R.java
, such as
R.drawable.myimage
@drawable/myimage
Resources
Once you provide a resource in your application (discussed in Providing Resources), you can apply it by
referencing its resource ID. All resource IDs are defined in your project's R
class, which
the aapt
tool automatically generates.
When your application is compiled, aapt
generates the R
class, which contains
resource IDs for all the resources in your res/
directory. For each type of resource, there is an R
subclass (for example,
R.drawable
for all drawable resources) and for each resource of that type, there is a static
integer (for example, R.drawable.icon
). This integer is the resource ID that you can use
to retrieve your resource.
Although the R
class is where resource IDs are specified, you should never need to
look there to discover a resource ID. A resource ID is always composed of:
string
, drawable
, and layout
. For more about the different types, see Resource Types.
android:name
attribute, if the
resource is a simple value (such as a string).There are two ways you can access a resource:
R
class, such as:
R.string.hello
string
is the resource type and hello
is the resource name. There are many
Android APIs that can access your resources when you provide a resource ID in this format. See
Accessing Resources in Code.
R
class, such as:
@string/hello
string
is the resource type and hello
is the resource name. You can use this
syntax in an XML resource any place where a value is expected that you provide in a resource. See Accessing Resources from XML.
You can use a resource in code by passing the resource ID as a method parameter. For
example, you can set an ImageView
to use the res/drawable/myimage.png
resource using setImageResource()
:
ImageView imageView = (ImageView) findViewById(R.id.myimageview); imageView.setImageResource(R.drawable.myimage);
You can also retrieve individual resources using methods in Resources
, which you can get an instance of
with getResources()
.
While uncommon, you might need access your original files and directories. If you do, then
saving your files in res/
won't work for you, because the only way to read a resource from
res/
is with the resource ID. Instead, you can save your resources in the
assets/
directory.
Files saved in the assets/
directory are not given a resource
ID, so you can't reference them through the R
class or from XML resources. Instead, you can
query files in the assets/
directory like a normal file system and read raw data using
AssetManager
.
However, if all you require is the ability to read raw data (such as a video or audio file),
then save the file in the res/raw/
directory and read a stream of bytes using openRawResource()
.
Here's the syntax to reference a resource in code:
[<package_name>.]R.<resource_type>.<resource_name>
<package_name>
is the name of the package in which the resource is located (not
required when referencing resources from your own package).<resource_type>
is the R
subclass for the resource type.<resource_name>
is either the resource filename
without the extension or the android:name
attribute value in the XML element (for simple
values).See Resource Types for more information about each resource type and how to reference them.
There are many methods that accept a resource ID parameter and you can retrieve resources using
methods in Resources
. You can get an instance of Resources
with Context.getResources()
.
Here are some examples of accessing resources in code:
// Load a background for the current screen from a drawable resourcegetWindow()
.setBackgroundDrawableResource
(R.drawable.my_background_image) ; // Set the Activity title by getting a string from the Resources object, because // this method requires a CharSequence rather than a resource IDgetWindow()
.setTitle
(getResources().getText
(R.string.main_title)); // Load a custom layout for the current screensetContentView
(R.layout.main_screen); // Set a slide in animation by getting an Animation from the Resources object mFlipper.setInAnimation
(AnimationUtils.loadAnimation(this, R.anim.hyperspace_in)); // Set the text on a TextView object using a resource ID TextView msgTextView = (TextView) findViewById(R.id.msg); msgTextView.setText
(R.string.hello_message);
Caution: You should never modify the R.java
file by hand—it is generated by the aapt
tool when your project is
compiled. Any changes are overridden next time you compile.
You can define values for some XML attributes and elements using a reference to an existing resource. You will often do this when creating layout files, to supply strings and images for your widgets.
For example, if you add a Button
to your layout, you should use
a string resource for the button text:
<Button android:layout_width="fill_parent" android:layout_height="wrap_content" android:text="@string/submit" />
Here is the syntax to reference a resource in an XML resource:
@[<package_name>:]<resource_type>/<resource_name>
<package_name>
is the name of the package in which the resource is located (not
required when referencing resources from the same package)<resource_type>
is the
R
subclass for the resource type<resource_name>
is either the resource filename
without the extension or the android:name
attribute value in the XML element (for simple
values).See Resource Types for more information about each resource type and how to reference them.
In some cases you must use a resource for a value in XML (for example, to apply a drawable image to a widget), but you can also use a resource in XML any place that accepts a simple value. For example, if you have the following resource file that 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 the following layout file to set the text color and text string:
<?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" />
In this case you don't need to specify the package name in the resource reference because the resources are from your own package. To reference a system resource, you would need to include the package name. For example:
<?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="@android:color/secondary_text_dark" android:text="@string/hello" />
Note: You should use string resources at all times, so that your application can be localized for other languages. For information about creating alternative resources (such as localized strings), see Providing Alternative Resources.
You can even use resources in XML to create aliases. For example, you can create a drawable resource that is an alias for another drawable resource:
<?xml version="1.0" encoding="utf-8"?> <bitmap xmlns:android="http://schemas.android.com/apk/res/android" android:src="@drawable/other_drawable" />
This sounds redundant, but can be very useful when using alternative resource. Read more about Creating alias resources.
A style attribute resource allows you to reference the value of an attribute in the currently-applied theme. Referencing a style attribute allows you to customize the look of UI elements by styling them to match standard variations supplied by the current theme, instead of supplying a hard-coded value. Referencing a style attribute essentially says, "use the style that is defined by this attribute, in the current theme."
To reference a style attribute, the name syntax is almost identical to the normal resource
format, but instead of the at-symbol (@
), use a question-mark (?
), and the
resource type portion is optional. For instance:
?[<package_name>:][<resource_type>/]<resource_name>
For example, here's how you can reference an attribute to set the text color to match the "primary" text color of the system theme:
<EditText id="text" android:layout_width="fill_parent" android:layout_height="wrap_content" android:textColor="?android:textColorSecondary" android:text="@string/hello_world" />
Here, the android:textColor
attribute specifies the name of a style attribute
in the current theme. Android now uses the value applied to the android:textColorSecondary
style attribute as the value for android:textColor
in this widget. Because the system
resource tool knows that an attribute resource is expected in this context,
you do not need to explicitly state the type (which would be
?android:attr/textColorSecondary
)—you can exclude the attr
type.
Android contains a number of standard resources, such as styles, themes, and layouts. To
access these resource, qualify your resource reference with the
android
package name. For example, Android provides a layout resource you can use for
list items in a ListAdapter
:
setListAdapter
(newArrayAdapter
<String>(this, android.R.layout.simple_list_item_1, myarray));
In this example, simple_list_item_1
is a layout resource defined by the
platform for items in a ListView
. You can use this instead of creating
your own layout for list items. (For more about using ListView
, see the
List View Tutorial.)