Resources

Resources play a very important role in the app: they define layouts, animations, icons, styles, and more. You will probably use them very often, so you have to understand how to work with them. In this topic, we will look at what resources are and how to access them.

Resources

A resource in an Android app is a file (for example, a layout file), or some value like a simple string. Resources can be layout files, strings, sound files, image files, and so on. All those resources are stored in the res folder.

If you open an Android project, you will notice several folders for various resources in the res directory:

Subdirectories are created in the res directory according to the resource types:

Some subdirectories are present by default, while some others can be added manually.

There are two ways to access resources: in Java or Kotlin files, and in XML files. Let's take a look at both ways in detail.

Accessing resources in XML

Sometimes it is necessary to access a resource in an XML file. For example, imagine that you need to write the path to your picture in the XML file, or set the color of the text from the file colors.xml.

Links to the resources in XML files have the following syntax: @[package_name:]resource_type/resource_name, where:

• package_name represents the name of the package where the resource is located. If your resource is in the same package, package_name is optional.

• resource_type matches a resource subdirectory like drawable or layout, or a value resource type like color or string.

• resource_name is the filename without extension or the value of android:name attribute in the XML element.

For example, say we have the following strings.xml file:

<resources>
    <string name="app_name">App Resource</string>
</resources>

To make a TextView show this string, we make the following:

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/app_name" />

In this case, text attribute will receive the app_name string resource value.

We can also use built-in Android resources by specifying android package: android:text="@android:string/ok".

Accessing resources in code

The build system generates R.java file containing identifiers for all application resources.

Even an empty project contains res/layout/activity_main.xml, and MainActivity makes it show in onCreate() method:

setContentView(R.layout.activity_main)

Through R.layout.activity_main we get access to activity_main.xml. Layout is the type of the resource and activity_main is the name of the resource.

As another example, you can get an ImageView with id my_image_view using findViewById method and make it show res/drawable/my_image.png resource using setImageResource(Int) method:

setContentView(R.layout.activity_main)
...
val imageView = findViewById<ImageView>(R.id.my_image_view)
imageView.setImageResource(R.drawable.my_image)

Similarly, we can receive other resources. For example, we use the same app_name from res/values/strings.xml.

val appTextView = findViewById<TextView>(R.id.app)
appTextView.setText(R.string.app_name)

Keep in mind: R class contains integer identifiers, not values. This is especially important when dealing with colors: they are integers, too.

You can use resources accessible in Activity, View, and some other classes to fetch individual values:

appTextView.setColor(resources.getColor(R.color.dark))

Conclusion

Now you know what kinds of resources do exist and how to declare and use them in your project. To access resources from other XML resources, we use @[package:]type/name syntax, and in Kotlin code we use identifiers from R class. Now you know how to make certain views use some resources, and how to access individual resource values.