One of the things that's really helpful about themes, is that we can use them to

make our app look pretty consistent across many different versions of Android.

We used to have to do this,

using multiple styles that targeted different versions of the SDK.

But we get a lot of help now by way of the Android Support Libraries.

Check out how this is described here in the Android developer docs.

You may want a standard way to provide newer features on

earlier versions of Android, or gracefully fall back to equivalent functionality.

Rather than building code to handle earlier versions of the platform,

you can leverage these libraries to provide that compatibility layer.

So what that's telling us is that we can plug in this support library to our

project, and there are some things available for us that make it easy for

our app to be backwards-compatible on older versions of Android.

That's important because some of those older versions don't have

some of the newer features that we're using in our app.

So by default, we are taking advantage of a few compatibility features

provided by the app components package of the Support Library.

If we click on features over here,

we can read about some of the common useful components available to us.

For example, this dark gray bar here at the top of the screen is called the app

bar, and it wasn't always a part of Android.

So this was introduced as the action bar in a previous version of Android.

And then more recently, it was updated and renamed the app bar.

For example, this dark gray bar here at the top of our screen

is called the app bar, and it wasn't always a part of Android,

it was first introduced in an older version of Android and

it was called the action bar, and then it was updated and renamed to the app bar.

So some older versions of Android that people are still using

don't have an app bar or an action bar, but it's really useful for

certain actions and app navigation, which we'll learn more about later.

But we can include the app bar on older versions of Android

by using the AppCompat Support Library.

Let's take a look at what was created automatically for us.

So first, let's dig into our build.gradle file,

remember to click on the second one here, the module app one, and

down here at the bottom we see a dependency for appcompact.

So this first part, com.android.support is how we include the Support Library.

Then different sections of the Support Library, like appcompat,

are specified after the colon, and then the specific version is appended last.

So, this was included automatically for us, but

if you ever need to add a new Support Library package yourself,

you can look it up in the developer docs, let's go back and take a look.

Normally, I would just search for the Support Library or

the specific package up here at the top, but we're already at the right page.

So if we click on packages here on the left,

we now see all the available packages in here on the right.

If we click on one, like v7 appcompat library, we can find the exact

value that we should use in our gradle file, here it is down here.

Note that this will always be the latest version of the Support Library,

which you can update by using the SDK Manager that we briefly explored earlier.

To do so, just come up here back to the SDK Manager, click on SDK Tools here,

and then make sure that Android Support Library is checked, and

that it's completely installed and up to date.

Okay, so this includes the appcompat package, and

that means that we can use its features in code.

Back in styles.xml,

we saw that we're already extending an AppCompat theme right here.

Next if we open up MainActivity.java, we see that

it's extending AppCompatActivity instead of a regular activity class.

Again, this is used to make sure that the app bar works on older

versions of Android.

This AppCompatActivity provides an app bar and also some material design

aesthetic changes, which makes things look more consistent across versions.

We will explore some of the other Support Library features in future projects,

like a RecyclerView.

But it's important to know what the Support Library is, and

how to include different packages.

Remember that its main function is to make your app look and

work virtually the same on both old and new versions of Android.