Android fundamentals
Android apps can be written using Kotlin, Java, and C++ languages. The Android SDK tools compile your code along with any data and resource files into an APK or an Android App Bundle.
Each Android app lives in its own security sandbox, protected by the following Android security features:
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The Android operating system is a multi-user Linux system in which each app is a different user.
- By default, the system assigns each app a unique Linux user ID (the ID is used only by the system and is unknown to the app). The system sets permissions for all the files in an app so that only the user ID assigned to that app can access them.
- Each process has its own virtual machine (VM), so an app’s code runs in isolation from other apps.
- By default, every app runs in its own Linux process. The Android system starts the process when any of the app’s components need to be executed, and then shuts down the process when it’s no longer needed or when the system must recover memory for other apps.
App components
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. Some components depend on others.
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. You implement an activity as a subclass of the Activity class.
- Services A service is a general-purpose entry point for keeping an app running in the background for all kinds of reasons There are actually two very distinct semantics services tell the system about how to manage an app: Started services tell the system to keep them running until their work is completed. This could be to sync some data in the background or play music even after the user leaves the app.
- 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.
The manifest file
Before the Android system can start an app component, the system must know that the component exists by reading the app’s manifest file, AndroidManifest.xml. Your app must declare all its components in this file, which must be at the root of the app project directory.
Declaring components
You must declare all app components using the following elements:
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elements for activities. -
elements for services. -
elements for broadcast receivers. -
elements for content providers.
<?xml version=”1.0” encoding=”utf-8”?> <manifest … > <application android:icon=”@drawable/app_icon.png” … > <activity android:name=”com.example.project.ExampleActivity” android:label=”@string/example_label” … > </activity> … </application> </manifest>
Declaring component capabilities
you can use an Intent to start activities, services, and broadcast receivers. You can use an Intent by explicitly naming the target component (using the component class name) in the intent