app
   • Music playback is something a user is directly aware of
   • A regular background service is something tthat a user is unaware of

2. Bound services run because another app or the system has said that it wants to make use of the service. It is like providing an API for an app to use in the background.

Activating Components

Activities and services are activated by an asynchronous message called an intent, which binds individual components to each other at runtime. Intent defines the action to perform and may specify the URI of the data to act on. An intent may convey a request for an activity to show an image or to open a webpage. You can also start an activity to receive a result in which case the activity also returns the reuslt in an intent

AndroidManifest.xml

If a component is declared in AndroidManifest.xml without a specific package name but uses a . instead, it assumes that the app’s package name must be used. An Intent object is defined with <intent-filter> element in AndroidManifext.xml