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Android Development
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Creating Resources

Android supports string, bitmap, and many other types of resource. The syntax and format of each, and where they're stored, depends upon the type of object. In general, though, you create resources from three types of files: XML files (everything but bitmaps and raw), bitmap files(for images) and Raw files (anything else, for example sound files, etc.). In fact, there are two different types of XML file as well, those that get compiled as-is into the package, and those that are used to generate resources by aapt. Here is a list of each resource type, the format of the file, a description of the file, and details of any XML files.

You will create and store your resource files under the appropriate subdirectory under the res/ directory in your project. Android has a resource compiler (aapt) that compiles resources according to which subfolder they are in, and the format of the file. Table 1 shows a list of the file types for each resource. See the Available Resources for descriptions of each type of object, the syntax, and the format or syntax of the containing file.

Table 1

Directory Resource Types
res/anim/ XML files that are compiled into frame by frame animation or tweened animation objects
res/drawable/

.png, .9.png, .jpg files that are compiled into the following Drawable resource subtypes:

To get a resource of this type, use mContext.getResources().getDrawable(R.drawable.imageId)

Note: Image resources placed in here may be automatically optimized with lossless image compression by the aapt tool. For example, a true-color PNG that does not require more than 256 colors may be converted to an 8-bit PNG with a color palette. This will result in an image of equal quality but which requires less memory. So be aware that the image binaries placed in this directory can change during the build. If you plan on reading an image as a bit stream in order to convert it to a bitmap, put your images in the res/raw/ folder instead, where they will not be optimized.

res/layout/ XML files that are compiled into screen layouts (or part of a screen). See Declaring Layout.
res/values/

XML files that can be compiled into many kinds of resource.

Note: Unlike the other res/ folders, this one can hold any number of files that hold descriptions of resources to create rather than the resources themselves. The XML element types control where these resources are placed under the R class.

While the files can be named anything, these are the typical files in this folder (the convention is to name the file after the type of elements defined within):

  • arrays.xml to define arrays
  • colors.xml to define color drawables and color string values. Use Resources.getDrawable() and Resources.getColor(), respectively, to get these resources.
  • dimens.xml to define dimension value. Use Resources.getDimension() to get these resources.
  • strings.xml to define string values (use either Resources.getString or preferably Resources.getText() to get these resources. getText() will retain any rich text styling which is usually desirable for UI strings.
  • styles.xml to define style objects.
res/xml/ Arbitrary XML files that are compiled and can be read at run time by calling Resources.getXML().
res/raw/ Arbitrary files to copy directly to the device. They are added uncompiled to the compressed file that your application build produces. To use these resources in your application, call Resources.openRawResource() with the resource ID, which is R.raw.somefilename.

Resources are compiled into the final APK file. Android creates a wrapper class, called R, that you can use to refer to these resources in your code. R contains subclasses named according to the path and file name of the source file

Android Development
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