Android Resources Access

There are many elements that contribute to building an excellent Android application. Beyond the application's coding, you need to focus on various resources such as the static content you use, like bitmaps, colors, layout definitions, UI strings, animations, and more. These resources are typically placed in separate subdirectories under the project's res/ directory.

This tutorial will guide you on how to organize application resources, specify alternative resources, and access them within your application.

Organizing Resources in Eclipse

You need to place each type of resource in a specific subdirectory under the project's res/ directory. For example, here is the file hierarchy of a simple project:

MyProject/
    src/  
        MyActivity.java  
    res/
        drawable/  
            icon.png  
        layout/  
            activity_main.xml
            info.xml
        values/  
            strings.xml

The res/ directory contains all resources in various subdirectories. Here, there is an image resource, two layout resources, and a string resource file. The following table details the supported resources in the res/ directory of a project.

Alternative Resources

Your application may need to provide alternative resources for specific device configurations, such as different screen resolutions or languages. At runtime, Android detects the current device configuration and loads the appropriate resources.

To specify a set of alternative resources for a specific configuration, follow these steps:

• Create a new directory under res/ named in the format <resource_name>_<config_qualifier>. Here, resource_name is any resource mentioned in the table above, such as layouts or images, and qualifier specifies which configuration to use. Refer to the official documentation for a complete list of qualifiers for different types of resources.

• Save the corresponding alternative resources in this directory. These resource files must have the same name as the default resource files but contain alternative content. For example, although the image filenames are the same, the images for high-resolution screens have higher resolutions.

Here is an example specifying default screen images and alternative images for high-resolution screens:

MyProject/
   src/
    main/
    java/
       MyActivity.java  
       res/
          drawable/  
            icon.png
            background.png
        drawable-hdpi/  
            icon.png
            background.png  
        layout/  
            activity_main.xml
            info.xml
        values/  
            strings.xml

Another example specifying default language layouts and alternative layouts for Arabic:

MyProject/
   src/
    main/
    java/
       MyActivity.java  
      res/
         drawable/  
            icon.png
            background.png
        drawable-hdpi/  
            icon.png
            background.png  
        layout/  
            activity_main.xml
            info.xml
        layout-ar/
            main.xml
        values/  
            strings.xml

Accessing Resources

In application development, you need to access defined resources, either through code or XML files. The following sections explain how to access resources in both scenarios.

Accessing Resources in Code

When an Android application is compiled, an R class is generated, which contains IDs for all resources in the res/ directory. You can access resources using the R class, either by subclass and resource name or directly by resource ID.

Example

To access res/drawable/myimage.png and set it to an ImageView, you would use the following code:

ImageView imageView = (ImageView) findViewById(R.id.myimageview);
imageView.setImageResource(R.drawable.myimage);

Here, the first line retrieves the ImageView defined as myimageview in the layout file using R.id.myimageview. The second line retrieves the image named myimage in the drawable subdirectory of res/ using R.drawable.myimage.

Example

Consider the following definition in res/values/strings.xml:

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <string name="hello">Hello, World!</string>
</resources>

You can now use the resource ID to set the text on a TextView object with ID msg as follows:

TextView msgTextView = (TextView) findViewById(R.id.msg);
msgTextView.setText(R.string.hello);

Example

Consider the following layout definition in res/layout/activity_main.xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
   android:layout_width="fill_parent" 
   android:layout_height="fill_parent" 
   android:orientation="vertical" >

   <TextView android:id="@+id/text"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="Hello, I am a TextView" />

   <Button android:id="@+id/button"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="Hello, I am a Button" />

</LinearLayout>

This application code will load this layout in the onCreate() method as follows:

public void onCreate(Bundle savedInstanceState) {
   super.onCreate(savedInstanceState);
   setContentView(R.layout.main_activity);
}

Accessing Resources in XML

Consider the following XML resource file res/values/strings.xml, which includes a color resource and a string resource:

<?xml version="1.0" encoding="utf-8"?>
<resources>
   <color name="opaque_red">#f00</color>
   <string name="hello">Hello!</string>
</resources>

You can use these resources in a layout file to set text color and content as follows:

<?xml version="1.0" encoding="utf-8"?>
<EditText xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:textColor="@color/opaque_red"
    android:text="@string/hello" />

Now, if you revisit the "Hello World!" example from the previous chapter, you should have a better understanding of all the concepts covered in this section. I strongly recommend reviewing the previous example to see the basic usage of different resources.