Chapter 23

Working with Resources

Resources are static bits of information held outside the Java source code. You have seen one type of resource—the layout—frequently in the examples in this book. There are many other types of resources, such as images and strings, that you can take advantage of in your Android applications.

The Resource Lineup

Resources are stored as files under the res/ directory in your Android project layout. With the exception of raw resources (res/raw/), all the other types of resources are parsed for you, either by the Android packaging system or by the Android system on the device or emulator. So, for example, when you lay out an activity's UI via a layout resource (res/layout/), you do not have to parse the layout XML yourself because Android handles that for you.

In addition to layout resources (introduced in Chapter 8), there are several other types of resource available to you, including the following:

  • Images (res/drawable-mdpi/, res/drawable-ldpi, etc.), for putting static icons, images, photos, or other pictures in a user interface
  • Raw (res/raw/), for arbitrary files that have meaning to your application but not necessarily to Android frameworks
  • Strings, colors, arrays, and dimensions (res/values/), for both giving these sorts of constants symbolic names and keeping them separate from the rest of the code (e.g., for internationalization and localization)
  • XML (res/xml/), for static XML files containing your own data and structure

String Theory

Keeping your labels and other bits of text outside the main source code of your application is generally considered to be a very good idea. In particular, it helps with internationalization and localization, covered in the “Different Strokes for Different Folks” section later in this chapter. Even if you are not going to translate your strings to other languages, it is easier to make corrections if all the strings are in one spot instead of scattered throughout your source code.

Android supports regular externalized strings, along with string formats, where the string has placeholders for dynamically inserted information. On top of that, Android supports simple text formatting, called styled text, so you can make your words be bold or italic intermingled with normal text.

Plain Strings

Generally speaking, all you need for plain strings is an XML file in the res/values directory (typically named res/values/strings.xml), with a resources root element, and one child string element for each string you wish to encode as a resource. The string element takes a name attribute, which is the unique name for this string, and a single text element containing the text of the string, as shown in this example:

<resources>
  <string name="quick">The quick brown fox...</string>
  <string name="laughs">He who laughs last...</string>
</resources>

The only tricky part is if the string value contains a quote mark (") or an apostrophe ('). In those cases, you will want to escape those values, by preceding them with a backslash (e.g., These are the times that try men's souls.). Or, if it is just an apostrophe, you could enclose the value in quote marks (e.g., "These are the times that try men's souls.").

You can then reference this string from a layout file (as @string/..., where the ellipsis is the unique name, such as @string/laughs). Or you can get the string from your Java code by calling getString() with the resource ID of the string resource, which is the unique name prefixed with R.string. (e.g., getString(R.string.quick)).

String Formats

As with other implementations of the Java language, Android's Dalvik virtual machine supports string formats. Here, the string contains placeholders representing data to be replaced at runtime by variable information (e.g., My name is %1$s). Plain strings stored as resources can be used as string formats:

String strFormat=getString(R.string.my_name);
String strResult=String.format(strFormat, "Tim");
((TextView)findViewById(R.id.some_label)).setText(strResult);

There is also a flavor of getString() that does the String.format() call for you:

String strResult=getString(R.string.my_name, "Tim");
((TextView)findViewById(R.id.some_label)).setText(strResult);

It is very important that you use the version of the placeholders that takes an index—%1$s instead of just %s. Strategically, translations of your string resources may cause you to apply the variable data in a different order than did your original translation, and using nonindexed placeholders locks you into a particular order. Tactically, your project will fail to compile, as the Android build tools reject nonindexed placeholders nowadays.

Styled Text

If you want really rich text, you should have raw resources containing HTML, and then pour those into a WebKit widget. However, for light HTML formatting, using inline elements such as <b>, <i>, and <u>, you can just use them in a string resource:

<resources>
  <string name="b">This has <b>bold</b> in it.</string>
  <string name="i">Whereas this has <i>italics</i>!</string>
</resources>

You can access these via getText(), which gives you back an object supporting the android.text.Spanned interface and therefore has all of the formatting applied:

((TextView)findViewById(R.id.another_label))
            .setText(getText(R.string.b));

Styled Text and Formats

Where styled text gets tricky is with styled string formats, as String.format() works on String objects, not Spanned objects with formatting instructions. If you really want to have styled string formats, here is the workaround:

  1. Entity-escape the angle brackets in the string resource (e.g., this is &lt;b&gt;%1$s&lt;/b&gt;).
  2. Retrieve the string resource as normal, though it will not be styled at this point (e.g., getString(R.string.funky_format)).
  3. Generate the format results, being sure to escape any string values you substitute, in case they contain angle brackets or ampersands:
    String.format(getString(R.string.funky_format),
                  TextUtils.htmlEncode(strName));
  4. Convert the entity-escaped HTML into a Spanned object via Html.fromHtml():
    someTextView.setText(Html
                         .fromHtml(resultFromStringFormat));

To see this in action, let's look at the Resources/Strings demo. Here is the layout file:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:orientation="vertical"
  android:layout_width="fill_parent"
  android:layout_height="fill_parent"
  >
  <LinearLayout
    android:orientation="horizontal"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    >
    <Button android:id="@+id/format"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="@string/btn_name"
      android:onClick="applyFormat"
      />
    <EditText android:id="@+id/name"
      android:layout_width="fill_parent"
      android:layout_height="wrap_content"
      />
  </LinearLayout>
  <TextView android:id="@+id/result"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    />
</LinearLayout>

As you can see, it is just a button, a field, and a label. The idea is for users to enter their name in the field, and then click the button to cause the label to be updated with a formatted message containing their name.

The Button in the layout file references a string resource (@string/btn_name), so we need a string resource file (res/values/strings.xml):

<?xml version="1.0" encoding="utf-8"?>
<resources>
  <string name="app_name">StringsDemo</string>
  <string name="btn_name">Name:</string>
  <string name="funky_format">My name is &lt;b&gt;%1$s&lt;/b&gt;</string>
</resources>

The app_name resource is automatically created by the android create project command. The btn_name string is the caption of the Button, while our styled string format is in funky_format.

Finally, to hook all this together, we need a pinch of Java:

package com.commonsware.android.strings;

import android.app.Activity;
import android.os.Bundle;
import android.text.TextUtils;
import android.text.Html;
import android.view.View;
import android.widget.EditText;
 import android.widget.TextView;

public class StringsDemo extends Activity {
  EditText name;
  TextView result;

  @Override
  public void onCreate(Bundle icicle) {
    super.onCreate(icicle);
    setContentView(R.layout.main);

    name=(EditText)findViewById(R.id.name);
    result=(TextView)findViewById(R.id.result);
  }

  public void applyFormat(View v) {
    String format=getString(R.string.funky_format);
    String simpleResult=String.format(format,
                     TextUtils.htmlEncode(name.getText().toString()));
    result.setText(Html.fromHtml(simpleResult));
  }
}

The string resource manipulation can be found in applyFormat(), which is called when the button is clicked. First, we get our format via getString()—something we could have done at onCreate() time for efficiency. Next, we format the value in the field using this format, getting a String back, since the string resource is in entity-encoded HTML. Note the use of TextUtils.htmlEncode() to entity-encode the entered name, in case somebody decides to use an ampersand or something. Finally, we convert the simple HTML into a styled text object via Html.fromHtml() and update our label.

When the activity is first launched, we have an empty label, as shown in Figure 23–1.

images

Figure 23–1. The StringsDemo sample application, as initially launched

If we fill in a name and click the button, we get the result shown in Figure 23–2.

images

Figure 23–2. The same application, after filling in some heroic figure's name

Got the Picture?

Android supports images in the PNG, JPEG, BMP, WEBP, and GIF formats. GIF is officially discouraged, however. PNG is the most common format given its preference in earlier versions of Android, and growing popularity on the Web in general. WEBP is newly supported in Ice Cream Sandwich. It's a codec built on the VP8 technology acquired by Google in its purchase of On2 Technologies in 2010. WEBP (usually pronounced “weppy”) provides approximately 40 percent better compression than JPEG for the same image quality. Images can be used anywhere that you require a Drawable, such as the image and background of an ImageView.

Using images is simply a matter of putting your image files in res/drawable/ and then referencing them as a resource. Within layout files, images are referenced as @drawable/..., where the ellipsis is the base name of the file (e.g., for res/drawable/foo.png, the resource name is @drawable/foo). In Java, where you need an image resource ID, use R.drawable. plus the base name (e.g., R.drawable.foo).

So, let's update the previous example to use an icon for the button instead of the string resource. This can be found as Resources/Images. We slightly adjust the layout file, using an ImageButton and referencing a drawable named @drawable/icon, which refers to an image file in res/drawable with a base name of icon. In this case, we use a 32×32-pixel PNG file from the Nuvola icon set.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:orientation="vertical"
  android:layout_width="fill_parent"
  android:layout_height="fill_parent"
  >
  <LinearLayout
    android:orientation="horizontal"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    >
    <ImageButton android:id="@+id/format"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:src="@drawable/icon"
      android:onClick="applyFormat"
      />
    <EditText android:id="@+id/name"
      android:layout_width="fill_parent"
      android:layout_height="wrap_content"
      />
  </LinearLayout>
  <TextView android:id="@+id/result"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    />
</LinearLayout>

Now, our button has the desired icon, as shown in Figure 23–3.

images

Figure 23–3. The ImagesDemo sample application

XML: The Resource Way

If you wish to package static XML with your application, you can use an XML resource. Simply put the XML file in res/xml/, and you can access it by getXml() on a Resources object, supplying it a resource ID of R.xml. plus the base name of your XML file. For example, in an activity, with an XML file of words.xml, you could call getResources().getXml(R.xml.words). This returns an instance of an XmlPullParser, found in the org.xmlpull.v1 Java namespace.

An XML pull parser is event-driven: you keep calling next() on the parser to get the next event, which could be START_TAG, END_TAG, END_DOCUMENT, and so on. On a START_TAG event, you can access the tag's name and attributes; a single TEXT event represents the concatenation of all text nodes that are direct children of this element. By looping, testing, and invoking per-element logic, you parse the file.

To see this in action, let's rewrite the Java code for the Files/Static sample project to use an XML resource. This new project, Resources/XML, requires that you place the words.xml file from Static not in res/raw/, but in res/xml/. The layout stays the same, so all that needs to be replaced is the Java source:

package com.commonsware.android.resources;

import android.app.Activity;
import android.os.Bundle;
import android.app.ListActivity;
import android.view.View;
import android.widget.AdapterView;
 import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;
import java.io.InputStream;
import java.util.ArrayList;
import org.xmlpull.v1.XmlPullParser;
import org.xmlpull.v1.XmlPullParserException;

public class XMLResourceDemo extends ListActivity {
  TextView selection;
  ArrayList<String> items=new ArrayList<String>();

  @Override
  public void onCreate(Bundle icicle) {
    super.onCreate(icicle);
    setContentView(R.layout.main);
    selection=(TextView)findViewById(R.id.selection);

    try {
      XmlPullParser xpp=getResources().getXml(R.xml.words);

      while (xpp.getEventType()!=XmlPullParser.END_DOCUMENT) {
        if (xpp.getEventType()==XmlPullParser.START_TAG) {
          if (xpp.getName().equals("word")) {
            items.add(xpp.getAttributeValue(0));
          }
        }

        xpp.next();
      }
    }
    catch (Throwable t) {
      Toast
        .makeText(this, "Request failed: "+t.toString(), Toast.LENGTH_LONG)
        .show();
    }

    setListAdapter(new ArrayAdapter<String>(this,
                           android.R.layout.simple_list_item_1,
                           items));
  }

  public void onListItemClick(ListView parent, View v, int position,
                  long id) {
    selection.setText(items.get(position).toString());
  }
}

Now, inside our try...catch block, we get our XmlPullParser and loop until the end of the document. If the current event is START_TAG and the name of the element is word (xpp.getName().equals("word")), then we get the one and only attribute and pop that into our list of items for the selection widget. Since we have complete control over the XML file, it is safe enough to assume there is exactly one attribute. In other cases, if you are not sure that the XML is properly defined, you might consider checking the attribute count (getAttributeCount()) and the name of the attribute (getAttributeName()), instead of assuming the 0-index attribute is what you think it is.

The result looks the same as before, albeit with a different name in the title bar, as shown in Figure 23–4.

images

Figure 23–4. The XMLResourceDemo sample application

Miscellaneous Values

In the res/values/ directory, in addition to string resources, you can place one or more XML files describing other simple resources, such as dimensions, colors, and arrays. You have already seen uses of dimensions and colors in previous examples, where they were passed as simple strings (e.g., "10dip") as parameters to calls. You could set these up as Java static final objects and use their symbolic names, but that works only inside Java source, not in layout XML files. By putting these values in resource XML files, you can reference them from both Java and layouts, plus have them centrally located for easy editing.

Resource XML files have a root element of resources; everything else is a child of that root.

Dimensions

Dimensions are used in several places in Android to describe distances, such as a widget's padding. There are several different units of measurement available to you:

  • in and mm for inches and millimeters, respectively. These are based on the actual size of the screen.
  • pt for points. In publishing terms, a point is 1/72 inch (again, based on the actual physical size of the screen)
  • dip and sp for device-independent pixels and scale-independent pixels, respectively. One pixel equals one dip for a 160-dpi resolution screen, with the ratio scaling based on the actual screen pixel density. Scale-independent pixels also take into account the user's preferred font size.

To encode a dimension as a resource, add a dimen element, with a name attribute for your unique name for this resource, and a single child text element representing the value:

<resources>
  <dimen name="thin">10px</dimen>
  <dimen name="fat">1in</dimen>
</resources>

In a layout, you can reference dimensions as @dimen/..., where the ellipsis is a placeholder for your unique name for the resource (e.g., thin and fat from the preceding sample). In Java, you reference dimension resources by the unique name prefixed with R.dimen. (e.g., Resources.getDimen(R.dimen.thin)).

Colors

Colors in Android are hexadecimal RGB values, with the option to also specify an alpha channel. You have your choice of single-character hex values or double-character hex values, providing four styles:

  • #RGB
  • #ARGB
  • #RRGGBB
  • #AARRGGBB

These work similarly to their counterparts in Cascading Style Sheets (CSS).

You can, of course, put these RGB values as string literals in Java source or layout resources. If you wish to turn them into resources, though, all you need to do is add color elements to the resource file, with a name attribute for your unique name for this color, and a single text element containing the RGB value itself:

<resources>
  <color name="yellow_orange">#FFD555</color>
  <color name="forest_green">#005500</color>
  <color name="burnt_umber">#8A3324</color>
</resources>

In a layout, you can reference colors as @color/..., replacing the ellipsis with your unique name for the color (e.g., burnt_umber). In Java, you reference color resources by the unique name prefixed with R.color. (e.g., Resources.getColor(R.color.forest_green)).

Arrays

Array resources are designed to hold lists of simple strings, such as a list of honorifics (Mr., Mrs., Ms., Dr., etc.).

In the resource file, you need one string-array element per array, with a name attribute for the unique name you are giving the array. Then, add one or more child item elements, each with a single text element containing the value for that entry in the array:

<?xml version="1.0" encoding="utf-8"?>
<resources>
  <string-array name="cities">
    <item>Philadelphia</item>
    <item>Pittsburgh</item>
    <item>Allentown/Bethlehem</item>
    <item>Erie</item>
    <item>Reading</item>
    <item>Scranton</item>
    <item>Lancaster</item>
    <item>Altoona</item>
    <item>Harrisburg</item>
  </string-array>
  <string-array name="airport_codes">
    <item>PHL</item>
    <item>PIT</item>
    <item>ABE</item>
    <item>ERI</item>
    <item>RDG</item>
    <item>AVP</item>
    <item>LNS</item>
    <item>AOO</item>
    <item>MDT</item>
  </string-array>
</resources>

From your Java code, you can then use Resources.getStringArray() to get a String[] of the items in the list. The parameter to getStringArray() is your unique name for the array, prefixed with R.array. (e.g., Resources.getStringArray(R.array.honorifics)).

Different Strokes for Different Folks

One set of resources may not fit all situations where your application may be used. One obvious area comes with string resources and dealing with internationalization (I18N) and localization (L10N). Putting strings all in one language works fine—at least for the developer—but covers only one language.

That is not the only scenario where resources might need to differ, though. Here are others:

  • Screen orientation: Is the screen in a portrait or landscape orientation? Or is the screen square and, therefore, without an orientation?
  • Screen size: How many pixels does the screen have, so you can size your resources accordingly (e.g., large versus small icons)?
  • Touchscreen: Does the device have a touchscreen? If so, is the touchscreen set up to be used with a stylus or a finger?
  • Keyboard: Which keyboard does the user have (QWERTY, numeric, neither), either now or as an option?
  • Other input: Does the device have some other form of input, like a D-pad or click-wheel?

The way Android currently handles this is by having multiple resource directories, with the criteria for each embedded in its name.

Suppose, for example, you want to support strings in both English and Spanish. Normally, for a single-language setup, you would put your strings in a file named res/values/strings.xml. To support both English and Spanish, you would create two folders, res/values-en/ and res/values-es/, where the value after the hyphen is the ISO 639-1 two-letter code for the language. Your English strings would go in res/values-en/strings.xml and the Spanish ones would go in res/values-es/strings.xml. Android will choose the proper file based on the user's device settings.

An even better approach is for you to consider some language to be your default, and put those strings in res/values/strings.xml. Then, create other resource directories for your translations (e.g., res/values-es/strings.xml for Spanish). Android will try to match a specific language set of resources; failing that, it will fall back to the default of res/values/strings.xml.

Seems easy, right?

Where things start to get complicated is when you need to use multiple disparate criteria for your resources. For example, suppose you want to develop for the following devices:

  • HTC Nexus 1, which has a normal-size, high-density screen and no hardware keyboard
  • Samsung Galaxy Tab, which has a large-size, high-density screen and no hardware keyboard
  • Motorola Charm, which has a small-size, medium-density screen and a hardware keyboard

You may want to have somewhat different layouts for these devices, to take advantage of different screen real estate and different input options. Specifically, you may want the following:

  • Different layouts for each combination of size, orientation, and keyboard
  • Different drawables for each density

Once you get into these sorts of situations, though, all sorts of rules come into play, such as the following:

  • The configuration options (e.g., -en) have a particular order of precedence, and they must appear in the directory name in that order. The Android documentation outlines the specific order in which these options can appear. For the purposes of this example, screen size is more important than screen orientation, which is more important than screen density, which is more important than whether or not the device has a keyboard.
  • There can be only one value of each configuration option category per directory.
  • Options are case sensitive.

So, for the sample scenario, in theory, we would need the following directories, representing the possible combinations:

  • res/layout-large-port-mdpi-qwerty
  • res/layout-large-port-mdpi-nokeys
  • res/layout-large-port-hdpi-qwerty
  • res/layout-large-port-hdpi-nokeys
  • res/layout-large-land-mdpi-qwerty
  • res/layout-large-land-mdpi-nokeys
  • res/layout-large-land-hdpi-qwerty
  • res/layout-large-land-hdpi-nokeys
  • res/layout-normal-port-mdpi-qwerty
  • res/layout-normal-port-mdpi-nokeys
  • res/layout-normal-port-finger-qwerty
  • res/layout-normal-port-hdpi-nokeys
  • res/layout-normal-land-mdpi-qwerty
  • res/layout-normal-land-mdpi-nokeys
  • res/layout-normal-land-hdpi-qwerty
  • res/layout-normal-land-hdpi-nokeys
  • res/drawable-large-port-mdpi-qwerty
  • res/drawable-large-port-mdpi-nokeys
  • res/drawable-large-port-hdpi-qwerty
  • res/drawable-large-port-hdpi-nokeys
  • res/drawable-large-land-mdpi-qwerty
  • res/drawable-large-land-mdpi-nokeys
  • res/drawable-large-land-hdpi-qwerty
  • res/drawable-large-land-hdpi-nokeys
  • res/drawable-normal-port-mdpi-qwerty
  • res/drawable-normal-port-mdpi-nokeys
  • res/drawable-normal-port-finger-qwerty
  • res/drawable-normal-port-hdpi-nokeys
  • res/drawable-normal-land-mdpi-qwerty
  • res/drawable-normal-land-mdpi-nokeys
  • res/drawable-normal-land-hdpi-qwerty
  • res/drawable-normal-land-hdpi-nokeys

Don't panic! We will shorten this list in just a moment!

Note that there is nothing preventing you from also having a directory with the unadorned base name (res/layout). In fact, this is really a good idea, in case future editions of the Android runtime introduce other configuration options you did not consider—having a default layout might make the difference between your application working or failing on that new device.

As promised, we can cut the number of required directories substantially. We do so by decoding the rules Android uses for determining which, among a set of candidates, is the correct resource directory to use:

  1. Android tosses out directories that are specifically invalid. So, for example, if the screen size of the device is normal, Android drops the -large directories as candidates, since they call for some other size.
  2. Android counts the number of matches for each folder, and pays attention to only those with the most matches.
  3. Android goes in the order of precedence of the options; in other words, it goes from left to right in the directory name.

Also, our drawables vary only by density, and our layouts do not vary by density, so we can clear out a lot of combinations by focusing on only the relevant platform differences.

So, we could skate by with only the following configurations:

  • res/layout-large-land-qwerty
  • res/layout-large-qwerty
  • res/layout-large-land
  • res/layout-large
  • res/layout-normal-land-qwerty
  • res/layout-normal-qwerty
  • res/layout-normal-land
  • res/layout
  • res/drawable-hdpi
  • res/drawable

Here, we take advantage of the fact that specific matches take precedence over unspecified values. So, a device with a QWERTY keyboard will choose a resource with qwerty in the directory over a resource that does not specify its keyboard type.

We could refine this even further, to cover only the specific devices we are targeting (e.g., there is no large device with qwerty):

  • res/layout-large-land
  • res/layout-large
  • res/layout-land-qwerty
  • res/layout-qwerty
  • res/layout-land
  • res/layout
  • res/drawable-hdpi
  • res/drawable

If we did not care about having different layouts depending on whether the device had a hardware keyboard, we could drop the two -qwerty resource sets.

We will see these resource sets again in Chapter 25, which describes how to support multiple screen sizes.

RTL Languages: Going Both Ways

Android 2.3 added support for many more languages than it supported in previous versions of the platform. As such, you now have greater opportunity to localize your application where it is needed.

In particular, Android 2.3 added support for right-to-left (RTL) languages, notably Hebrew and Arabic. Previously, Android supported only languages written horizontally from left to right, such as English. This means you may create localized versions for RTL languages, but first you need to consider whether your UI in general will work properly for RTL languages. For example:

  • Are your TextView widgets aligned on the left side with other widgets or containers? If so, is that the right configuration for your RTL users?
  • Will there be any issues with your EditText widgets when users start entering RTL text, such as inappropriate scrolling because you have not properly constrained the EditText widget's width?
  • If you created your own forms of text input, outside of EditText and the input method framework (e.g., custom onscreen virtual keyboards), will they support RTL languages?

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