Android 开发指南 翻译6 Accessing Resources

时间：2022-07-17 05:34:51

Android 开发指南 翻译6 Accessing Resources

处理资源


Once you provide a resource in your application (discussed in Providing Resources), you can apply it by referencing its resource ID. All resource IDs are defined in your project's R class, which the aapt tool automatically generates.

在应用中提够资源后，可通过资源ID引用他们。所有的资源ID定义在项目的R类中。

When your application is compiled, aapt generates the R class, which contains resource IDs for all the resources in your res/ directory. For each type of resource, there is an R subclass (for example, R.drawable for all drawable resources) and for each resource of that type, there is a static integer (for example, R.drawable.icon). This integer is the resource ID that you can use to retrieve your resource.

编译应用时，产生一个R类，包括所有res下的资源。资源ID是一个静态的整数值。

Although the R class is where resource IDs are specified, you should never need to look there to discover a resource ID. A resource ID is always composed of:

  • The resource type: Each resource is grouped into a "type," such as string, drawable, andlayout. For more about the different types, see Resource Types.
  • The resource name, which is either: the filename, excluding the extension; or the value in the XML android:name attribute, if the resource is a simple value (such as a string).

There are two ways you can access a resource:

  • In code: Using an static integer from a sub-class of your R class, such as:
    R.string.hello

    string is the resource type and hello is the resource name. There are many Android APIs that can access your resources when you provide a resource ID in this format. See Accessing Resources in Code.

  • In XML: Using a special XML syntax that also corresponds to the resource ID defined in your R class, such as:
    @string/hello

    string is the resource type and hello is the resource name. You can use this syntax in an XML resource any place where a value is expected that you provide in a resource. See Accessing Resources from XML.

Accessing Resources in Code

在代码中引用资源

You can use a resource in code by passing the resource ID as a method parameter. For example, you can set an ImageView to use theres/drawable/myimage.png resource using setImageResource():

在代码中使用资源，可以传递资源ID给方法的参数。

ImageView imageView = (ImageView) findViewById(R.id.myimageview);
imageView.setImageResource(R.drawable.myimage);

You can also retrieve individual resources using methods in Resources, which you can get an instance of with getResources().

Access to Original Files

While uncommon, you might need access your original files and directories. If you do, then saving your files inres/ won't work for you, because the only way to read a resource from res/ is with the resource ID. Instead, you can save your resources in the assets/ directory.

Files saved in the assets/ directory are not given a resource ID, so you can't reference them through the Rclass or from XML resources. Instead, you can query files in the assets/ directory like a normal file system and read raw data using AssetManager.

However, if all you require is the ability to read raw data (such as a video or audio file), then save the file in theres/raw/ directory and read a stream of bytes usingopenRawResource().

Syntax

Here's the syntax to reference a resource in code:

在代码中引用资源的语法：

[<package_name>.]R.<resource_type>.<resource_name>
  • <package_name> is the name of the package in which the resource is located (not required when referencing resources from your own package).
  • <resource_type> is the R subclass for the resource type.
  • <resource_name> is either the resource filename without the extension or the android:nameattribute value in the XML element (for simple values).

See Resource Types for more information about each resource type and how to reference them.

Use cases

There are many methods that accept a resource ID parameter and you can retrieve resources using methods in Resources. You can get an instance of Resources with Context.getResources().

很多方法可以接受资源ID参数。也可以使用Resouces的方法获得资源，通过Conext.getResouces()获得一个Resources实例。

Here are some examples of accessing resources in code:

// Load a background for the current screen from a drawable resource
getWindow().setBackgroundDrawableResource(R.drawable.my_background_image) ;// Set the Activity title by getting a string from the Resources object, because//  this method requires a CharSequence rather than a resource IDgetWindow().setTitle(getResources().getText(R.string.main_title));// Load a custom layout for the current screensetContentView(R.layout.main_screen);// Set a slide in animation by getting an Animation from the Resources objectmFlipper.setInAnimation(AnimationUtils.loadAnimation(this,        R.anim.hyperspace_in));// Set the text on a TextView object using a resource IDTextView msgTextView = (TextView) findViewById(R.id.msg);msgTextView.setText(R.string.hello_message);

Caution: You should never modify the R.java file by hand—it is generated by the aapt tool when your project is compiled. Any changes are overridden next time you compile.

注意：不应该手工修改R.java文件。

Accessing Resources from XML

You can define values for some XML attributes and elements using a reference to an existing resource. You will often do this when creating layout files, to supply strings and images for your widgets.

使用一个存在的资源，定义XML属性和元素。

For example, if you add a Button to your layout, you should use a string resource for the button text:

<Button
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="@string/submit" />

Syntax

语法

Here is the syntax to reference a resource in an XML resource:

在XML资源文件中引用一个资源

@[<package_name>:]<resource_type>/<resource_name>
  • <package_name> is the name of the package in which the resource is located (not required when referencing resources from the same package)
  • <resource_type> is the R subclass for the resource type
  • <resource_name> is either the resource filename without the extension or the android:name attribute value in the XML element (for simple values).

See Resource Types for more information about each resource type and how to reference them.

Use cases

In some cases you must use a resource for a value in XML (for example, to apply a drawable image to a widget), but you can also use a resource in XML any place that accepts a simple value. For example, if you have the following resource file that includes a color resource and a string resource:

<?xml version="1.0" encoding="utf-8"?>
<resources>
   <color name="opaque_red">#f00</color>
   <string name="hello">Hello!</string>
</resources>

You can use these resources in the following layout file to set the text color and text string:

<?xml version="1.0" encoding="utf-8"?>
<EditText xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:textColor="@color/opaque_red"
    android:text="@string/hello" />

In this case you don't need to specify the package name in the resource reference because the resources are from your own package. To reference a system resource, you would need to include the package name. For example:

这个例子中，在引用资源时，没有必要指定包名，因为资源来自你自己的程序包。要引用系统资源，必须包括包名。

<?xml version="1.0" encoding="utf-8"?>
<EditText xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:textColor="@android:color/secondary_text_dark"
    android:text="@string/hello" />

Note: You should use string resources at all times, so that your application can be localized for other languages. For information about creating alternative resources (such as localized strings), see Providing Alternative Resources.

You can even use resources in XML to create aliases. For example, you can create a drawable resource that is an alias for another drawable resource:

<?xml version="1.0" encoding="utf-8"?>
<bitmap xmlns:android="http://schemas.android.com/apk/res/android"
    android:src="@drawable/other_drawable" />

This sounds redundant, but can be very useful when using alternative resource. Read more about Creating alias resources.

Referencing style attributes

引用style属性

A style attribute resource allows you to reference the value of an attribute in the currently-applied theme. Referencing a style attribute allows you to customize the look of UI elements by styling them to match standard variations supplied by the current theme, instead of supplying a hard-coded value. Referencing a style attribute essentially says, "use the style that is defined by this attribute, in the current theme."

风格属性资源允许引用当前主题的属性值。“使用在当前主题中定义的一个属性”

To reference a style attribute, the name syntax is almost identical to the normal resource format, but instead of the at-symbol (@), use a question-mark (?), and the resource type portion is optional. For instance:

引用style属性，语法和普通资源相同，只有用?代替@。

?[<package_name>:][<resource_type>/]<resource_name>

For example, here's how you can reference an attribute to set the text color to match the "primary" text color of the system theme:

<EditText id="text"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:textColor="?android:textColorSecondary"
    android:text="@string/hello_world" />

Here, the android:textColor attribute specifies the name of a style attribute in the current theme. Android now uses the value applied to theandroid:textColorSecondary style attribute as the value for android:textColor in this widget. Because the system resource tool knows that an attribute resource is expected in this context, you do not need to explicitly state the type (which would be ?android:attr/textColorSecondary)—you can exclude theattr type.

由于系统资源工具知道是属性资源，所以不必生命类型-可以不写attr类型标识。

Accessing Platform Resources

使用平台资源 使用Android平台资源，必须指定android包名。

Android contains a number of standard resources, such as styles, themes, and layouts. To access these resource, qualify your resource reference with theandroid package name. For example, Android provides a layout resource you can use for list items in a ListAdapter:

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

In this example, simple_list_item_1 is a layout resource defined by the platform for items in a ListView. You can use this instead of creating your own layout for list items. (For more about using ListView, see the List View Tutorial.)

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