Android Getting started with Android Application Fundamentals
Example
Android Apps are written in Java. The Android SDK tools compile the code, data and resource files into an APK (Android package). Generally, one APK file contains all the content of the app.
Each app runs on its own virtual machine(VM) so that app can run isolated from other apps. Android system works with the principle of least privilege. Each app only has access to the components which it requires to do its work, and no more. However, there are ways for an app to share data with other apps, such as by sharing Linux user id between app, or apps can request permission to access device data like SD card, contacts etc.
App Components
App components are the building blocks of an Android app. Each components plays a specific role in an Android app which serves a distinct purpose and has distinct life-cycles(the flow of how and when the component is created and destroyed). Here are the four types of app components:
- Activities: An activity represents a single screen with a User Interface(UI). An Android app may have more than one activity. (e.g. an email app might have one activity to list all the emails, another to show the contents of each email, and another to compose new email.) All the activities in an App work together to create a User eXperience (UX).
- Services: A service runs in the background to perform long-running operations or to perform work for a remote processes. A service does not provide any UI, it runs only in the background with the User's input. (e.g. a service can play music in the background while the user is in a different App, or it might download data from the internet without blocking user's interaction with the Android device.)
- Content Providers: A content provider manages shared app data. There are four ways to store data in an app: it can be written to a file and stored in the file system, inserted or updated to a SQLite database, posted to the web, or saved in any other persistent storage location the App can access. Through content providers, other Apps can query or even modify the data. (e.g. Android system provides a content provider that manages the user's contact information so that any app which has permission can query the contacts.) Content providers can also be used to save the data which is private to the app for better data integrity.
- Broadcast receivers: A broadcast receiver responds to the system-wide broadcasts of announcements (e.g. a broadcast announcing that the screen has turned off, the battery is low, etc.) or from Apps (e.g. to let other apps know that some data has been downloaded to the device and is available for them to use). Broadcast receivers don't have UIs but they can show notification in the status bar to alert the user. Usually broadcast receivers are used as a gateway to other components of the app, consisting mostly of activities and services.
One unique aspect of the Android system is that any app can start another app's component (e.g. if you want to make call, send SMS, open a web page, or view a photo, there is an app which already does that and your app can make use of it, instead of developing a new activity for the same task).
When the system starts a component, it starts the process for that app (if it isn't already running, i.e. only one foreground process per app can run at any given time on an Android system) and instantiates the classes needed for that component. Thus the component runs on the process of that App that it belongs to. Therefore, unlike apps on other systems, Android apps don't have a single entry point(there is no main() method).
Because the system runs each app in a separate process, one app cannot directly activate another app's components, however the Android system can. Thus to start another app's component, one app must send a message to the system that specifies an intent to start that component, then the system will start that component.
Context
Instances of the class android.content.Context provide the connection to the Android system which executes the application. Instance of Context is required to get access to the resources of the project and the global information about the app's environment.
Let's have an easy to digest example: Consider you are in a hotel, and you want to eat something. You call room-service and ask them to bring you things or clean up things for you. Now think of this hotel as an Android app, yourself as an activity, and the room-service person is then your context, which provides you access to the hotel resources like room-service, food items etc.
Yet an other example, You are in a restaurant sitting on a table, each table has an attendant, when ever you want to order food items you ask the attendant to do so. The attendant then places your order and your food items gets served on your table. Again in this example, the restaurant is an Android App, the tables or the customers are App components, the food items are your App resources and the attendant is your context thus giving you a way to access the resources like food items.
Activating any of the above components requires the context's instance. Not just only the above, but almost every system resource: creation of the UI using views(discussed later), creating instance of system services, starting new activities or services -- all require context.
More detailed description is written here.
