Android Context 101 with Class Diagram

Beginner’s Guide to understand the fundamental of Context with class diagram in Android app development

You may find Context in Android app development is confusing. For example, which one you should use?

Before answering that, let’s look at the Context class diagram below to understand its relationship with other classes/components.

Context Class Diagram

From the class diagram above, you can tell Activity, Service and Application inherits/extends Context indirectly.

As you can see, there are ContextThemeWrapper that extends ContextWrapper that extends Context. Here are the brief descriptions of all 3 classes.

• Context is an abstract class that allows an Android application to access system resources and interact with the operating system
• ContextWrapper is a convenient way to modify the behavior of a Context object, such as adding additional functionality or overriding existing behavior, without changing the original Context itself.
• ContextThemeWrapper allows you to change the visual appearance of an activity or view by applying a different theme to it

The most important thing to understand here is ContextWrapper has a mBase member variable which holds the Context reference (called Base Context) from the Android system which creates it.

For example, when an Activity is created by an Android system, a newly created Context (from the Android system) is passed into the ContextWrapper (held by the mBase member variable).

So after an Activity is created, it consists of 3 context objects now:

• Application Context — Can be retrieved by calling GetApplicationContext(). This is the same object instance throughout the whole application.
• Base Context — Can be retrieved by calling GetBaseContext(). This is a newly created Context every time an activity is created.
• Activity Context — The activity…