Resources and InternationalizationResources are external files (that is, non-code files) that are used by your code and compiled into your application at build time. Android supports a number of different kinds of resource files, including XML, PNG, and JPEG files. The XML files have very different formats depending on what they describe. This document describes what kinds of files are supported, and the syntax or format of each. Resources are externalized from source code, and XML files are compiled into a binary, fast loading format for efficiency reasons. Strings, likewise, are compressed into a more efficient storage form. It is for these reasons that we have these different resource types in the Android platform. This is a fairly technically dense document, and together with the Available Resources document, they cover a lot of information about resources. It is not necessary to know this document by heart to use Android, but rather to know that the information is here when you need it. IntroductionThis topic includes a terminology list associated with resources, and a series of examples of using resources in code. For a complete guide to the supported Android resource types, see Available Resources. The Android resource system keeps track of all non-code assets associated with an application. You use the An application's resources are compiled into the application binary at build time for you by the build system. To use a resource, you must install it correctly in the source tree and build your application. As part of the build process, symbols for each of the resources are generated that you can use in your source code -- this allows the compiler to verify that your application code matches up with the resources you defined. The rest of this section is organized as a tutorial on how to use resources in an application. Creating ResourcesAndroid 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
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 Global Resource Notes
Using ResourcesThis section describes how to use the resources you've created. It includes the following topics:
At compile time, Android generates a class named R that contains resource identifiers to all the resources in your program. This class contains several subclasses, one for each type of resource supported by Android, and for which you provided a resource file. Each class contains one or more identifiers for the compiled resources, that you use in your code to load the resource. Here is a small resource file that contains string, layout (screens or parts of screens), and image resources. Note: the R class is an auto-generated file and is not designed to be edited by hand. It will be automatically re-created as needed when the resources are updated. package com.android.samples; public final class R { public static final class string { public static final int greeting=0x0204000e; public static final int start_button_text=0x02040001; public static final int submit_button_text=0x02040008; public static final int main_screen_title=0x0204000a; }; public static final class layout { public static final int start_screen=0x02070000; public static final int new_user_pane=0x02070001; public static final int select_user_list=0x02070002; }; public static final class drawable { public static final int company_logo=0x02020005; public static final int smiling_cat=0x02020006; public static final int yellow_fade_background=0x02020007; public static final int stretch_button_1=0x02020008; }; }; Using Resources in CodeUsing resources in code is just a matter of knowing the full resource ID and what type of object your resource has been compiled into. Here is the syntax for referring to a resource:
or
Where Here are some good and bad examples of using compiled resources in code: // Load a background for the current screen from a drawable resource. this.getWindow().setBackgroundDrawableResource(R.drawable.my_background_image); // WRONG Sending a string resource reference into a // method that expects a string. this.getWindow().setTitle(R.string.main_title); // RIGHT Need to get the title from the Resources wrapper. this.getWindow().setTitle(Resources.getText(R.string.main_title)); // Load a custom layout for the current screen. setContentView(R.layout.main_screen); // Set a slide in animation for a ViewFlipper object. mFlipper.setInAnimation(AnimationUtils.loadAnimation(this, R.anim.hyperspace_in)); // Set the text on a TextView object. TextView msgTextView = (TextView)findViewByID(R.id.msg); msgTextView.setText(R.string.hello_message); References to ResourcesA value supplied in an attribute (or resource) can also be a reference to a resource. This is often used in layout files to supply strings (so they can be localized) and images (which exist in another file), though a reference can be any resource type including colors and integers. For example, if we have color resources, we can write a layout file that sets the text color size to be the value contained in one of those resources: <?xml version="1.0" encoding="utf-8"?> <EditText id="text" xmlns:android="http://schemas./apk/res/android" android:layout_width="fill_parent" android:layout_height="fill_parent" android:textColor="@color/opaque_red" android:text="Hello, World!" /> Note here the use of the '@' prefix to introduce a resource reference -- the text following that is the name of a resource in the form of <?xml version="1.0" encoding="utf-8"?> <EditText id="text" xmlns:android="http://schemas./apk/res/android" android:layout_width="fill_parent" android:layout_height="fill_parent" android:textColor="@android:color/opaque_red" android:text="Hello, World!" /> As another example, you should always use resource references when supplying strings in a layout file so that they can be localized: <?xml version="1.0" encoding="utf-8"?> <EditText id="text" xmlns:android="http://schemas./apk/res/android" android:layout_width="fill_parent" android:layout_height="fill_parent" android:textColor="@android:color/opaque_red" android:text="@string/hello_world" /> This facility can also be used to create references between resources. For example, we can create new drawable resources that are aliases for existing images: <?xml version="1.0" encoding="utf-8"?> <resources> <drawable id="my_background">@android:drawable/theme2_background</drawable> </resources> References to Theme AttributesAnother kind of resource value allows you to reference the value of an attribute in the current theme. This attribute reference can only be used in style resources and XML attributes; it allows you to customize the look of UI elements by changing them to standard variations supplied by the current theme, instead of supplying more concrete values. As an example, we can use this in our layout to set the text color to one of the standard colors defined in the base system theme: <?xml version="1.0" encoding="utf-8"?> <EditText id="text" xmlns:android="http://schemas./apk/res/android" android:layout_width="fill_parent" android:layout_height="fill_parent" android:textColor="?android:textDisabledColor" android:text="@string/hello_world" /> Note that this is very similar to a resource reference, except we are using an '?' prefix instead of '@'. When you use this markup, you are supplying the name of an attribute resource that will be looked up in the theme -- because the resource tool knows that an attribute resource is expected, you do not need to explicitly state the type (which would be Other than using this resource identifier to find the value in the theme instead of raw resources, the name syntax is identical to the '@' format: Using System ResourcesMany resources included with the system are available to applications. All such resources are defined under the class "android.R". For example, you can display the standard application icon in a screen with the following code: public class MyActivity extends Activity { public void onStart() { requestScreenFeatures(FEATURE_BADGE_IMAGE); super.onStart(); setBadgeResource(android.R.drawable.sym_def_app_icon); } } In a similar way, this code will apply to your screen the standard "green background" visual treatment defined by the system: public class MyActivity extends Activity { public void onStart() { super.onStart(); setTheme(android.R.style.Theme_Black); } } Alternate Resources (for alternate languages and configurations)You can supply different resources for your application to use depending on the UI language or hardware configuration on the device. Note that although you can include different string, layout, and other resources, the SDK does not expose methods to let you specify which alternate resource set to load. Android detects the proper set for the hardware and location, and loads them as appropriate. Users can select alternate language settings using the settings panel on the device. To include alternate resources, create parallel resource folders with qualifiers appended to the folder names, indicating the configuration it applies to (language, screen orientation, and so on). For example, here is a project that holds one string resource file for English, and another for French: MyApp/ res/ values-en/ strings.xml values-fr/ strings.xml Android supports several types of qualifiers, with various values for each. Append these to the end of the resource folder name, separated by dashes. You can add multiple qualifiers to each folder name, but they must appear in the order they are listed here. For example, a folder containing drawable resources for a fully specified configuration would look like this: MyApp/ res/ drawable-en-rUS-large-long-port-mdpi-finger-keysexposed-qwerty-navexposed-dpad-480x320/ More typically, you will only specify a few specific configuration options. You may drop any of the values from the complete list, as long as the remaining values are still in the same order: MyApp/ res/ drawable-en-rUS-finger/ drawable-port/ drawable-port-mdpi/ drawable-qwerty/ Table 2 lists the valid folder-name qualifiers, in order of precedence. Qualifiers that are listed higher in the table take precedence over those listed lower, as described in How Android finds the best matching directory.
This list does not include device-specific parameters such as carrier, branding, device/hardware, or manufacturer. Everything that an application needs to know about the device that it is running on is encoded via the resource qualifiers in the table above. All resource directories, qualified and unqualified, live under the
How resources are referenced in codeAll resources will be referenced in code or resource reference syntax by their simple, undecorated names. So if a resource were named this: If several drawable directories are available, Android will select one of them (as described below) and load How Android finds the best matching directoryAndroid will pick which of the various underlying resource files should be used at runtime, depending on the current configuration of the device. The example used here assumes the following device configuration:
Here is how Android makes the selection:
Exception: If the qualifier in question is screen pixel density, Android will select the option that most closely matches the device, and the selection process will be complete. In general, Android will prefer scaling down a larger original image to scaling up a smaller original image. Tip: The precedence of the qualifiers is more important than the number of qualifiers that exactly match the device. For example, in step 4 above, the last choice on the list includes three qualifiers that exactly match the device (orientation, touchscreen type, and input method), while This flowchart summarizes how Android selects resource directories to load. TerminologyThe resource system brings a number of different pieces together to form the final complete resource functionality. To help understand the overall system, here are some brief definitions of the core concepts and components you will encounter in using it: Asset: A single blob of data associated with an application. This includes object files compiled from the Java source code, graphics (such as PNG images), XML files, etc. These files are organized in a directory hierarchy that, during final packaging of the application, is bundled together into a single ZIP file. aapt: Android Asset Packaging Tool. The tool that generates the final ZIP file of application assets. In addition to collecting raw assets together, it also parses resource definitions into binary asset data. Resource Table: A special asset that aapt generates for you, describing all of the resources contained in an application/package. This file is accessed for you by the Resources class; it is not touched directly by applications. Resource: An entry in the Resource Table describing a single named value. Broadly, there are two types of resources: primitives and bags. Resource Identifier: In the Resource Table all resources are identified by a unique integer number. In source code (resource descriptions, XML files, Java source code) you can use symbolic names that stand as constants for the actual resource identifier integer. Primitive Resource: All primitive resources can be written as a simple string, using formatting to describe a variety of primitive types included in the resource system: integers, colors, strings, references to other resources, etc. Complex resources, such as bitmaps and XML describes, are stored as a primitive string resource whose value is the path of the underlying Asset holding its actual data. Bag Resource: A special kind of resource entry that, instead of a simple string, holds an arbitrary list of name/value pairs. Each name is itself a resource identifier, and each value can hold the same kinds of string formatted data as a normal resource. Bags also support inheritance: a bag can inherit the values from another bag, selectively replacing or extending them to generate its own contents. Kind: The resource kind is a way to organize resource identifiers for various purposes. For example, drawable resources are used to instantiate Drawable objects, so their data is a primitive resource containing either a color constant or string path to a bitmap or XML asset. Other common resource kinds are string (localized string primitives), color (color primitives), layout (a string path to an XML asset describing a view layout), and style (a bag resource describing user interface attributes). There is also a standard "attr" resource kind, which defines the resource identifiers to be used for naming bag items and XML attributes Style: The name of the resource kind containing bags that are used to supply a set of user interface attributes. For example, a TextView class may be given a style resource that defines its text size, color, and alignment. In a layout XML file, you associate a style with a bag using the "style" attribute, whose value is the name of the style resource. Style Class: Specifies a related set of attribute resources. This data is not placed in the resource table itself, but used to generate constants in the source code that make it easier for you to retrieve values out of a style resource and/or XML tag's attributes. For example, the Android platform defines a "View" style class that contains all of the standard view attributes: padding, visibility, background, etc.; when View is inflated it uses this style class to retrieve those values from the XML file (at which point style and theme information is applied as approriate) and load them into its instance. Configuration: For any particular resource identifier, there may be multiple different available values depending on the current configuration. The configuration includes the locale (language and country), screen orientation, etc. The current configuration is used to select which resource values are in effect when the resource table is loaded. Theme: A standard style resource that supplies global attribute values for a particular context. For example, when writing an Activity the application developer can select a standard theme to use, such as the Theme.White or Theme.Black styles; this style supplies information such as the screen background image/color, default text color, button style, text editor style, text size, etc. When inflating a layout resource, most values for widgets (the text color, selector, background) if not explicitly set will come from the current theme; style and attribute values supplied in the layout can also assign their value from explicitly named values in the theme attributes if desired. Overlay: A resource table that does not define a new set of resources, but instead replaces the values of resources that are in another resource table. Like a configuration, this is applied at load time to the resource data; it can add new configuration values (for example strings in a new locale), replace existing values (for example change the standard white background image to a "Hello Kitty" background image), and modify resource bags (for example change the font size of the Theme.White style to have an 18 pt font size). This is the facility that allows the user to select between different global appearances of their device, or download files with new appearances. Resource ReferenceThe Available Resources document provides a detailed list of the various types of resource and how to use them from within the Java source code, or from other references. Internationalization and LocalizationComing Soon: Internationalization and Localization are critical, but are also not quite ready yet in the current SDK. As the SDK matures, this section will contain information on the Internationalization and Localization features of the Android platform. In the meantime, it is a good idea to start by externalizing all strings, and practicing good structure in creating and using resources. |
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来自: lao_o > 《Resources and Assets》