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Tools Attributes Reference

Android Studio supports a variety of XML attributes in the tools namespace that enable design-time features (such as which layout to show in a fragment) or compile-time behaviors (such as which shrinking mode to apply to your XML resources). When you build your app, the build tools remove these attributes so there is no effect on your APK size or runtime behavior.

To use these attributes, add the tools namespace to the root element of each XML file where you'd like to use them, as shown here:

<RootTag xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" >

Error handling attributes

The following attributes help suppress lint warning messages.

tools:ignore

Intended for: Any element

Used by: Lint

This attribute accepts a comma-separated list of lint issue ID's that you'd like the tools to ignore on this element or any of its decendents.

For example, you can tell the tools to ignore the MissingTranslation error:

<string name="show_all_apps" tools:ignore="MissingTranslation">All</string>

tools:targetApi

Intended for: Any element

Used by: Lint

This attribute works the same as the @TargetApi annotation in Java code: it lets you specify the API level (either as an integer or as a code name) that supports this element.

This tells the tools that you believe this element (and any children) will be used only on the specified API level or higher. This stops lint from warning you if that element or its attributes are not available on the API level you specify as your minSdkVersion.

For example, you might use this because GridLayout is available only on API level 14 and higher, but you know this layout is not used for any lower versions:

<GridLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    tools:targetApi="14" >

However, you should instead use GridLayout from the support library.

tools:locale

Intended for: <resources>

Used by: Lint, Android Studio editor

This tells the tools what the default language/locale is for the resources in the given <resources> element (because the tools otherwise assume English) in order to avoid warnings from the spell checker. The value must be a valid locale qualifier.

For example, you can add this to your values/strings.xml file (the default string values) to indicate that the language used for the default strings is Spanish rather than English:

<resources xmlns:tools="http://schemas.android.com/tools"
    tools:locale="es">

Design-time view attributes

The following attributes define layout characteristics that are visible only in the Android Studio layout preview.

tools: instead of android:

Intended for: <View>

Used by: Android Studio layout editor

You can insert sample data in your layout preview by using the tools: prefix instead of android: with any <View> attribute from the Android framework. This is useful when the attribute's value isn't populated until runtime but you want to see the effect beforehand, in the layout preview.

For example, if the android:text attribute value is set at runtime or you want to see the layout with a value different than the default, you can add tools:text to specify some text for the layout preview only.

Figure 1. The tools:text attribute sets "Google Voice" as the value for the layout preview

You can add both the android: namespace attribute (which is used at runtime) and the matching tools: attribute (which overrides the runtime attribute in the layout preview only).

You can also use a tools: attribute to unset an attribute only for the layout preview. For example, if you have a FrameLayout with multiple children but you want to see only one child in the layout preview, you can set one of them to be invisible in the layout preview, as shown here:

<Button
    android:id="@+id/button"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="First" />

<Button
    android:id="@+id/button2"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Second"
    tools:visibility="invisible"  />

When using the Layout Editor in design view, the Properties window also allows you to edit some design-time view attributes. Each design-time attribute is indicated with a wrench icon next to the attribute name to distinguish it from the real attribute of the same name.

tools:context

Intended for: Any root <View>

Used by: Lint, Android Studio layout editor

This attribute declares which activity this layout is associated with by default. This enables features in the editor or layout preview that require knowledge of the activity, such as what the layout theme should be in the preview and where to insert onClick handlers when you make those from a quickfix (figure 2).

Figure 2. Quickfix for the onClick attribute works only if you've set tools:context

You can specify the activity class name using the same dot prefix as in the manifest file (excluding the full package name). For example:

<android.support.constraint.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    tools:context=".MainActivity" >

Tip: You can also select the theme for the layout preview from the Layout Editor toolbar.

tools:layout

Intended for: <fragment>

Used by: Android Studio layout editor

This attribute declares which layout you want the layout preview to draw inside the fragment (because the layout preview cannot execute the activity code that normally applies the layout).

For example:

<fragment android:name="com.example.master.ItemListFragment"
    tools:layout="@layout/list_content" />

tools:listitem / tools:listheader / tools:listfooter

Intended for: <AdapterView> (and subclasses like <ListView>)

Used by: Android Studio layout editor

These attributes specify which layout to show in the layout preview for a list's items, header, and footer. Any data fields in the layout are filled with numeric contents such as "Item 1" so that the list items are not repetitive.

For example:

<ListView xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@android:id/list"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:listitem="@layout/sample_list_item"
    tools:listheader="@layout/sample_list_header"
    tools:listfooter="@layout/sample_list_footer" />

tools:showIn

Intended for: Any root <View> in a layout that's referred to by an <include>

Used by: Android Studio layout editor

This attribute allows you to point to a layout that uses this layout as an include, so you can preview (and edit) this file as it appears while embedded in its parent layout.

For example:

<TextView xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:text="@string/hello_world"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    tools:showIn="@layout/activity_main" />

Now the layout preview shows this TextView layout as it appears inside the activity_main layout.

tools:menu

Intended for: Any root <View>

Used by: Android Studio layout editor

This attribute specifies which menu the layout preview should show in the app bar. The value can be one or more menu IDs, separated by commas (without @menu/ or any such ID prefix and without the .xml extension). For example:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:menu="menu1,menu2" />

Resource shrinking attributes

The following attributes allow you to enable strict reference checks and declare whether to keep or discard certain resources when using resource shrinking.

To enable resource shrinking, set the shrinkResources property to true in your build.gradle file (alongside minifyEnabled for code shrinking). For example:

android {
    ...
    buildTypes {
        release {
            shrinkResources true
            minifyEnabled true
            proguardFiles getDefaultProguardFile('proguard-android.txt'),
                    'proguard-rules.pro'
        }
    }
}

tools:shrinkMode

Intended for: <resources>

Used by: Build tools with resource shrinking

This attribute allows you to specify whether the build tools should use "safe mode" (play it safe and keep all resources that are explicitly cited and that might be referenced dynamically with a call to Resources.getIdentifier()) or "strict mode" (keep only the resources that are explicitly cited in code or in other resources).

The default is to use safe mode (shrinkMode="safe"). To instead use strict mode, add shrinkMode="strict" to the <resources> tag as shown here:

<?xml version="1.0" encoding="utf-8"?>
<resources xmlns:tools="http://schemas.android.com/tools"
    tools:shrinkMode="strict" />

When you enable strict mode, you may need to use tools:keep to keep resources that were removed but that you actually want, and use tools:discard to explicitly remove even more resources.

For more information, see Shrink your resources.

tools:keep

Intended for: <resources>

Used by: Build tools with resource shrinking

When using resource shrinking to remove unused resources, this attribute allows you to specify resources to keep (typically because they are referenced in an indirect way at runtime, such as by passing a dynamically generated resource name to Resources.getIdentifier()).

To use, create an XML file in your resources directory (for example, at res/raw/keep.xml) with a <resources> tag and specify each resource to keep in the tools:keep attribute as a comma-separated list. You can use the asterisk character as a wild card. For example:

<?xml version="1.0" encoding="utf-8"?>
<resources xmlns:tools="http://schemas.android.com/tools"
    tools:keep="@layout/used_1,@layout/used_2,@layout/*_3" />

For more information, see Shrink your resources.

tools:discard

Intended for: <resources>

Used by: Build tools with resource shrinking

When using resource shrinking to strip out unused resources, this attribute allows you to specify resources you want to manually discard (typically because the resource is referenced but in a way that does not affect your app, or because the Gradle plugin has incorrectly deduced that the resource is referenced).

To use, create an XML file in your resources directory (for example, at res/raw/keep.xml) with a <resources> tag and specify each resource to keep in the tools:discard attribute as a comma-separated list. You can use the asterisk character as a wild card. For example:

<?xml version="1.0" encoding="utf-8"?>
<resources xmlns:tools="http://schemas.android.com/tools"
    tools:discard="@layout/unused_1" />

For more information, see Shrink your resources.

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