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Cyril Sermon (@admin) |
Application resources are stored under the res/ folder of your project hierarchy. In this folder, each of the available resource types can have a subfolder containing its resources.
If you start a project using the ADT wizard, it will create a res folder that contains subfolders for the values, drawable, and layout resources that contain the default layout, application icon, and string resource definitions, respectively, as shown in Figure 3-4.
There are seven primary resource types that have different folders: simple values, drawables, layouts, animations, XML, styles, and raw resources. When your application is built, these resources will be compiled as efficiently as possible and included in your application package.
This process also creates an R class file that contains references to each of the resources you include in your project. This lets you reference the resources in your code, with the advantage of design time syn-tax checking.
The following sections describe the specific resource types available within these categories and how to create them for your applications.
In all cases, the resource filenames should contain only lowercase letters, numbers, and the period (.) and underscore (_) symbols.
Supported simple values include strings, colors, dimensions, and string or integer arrays. All simple values are stored within XML files in the res/values folder.
Within each XML file, you indicate the type of value being stored using tags as shown in the sample
XML file below:
<?xml version=”1.0” encoding=”utf-8”?>
<resources>
<string name=”app_name”>To Do List</string> <color name=”app_background”>#FF0000FF</color> <dimen name=”default_border”>5px</dimen> <array name=”string_array”>
<item>Item 1</item>
<item>Item 2</item>
<item>Item 3</item>
</array>
<array name=”integer_array”>
<item>3</item>
<item>2</item>
<item>1</item>
</array>
</resources>This example includes all of the simple value types. By convention, resources are separated into sepa-rate files for each type; for example, res/values/strings.xml would contain only string resources.
The following sections detail the options for defining simple resources.
Externalizing your strings helps maintain consistency within your application and makes it much easier to create localized versions.
String resources are specified using the string tag as shown in the following XML snippet:
<string name=”stop_message”>Stop.</string>Android supports simple text styling, so you can use the HTML tags <b>, <i>, and <u> to apply bold, italics, or underlining to parts of your text strings as shown in the example below:
<string name=”stop_message”><b>Stop.</b></string>You can use resource strings as input parameters for the String.format method. However, String.format does not support the text styling described above. To apply styling to a format string, you have to escape the HTML tags when creating your resource, as shown below:
<string name=”stop_message”><b>Stop</b>. %1$s</string>Within your code, use the Html.fromHtml method to convert this back into a styled character sequence:
String rString = getString(R.string.stop_message);
String fString = String.format(rString, “Collaborate and listen.”);
CharSequence styledString = Html.fromHtml(fString);Use the color tag to define a new color resource. Specify the color value using a # symbol followed by the (optional) alpha channel, then the red, green, and blue values using one or two hexadecimal num-bers with any of the following notations:
❑#RGB
❑#RRGGBB
❑#ARGB
❑#ARRGGBB
The following example shows how to specify a fully opaque blue and a partially transparent green:
<color name=”opaque_blue”>#00F</color>
<color name=”transparent_green”>#7700FF00</color>Dimensions are most commonly referenced within style and layout resources. They’re useful for creat-ing layout constants such as borders and font heights.
To specify a dimension resource, use the dimen tag, specifying the dimension value, followed by an identifier describing the scale of your dimension:
❑ px Screen pixels ❑ in Physical inches ❑ pt Physical points
❑mm Physical millimeters
❑ dp Density-independent pixels relative to a 160-dpi screen ❑ sp Scale-independent pixels
These alternatives let you define a dimension not only in absolute terms, but also using relative scales that account for different screen resolutions and densities to simplify scaling on different hardware.
The following XML snippet shows how to specify dimension values for a large font size and a stan-dard border:
<dimen name=”standard_border”>5px</dimen>
<dimen name=”large_font_size”>16sp</dimen>Style resources let your applications maintain a consistent look and feel by specifying the attribute values used by Views. The most common use of themes and styles is to store the colors and fonts for an application.
You can easily change the appearance of your application by simply specifying a different style as the theme in your project manifest.
To create a style, use a style tag that includes a name attribute, and contains one or more item tags. Each item tag should include a name attribute used to specify the attribute (such as font size or color) being defined. The tag itself should then contain the value, as shown in the skeleton code below:
<?xml version=”1.0” encoding=”utf-8”?>
<resources>
<style name=”StyleName”>
<item name=”attributeName”>value</item>
</style>
</resources>Styles support inheritance using the parent attribute on the style tag, making it easy to create simple variations.
The following example shows two styles that can also be used as a theme; a base style that sets several text properties and a second style that modifies the first to specify a smaller font.
<?xml version=”1.0” encoding=”utf-8”?>
<resources>
<style name=”BaseText”>
<item name=”android:textSize”>14sp</item>
<item name=”android:textColor”>#111</item>
</style>
<style name=”SmallText” parent=”BaseText”>
<item name=”android:textSize”>8sp</item>
</style>
</resources>Drawable resources include bitmaps and NinePatch (stretchable PNG) images. They are stored as indi-vidual files in the res/drawable folder.
The resource identifier for a bitmap resource is the lowercase filename without an extension.
The preferred format for a bitmap resource is PNG, although JPG and GIF files are also supported.
NinePatch (or stretchable) images are PNG files that mark the parts of an image that can be stretched. NinePatch images must be properly defined PNG files that end in .9.png. The resource identifier for NinePatches is the filename without the trailing .9.png.
A NinePatch is a variation of a PNG image that uses a 1-pixel border to define the area of the image that can be stretched if the image is enlarged. To create a NinePatch, draw single-pixel black lines that represent stretchable areas along the left and top borders of your image. The unmarked sections won’t be resized, and the relative size of each of the marked sections will remain the same as the image size changes.
NinePatches are a powerful technique for creating images for the backgrounds of Views or Activities that may have a variable size; for example, Android uses NinePatches for creating button backgrounds.
Layout resources let you decouple your presentation layer by designing user-interface layouts in XML rather than constructing them in code.
The most common use of a layout is to define the user interface for an Activity. Once defined in XML, the layout is “inflated” within an Activity using setContentView, usually within the onCreate method.
You can also reference layouts from within other layout resources, such as layouts for each row in a List View. More detailed information on using and creating layouts in Activities can be found in Chapter 4.
Using layouts to create your screens is best-practice UI design in Android. The decoupling of the layout from the code lets you create optimized layouts for different hardware configurations, such as varying screen sizes, orientation, or the presence of keyboards and touch screens.
Each layout definition is stored in a separate file, each containing a single layout, in the res/layout folder. The filename then becomes the resource identifier.
A thorough explanation of layout containers and View elements is included in the next chapter, but as an example, the following code snippet shows the layout created by the New Project Wizard. It uses a LinearLayout as a layout container for a TextView that displays the “Hello World” greeting.
<?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”>
<TextView
android:layout_width=”fill_parent”
android:layout_height=”wrap_content”
android:text=”Hello World!”
/>
</LinearLayout>Android supports two types of animation. Tweened animations can be used to rotate, move, stretch, and fade a View; or you can create frame-by-frame animations to display a sequence of drawable images. A comprehensive overview of creating, using, and applying animations can be found in Chapter 11.
Defining animations as external resources allows you to reuse the same sequence in multiple places and provides you with the opportunity to present an alternative animation based on device hardware or orientation.
Each tweened animation is stored in a separate XML file in the project’s res/anim folder. As with lay-outs and drawable resources, the animation’s filename is used as its resource identifier.
An animation can be defined for changes in alpha (fading), scale (scaling), translate (moving), or rotate (rotating).
Each of these animation types supports attributes to define what the sequence will do:
Alpha
fromAlpha and toAlpha
Float from 0 to 1
Scale
fromXScale/toXScale
Float from 0 to 1
fromYScale/toYScale
Float from 0 to 1
pivotX/pivotY
String of the percentage of graphic width/height from 0% to 100%
Translate
fromX/toX
Float from 0 to 1
fromY/toY
Float from 0 to 1
Rotate
fromDegrees/toDegrees
Float from 0 to 360
pivotX/pivotY
String of the percentage of graphic width/height from 0% to 100%
You can create a combination of animations using the set tag. An animation set contains one or more animation transformations and supports various additional tags and attributes to customize when and how each animation within the set is run.
The following list shows some of the set tags available:
❑duration Duration of the animation in milliseconds.
❑startOffset Millisecond delay before starting this animation.
❑ fillBefore True to apply the animation transformation before it begins. ❑ fillAfter True to apply the animation transformation after it begins.
❑interpolator Interpolator to set how the speed of this effect varies over time. Chapter 11 explores the interpolators available. To specify an interpolator, reference the system animation resources at android:anim/interpolatorName.
If you do not use the startOffset tag, all the animation effects within a set will execute simultaneously.
The following example shows an animation set that spins the target 360 degrees while it shrinks and fades out:
<?xml version=”1.0” encoding=”utf-8”?>
<set xmlns:android=”http://schemas.android.com/apk/res/android” android:interpolator=”@android:anim/accelerate_interpolator”>
<rotate
android:fromDegrees=”0”
android:toDegrees=”360”
android:pivotX=”50%”
android:pivotY=”50%”
android:startOffset=”500”
android:duration=”1000” />
<scale
android:fromXScale=”1.0”
android:toXScale=”0.0”
android:fromYScale=”1.0”
android:toYScale=”0.0”
android:pivotX=”50%”
android:pivotY=”50%”
android:startOffset=”500”
android:duration=”500” />
<alpha
android:fromAlpha=”1.0”
android:toAlpha=”0.0”
android:startOffset=”500”
android:duration=”500” />
</set>Frame-by-frame animations let you create a sequence of drawables, each of which will be displayed for a specified duration, on the background of a View.
Because frame-by-frame animations represent animated drawables, they are stored in the res/drawble folder, rather than with the tweened animations, and use their filenames (without the xml extension) as their resource IDs.
The following XML snippet shows a simple animation that cycles through a series of bitmap resources, displaying each one for half a second. In order to use this snippet, you will need to create new image resources rocket1 through rocket3.
<animation-list
xmlns:android=”http://schemas.android.com/apk/res/android”
android:oneshot=”false”>
<item android:drawable=”@drawable/rocket1” android:duration=”500” /> <item android:drawable=”@drawable/rocket2” android:duration=”500” /> <item android:drawable=”@drawable/rocket3” android:duration=”500” />
</animation-list>