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Understanding App Resources

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Nathan Esquenazi edited this page Feb 8, 2016 · 37 revisions

Overview

In Android, almost everything is a resource. Defining resources that you can then access in your app is an essential part of Android development.

Resources are used for anything from defining colors, images, layouts, menus, and string values. The value of this is that nothing is hardcoded. Everything is defined in these resource files and then can be referenced within your application's code. The simplest of these resources and the most common is using string resources to allow for flexible, localized text.

Types of Resources

The following are the most common types of resources within Android apps:

Name Folder Description
Property Animations animator XML files that define property animations.
Tween Animations anim XML files that define tween animations.
Drawables drawable Bitmap files or XML files that act as graphics
Layout layout XML files that define a user interface layout
Menu menu XML files that define menus or action bar items
Values values XML files with values such as strings, integers, and colors.

In addition, note the following key files stored within the values folder mentioned above:

Name File Description
Colors res/values/colors.xml This is where we put color definitions
Dimensions res/values/dimens.xml This is where we put dimension values
Strings res/values/strings.xml This is where we put string values
Styles res/values/styles.xml This is where we put style values

For the full list of resource types, check out the Providing a Resource guide.

Defining a String Resource

For every piece of text you want to display within your application (i.e the label of a button, or the text inside a TextView), you should first define the text in the res/values/strings.xml file. Each entry is a key (representing the id of the text) and a value (the text itself). For example, if I want a button to display "Submit", I might add the following string resource to res/values/strings.xml:

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

Now if I ever reference the string resource for submit_label, the default will be for "Submit" to be displayed. Later though, you could create qualified resource files that change this value for different countries or between devices. We can also store more complex strings (with html or special characters) by using CDATA to escape the string such as:

<?xml version="1.0" encoding="utf-8"?>
<resources>
  <string name="feedback_label">
  <![CDATA[
    Please <a href="http://highlight.com">let us know</a> if you have feedback on this or if 
    you would like to log in with another identity service. Thanks! This is a longer string!  
  ]]>
  </string>
</resources>

For more details on defining string resources, check this guide. You can also refer to this guide for style resources and this guide for other types.

Referencing a Resource

Now that we have defined our string resource, we can access that resource in either our Java code or our XML layouts at any time. To access, the resource in the XML Layout file, simply use the @ syntax used to access any resource:

<Button
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="@string/submit_label" />

To access the resource directly in your Java code, simply use the getResources.getString or getString methods to access the value given the resource id:

String submitText = getResources().getString(R.string.submit_label)

And the string value will be retrieved. This similar pattern works for almost any resource from images (drawables) to colors. The getResources() method returns a Resources object with many resource fetching methods. Each resource is defined within different folders and files within the res directory depending on their type.

Dynamic Resource Retrieval

In certain cases, you might want to dynamically retrieve resources using just the key name rather than by "hardcoding" the resource id. For example, suppose I wanted to retrieve the "submit_label" string based on just that key name alone. This can be achieved using the getIdentifier method within an Activity:

public String getStringValue(String key) {
    // Retrieve the resource id
    String packageName = getBaseContext().getPackageName();
    Resources resources = getBaseContext().getResources();
    int stringId = resources.getIdentifier(key, "string", packageName);
    if (stringId == 0) { return null; }
    // Return the string value based on the res id
    return resources.getString(stringId);
}

Now you can reference your string resources dynamically using:

public String myKey = "submit_label"; // Maps to R.string.submit_label
public String myStringValue = getStringValue(myKey); // Returns string text

This can be done similarly for other types of resources as well. For example, for dynamically retrieving a view by a string ID:

// getViewById("tvTest");
public View getViewById(String id) {
    // Retrieve the resource id
    String packageName = getBaseContext().getPackageName();
    Resources resources = getBaseContext().getResources();
    int viewId = resources.getIdentifier(id, "id", packageName);
    if (viewId == 0) { return null; }
    // Return the string value based on the res id
    return findViewById(viewId);
}

Check out the getResources object and getIdentifier for more details.

Providing Alternate Resources

Responsive Design

In order to create an outstanding UI design, it is essential for app developers to create apps that work properly across a wide variety of devices. To do this, we first divide android mobile devices into various categories (buckets) based on screen size and display as shown below:

Apps must be designed to work across any different screen densities as well as screen sizes. This can be done leveraging a variety of systems provided by the Android framework.

Introducing Alternate Resources

One of the most powerful tools available to developers is the option to provide "alternate resources" based on specific qualifiers such as phone size, language, density, and others. Common uses for alternate resources include:

  • Alternative layout files for different form factors (i.e phone vs tablet)
  • Alternative string resources for different languages (i.e English vs Italian)
  • Alternative drawable resources for different screen densities (shown below)
  • Alternate style resources for different platform versions (Holo vs Material)
  • Alternate layout files for different screen orientations (i.e portrait vs landscape)

To specify configuration-specific alternatives for a set of resources, we create a new directory in res in the form of [resource]-[qualifiers]. For example, one best practice is to ensure that all images are provided for multiple screen densities.

Densities

This is achieved by having res/drawable-hdpi, res/drawable-xhdpi, and res/drawable-xxhdpi folders with different versions of the same image. The correct resource is then selected automatically by the system based on the device density. The directory listing might look like the following:

res/
    drawable/   
        icon.png
        background.png    
    drawable-hdpi/  
        icon.png
        background.png

Note the resource files need to all have the same name within the different folders. This system works for any type of resource with a wide variety of qualifiers.

Understanding Qualifiers

Android supports several configuration qualifiers and you can add multiple qualifiers to one directory name, by separating each qualifier with a dash. The most common qualifiers are listed below:

Configuration Examples Description
Language en, fr Language code selected on the device
Screen size sw480dp,sw600dp Minimum width of the screen's height or width.
Screen orientation port, land Screen is in portrait or landscape mode.
Screen density hdpi, xhdpi Screen density often used for alternate images.
Platform version v7, v11, v21 Platform version often used for styles.

You can specify multiple qualifiers for a single set of resources, separated by dashes. For example, drawable-en-sw600dp-land applies to English tablets in landscape orientation. Note that if you use multiple qualifiers for a resource directory, you must add them to the directory name in the order they are listed in the table above. See the official docs for the complete set of qualifiers available.

Alternate Layout Files

Often alternative resources are used to specify different layout files for phones and tablets. This can be done using the "smallest width" qualifer of sw. The folder structure might be setup as the following:

res/
    layout/   
        activity_main.xml
        item_photo.xml    
    layout-sw600dp/ 
        activity_main.xml
    layout-sw600dp-land/
        activity_main.xml 
    layout-sw720dp/ 
        activity_main.xml
        item_photo.xml

Generally speaking phones and phablets are between sw240 and sw480. 7" tablets are sw600 and 10" tablets are sw720. Here’s an illustrated example of the above procedure:

For a detailed tutorial on how to manage responsive layouts for tablets, review our Flexible User Interfaces guide. You can also review this article on UI design best practices and this official doc on resources for more details.

Layout Best Practices

Here is a quick checklist about how you can ensure that your application displays properly on different screens:

  • Avoid use hard coded pixel values in your application code
  • Use RelativeLayout properly and never use AbsoluteLayout
  • Use wrap_content, match_parent, or dp units when specifying dimensions
  • Use alternate layouts and drawables to ensure a responsive design when needed

Review the rest of the best practices for screen independence on the official guide.

Alias Resources

When you have a resource that you'd like to use for more than one device configuration, you do not need to put the same resource in more than one alternative resource directory. Instead, you can create an alternative resource that acts as an alias for a resource saved in your default resource directory.

Best Resource Match

When you request a resource for which you provide alternatives, Android selects which alternative resource to use at runtime, depending on the current device configuration. Read the official resource guide for a detailed overview of how the match is selected.

References

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