Supporting Multiple Screens | Android Developers

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5/4/11 7:30 AMSupporting Multiple Screens | Android Developers

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Quickview

Android runs on devices that have different

screen sizes and resolutions.The screen on which your application isdisplayed can affect its user interface.The platform handles most of the work of

adapting your app to the current screen.You can create screen-specific resources forprecise control of your UI, if needed.

Older applications run in a compatibility modethat provides best-effort rendering on thecurrent screen.

It's important to follow the best practicesdescribed in this document and test yourapplication in all supported screens.

In this document

Overview of Screen SupportRange of screens supported

How Android supports multiple screens

Density independence

Manifest attributes

Resource qualifiers

Best Practices for Screen Independence

Strategies for Legacy Apps

How to Test Your App

See Also

Providing Alternative Resources

Creating and Managing Virtual Devices

Supporting Multiple Screens

Android is designed to run on a variety of devices that offera range of screen sizes and resolutions. For applications,

the platform provides a consistent environment across

devices and handles much of the complexity of adapting an

application's UI to the screen on which it is being displayed.

At the same time, the platform exposes APIs that give

application developers precise control over their

application's UI when displayed on specific screen sizes

and resolutions.

This document explains the screens-support features

provided by the platform and how you use them in your

application. By following the practices described here, you

can easily create an application that displays properly on allsupported device screens and that you can deploy to any

device as a single .apk.

If you have already developed and published an application

for Android 1.5 orearlier, you should read this document

and consider how you may need to adapt your application

for proper display on new devices that offer different

screens and that are running Android 1.6 or later. In most

cases, only minor adjustments are needed, howeveryou

should make sure to test your application on all supported

screens.

Starting in Android 2.2, the platform includes support for

extra high density screens (xhdpi), and starting in Android2.3, the platform includes support for extra large screens

(xlarge). If you've already followed the guidance in this

document to support all other screen types, you should

consider providing additional support forxhdpiandxlarge

screens.

In particular, if you have an existing application that you would like to make available on small screens (such as

QVGA) or for which you would like to provide better support for extra large screens, please see Strategies for Legacy

Applications for more information about how to do that.

Overview of Screens Support

The sections below provide an overview of the Android platform's support for multiple screens, including an

introduction to terms and concepts used in this document and in the API, a summary of the screen configurations that

the platform supports, and an overview of the API and underlying screen-compatibility features.

Terms and Concepts

Screen size

Actual physical size, measured as the screen's diagonal.

For simplicity, Android collapses all actual screen sizes into four generalized sizes: small, normal, large, and extra

large. Applications can provide custom layouts for each of these four sizes the platform transparently handles

the rendering of the layouts at the actual screen size.

Aspect ratio
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The porportional relationship of the screen's physical width to its height. Applications can provide layout resources

for specific aspect ratios by using the resource qualifiers long and notlong.

Resolution

The total number of physical pixels on a screen. Note that, although resolution is often expressed as width x

height, resolution does not imply a specific aspect ratio. In Android, applications do not work directly with

resolution.

Density

Based on the screen resolution, the spread of pixels across the physical width and height of the screen.

A screen with lower density has fewer available pixels spread across the screen width and height, where a screen

with higher density has more sometimes significantly more pixels spread across the same area. The density

of a screen is important because, other things being equal, a UI element (such as a button) whose height and

width are defined in terms of screen pixels will appear larger on the lower density screen and smaller on the

higher density screen.

For simplicity, Android collapses all actual screen densities into four generalized densities: low, medium, large,

and extra large. Applications can provide custom resources for each of these densities the platform handles any

necessary scaling of the resources up or down to meet the specific screen density.

Density-independent pixel (dp)

A virtual pixel unit that applications can use in defining their UI, to express layout dimensions or position in a

density-independent way.

The density-independent pixel is equivalent to one physical pixel on a 160 dpi screen, the baseline density

assumed by the platform (as described later in this document). At run time, the platform transparently handles any

scaling of the dp units needed, based on the actual density of the screen in use. The conversion of dp units to

screen pixels is simple: pixels = dps * (density / 160). For example, on 240 dpi screen, 1 dp would

equal 1.5 physical pixels. Using dp units to define your application's UI is highly recommended, as a way of

ensuring proper display of your UI on different screens.

Range of screens supported

Starting from Android 1.6, the platform provides support for multiple screen sizes and resolutions, reflecting the many

new types and sizes of devices on which the platform runs. If you are developing an application that will run on

Android 1.6 or later, you can use the compatibility features of the Android platform to ensure that your application UIrenders properly across the range of supported screen sizes and resolutions.

To simplify the way that developers design their user interfaces for multiple devices and to allow more devices to

participate without affecting applications, the platform divides the range of actual supported screen sizes and

resolutions into:

A set of four generalized sizes: small, normal, large, andxlarge

A set of four generalized densities: ldpi(low), mdpi (medium), hdpi(high), andxhdpi (extra high)

Note: The xhdpi density category was added in Android 2.2 (API Level 8). Thexlarge size category was

added in Android 2.3 (API Level 9).

Applications can provide custom resources (primarily layouts) for any of the four generalized sizes and can provide

resources (primarily drawables such as images) for any of the four generalized densities. Applications do not need to

work with the actual physical size or density of the device screen. At run time, the platform handles the loading of the

correct size or density resources, based on the generalized size or density of the current device screen, and adapts

them to the actual pixel map of the screen.

The generalized size/density configurations are arranged around a baseline configuration that is assigned a size of

normaland a density ofmdpi(medium). All applications written for Android 1.5 or earlier are (by definition) designed

for the baseline HVGA screen used on the T-Mobile G1 and similar devices, which is size normaland density mdpi.

Each generalized screen configuration spans a range of actual screen densities and physical sizes. For example, that

means that multiple devices that report a screen size ofnormalmight offer screens that differ slightly in actual size or

aspect ratio. Similarly, devices that report a screen density ofhdpimight offer screens with slightly different pixel

densities. The platform makes these differences abstract, however applications can offer UI designed for the

generalized sizes and densities and let the system handle the actual rendering of the UI on the current device screenaccording to its characteristics.


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Using the alternative resources framework

The platform's support for loading screen size- and

density-specific resources at run time is based on

the alternative resources framework.

If you want to use size- or density-specific layouts or

drawables in your application and you are not

familiar with resource qualifiers or how the platform

uses them, please read Providing Alternative

Resources.

Figure 1. Illustration of how the Android platform maps actual screen densities and sizes to generalized density and

size configurations.

Although the platform lets your application provide customized resources for the various size and density

configurations, you do not need to do write custom code or provide custom resources for every combination of screen

size and density. The platform provides robust compatibility features, described in the sections below, that can handle

most of the work of rendering your application on any device screen, provided that you've implemented your

application UI properly. For more information about how to implement a UI that renders properly across device screens

and platform versions, see Best Practices for Screen Independence.

To help you test your applications, the Android SDK includes emulator skins that replicate the sizes and densities of

actual device screens on which your application is likely to run. You can also modify the default size and density of the

emulator skins to replicate the characteristics of any specific screen.

Table 1. Screen sizes and densities of emulator skins included in the Android SDK.

Low density (120), ldpi Medium density (160), mdpi High density (240), hdpi Extra high density (320),xhdpi

Smallscreen

QVGA (240x320)

Normalscreen

WQVGA400 (240x400)

WQVGA432 (240x432)

HVGA (320x480) WVGA800 (480x800)

WVGA854 (480x854)

Largescreen

WVGA800* (480x800)WVGA854* (480x854)

ExtraLargescreen

* To emulate this configuration, specify a custom density of 160 when creating an AVD that uses aWVGA800 or WVGA854 skin.

For an overview of the relative numbers of high (hdpi), medium (mdpi), and low (ldpi) density screens in Android-

powered devices available now, see the Screen Sizes and Densities dashboard.

How Android supports multiple screens

The foundation of Android's support for multiple screens is a

set of built-in compatibility features that together managethe rendering of application resources in an appropriate way

for the current device screen. The platform handles most of

the work of rendering your application, but also gives you

two key ways to control how your application is displayed, if

you need or want to use them:

The platform supports a set of resource qualifiers that

let you provide size- and density-specific resources, if

needed. The qualifiers for size-specific resources are

small, normal, large, and xlarge. Those for

density-specific resources are ldpi (low),mdpi

(medium), hdpi (high), and xhdpi (extra high). The qualifiers correspond to the generalized densities described

in Range of screens supported, above.

The platform also provides a manifest element, whose attributes

android:smallScreens , android:normalScreens, android:largeScreens , and
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Publishing to Small Screen Devices

To ensure the best experience for

users on small-screen devices,

Android Market only shows applications that

explicitly declare support for small screens. If

you developed an application on Android 1.5 or

earlier and published it on Android Market, you

need to test your application on small screens

and then upload an updated version that

explicitly indicates support for small screens.

android:xlargeScreens let you specify what generalized screen sizes your application supports. Another

attribute, android:anyDensity , lets you indicate whether or not your application includes built-in support for

multiple densities.

At run time, the platform provides three types of support to your application, to ensure the best possible display on the

current device screen:

1. Pre-scaling of resources (such as image assets)

Based on the density of the current screen, the platform automatically loads any size- or density-specific

resources from your application and displays them without scaling. If no matching resources are available, theplatform loads the default resources and scales them up or down as needed to match the current screen's

generalized density. The platform assumes that default resources are designed for proper display at the baseline

screen density of "medium" (160), unless they are loaded from a density-specific resource directory.

For example, if the current screen's density is "high", the platform loads resources that are tagged with the

qualifierhdpi and uses them without scaling. If no such resources are available, the platform uses the default

resources instead, scaling them from the baseline density ("medium") to "high".

For more information about how to create size- and density-specific resources, see Resource qualifiers.

2. Auto-scaling of pixel dimensions and coordinates

If the application states that it does not support different screen densities, the platform auto-scales any absolute

pixel coordinates, pixel dimension values, and pixel math used in the application (such as might be used for

specifying the width or padding for a view). It does this to ensure that pixel-defined screen elements are displayed

at approximately the same physical size as they would be at the baseline density of "medium" (160). The platform

handles this scaling transparently to the application and also reports scaled overall pixel dimensions to the

application, rather than physical pixel dimensions.

For instance, suppose a given device is using a WVGA high-denisty screen, which is 480x800 and about the

same size as a traditional HVGA screen, but it's running an app that states that it does not support multiple

densities. In this case, the system will "lie" to the application when it queries for screen dimensions, and report

320x533. Then, when the app does drawing operations, such as invalidating the rectangle from (10,10) to (100,

100), the system will likewise automatically transform the coordinates by scaling them the appropriate amount, and

actually invalidate the region (15,15) to (150, 150). The same thing happens in the other direction, if the

application is running on a lower-density screen, coordinates are scaled down.

For more information, see the android:anyDensity attribute in Manifest attributes for screens support.

3. Compatibility-mode display on larger screen-sizes

If the current screen's size is larger than your

application supports, as specified in the supports-

screens element, the platform displays the application

at the baseline size ("normal") and density ("medium).

For screens larger than baseline, the platform displays

the application in a baseline-sized portion of the overall

screen, against a black background.

For instance, suppose a given device is using a WVGA

medium density screen, classified as a "large" screen,

but the application states that it does not support large

screens; in this case, the system will again "lie" to the

application when it queries for screen dimensions, and

report 320x480. Instead of scaling the application,however, the application's 320x480 interface will be

placed as a "postage stamp" in the larger 480x800 screen.

For more information, see the android:anyDensity attribute in Manifest elements for screens support and the

Screen-Compatibility Examples section.

In general, these compatibility features ensure that all applications, including those written against Android 1.5 and

earlier platform versions, can display properly on most devices, especially when the device's screen is at the baseline

"normal" size or larger.

However, note that applications written for the baseline screen may need minor adjustments before they display

properly on smaller screens such as QVGA. With the reduced screen area of small screens, there may be tradeoffs in

design, content, and function that you, as the application developer, need to consider. For more information about how

to prepare an existing application for display on small screens, see Strategies for Legacy Applications.

The sections below provide more information how to take advantage of the platform's multiple-screens support.
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Density independence

The goal of density independence is to preserve the physical size, from the user's point of view, of user interface

elements declared in an application, when the application is displayed on screens with different densities. Density

independence applies to both layouts and drawables such as icons. Maintaining density-independence is important

because, other things being equal, a UI element (such as a button) whose height and width are defined in terms of

screen pixels will appear physically larger on the lower density screen and smaller on the higher density screen. Such

density-related size changes can cause problems in application layout, usability, and consistency with other

applications installed on the device.

The platform provides density independence to applications by default. It does this in three ways:

Through pre-scaling of drawable resources (scaled at resource loading time)

Through auto-scaling of density-independent pixel (dp) values used in layouts

Through auto-scaling of absolute pixel values used in the application (only needed if the application has set

android:anyDensity="false" in its manifest)

The example screens below illustrate the density independence provided by the platform. Note that both the layouts

and launcher icons are displayed at the same physical sizes, although screen sizes, aspect ratios, and densities are

different.

Figure 2. Examples of density independence on WVGA high density (left), HVGA medium density (center), and

QVGA low density (right).

In most cases, you can take advantage of density independence in your application simply by making sure that your

layouts specify all dimension values in density-independent pixels (dp ordp) or scale-independent pixels (sip orsp,

for text only). If you are using absolute pixel values in the application and manifest includes

android:anyDensity="true" , you will also need to scale the pixel values. See Converting dp units to pixel units

for more information.

Manifest attributes for screens support

Android 1.6 introduced a new manifest element, , whose attributes you can use to control the

display of your application on different classes of device screens, as listed in table 2. The smallScreens,

normalScreens, largeScreens and xlargeScreens attributes correspond to the generalized screen sizes

described in Range of screens supported, earlier in this document. Notice that the default values for each attributevary, depending on your minimum and targeted platform, as indicated in the android:minSdkVersion and

android:targetSdkVersion attributes of your manifest element.
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Table 2. Summary of attributes for the manifest element, including default values based on

platform version.

Attribute Description Default value, whenminSdkVersion ortargetSdkVersion

is 4 or lower

Default value, whenminSdkVersion ortargetSdkVersion

is 5 or higher

android:smallScreens Whether or not theapplication UI is designedfor use on smallscreens "true" if it is, and"false" if not.

"false" "true"

android:normalScreens Whether or not theapplication UI is designedfor use on normalscreens "true" if it is, and"false" if not. The defaultvalue is always "true".

"true" "true"

android:largeScreens Whether or not theapplication UI is designedfor use on large screens

"true" if it is, and "false"if not.

"false" "true"

android:anyDensity Whether or not the

application is designed to

manage its UI properly in

different density

environments "true" if

so, and "false" if not.

If set to "true", the

platform disables its

density-compatibility

features for all screen

densities specifically, the auto-

scaling of absolute

pixel units (px) and

math and relies on

the application to use

density-independent

pixel units (dp) and/or

math to manage the

adaptation of pixel

values according to

density of the current

screen. That is, as

long as yourapplication uses

density-independent

units (dp) for screen

layout sizes, then it

will perform properly

on different densities

when this attribute is

set to "true".

If set to "false", the

platform enables its

density-compatibility

features for all screen

densities. In this case,the platform provides

a scaled, virtual

"false" "true"
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screen pixel map to

the application, against

which it can layout and

draw its UI as though

against a medium-

density screen (160).

The platform then

transparently auto-

scales the

application's pixel unitsand math as needed

to match the actual

device screen density.

Note that the setting of this

attribute affects density-

compatibility only. It does

not affect size-compatibility

features such as display on

a virtual baseline screen.

Note: Android 2.3 (API Level 9) introduced a new attribute for the element:xlargeScreens, shown below. It works the same as the other screen attributes above, but, if neither your

minSdkVersion ortargetSdkVersion are set to "9", the default value is "false" when your application isinstalled on a device running Android 2.3.

Attribute Description Default value, whenminSdkVersion ortargetSdkVersion

is 8 or lower

Default value, whenminSdkVersion ortargetSdkVersion

is 9 or higher

android:xlargeScreens Whether or not theapplication UI is designedfor use on xlarge screens "true" if it is, and"false" if not.

"false" "true"

In general, when you declare a screen-size attribute (smallScreens, normalScreens , largeScreens, or

xlargeScreens) as "true", you are signaling to the platform that your application is designed to render properly on

that screen size. As a result, the platform does not apply any size-compatibility features (such as a virtual HVGA

display area). If you declare a screen-size attribute as "false", you are signaling that your application is notdesigned

for that screen size. In this case, the platform does apply size-compatibility features, rendering the application in an

HVGA baseline display area. If the current screen is larger than normalsize, the platform renders the application in a

virtual HVGA screen on the larger screen. See Screen-Compatibility Examples for an illustration of what an application

looks like when displayed in a virtual HVGA screen.

In other words, setting a attribute to "false" tells the platform to enable it's compatibility

features when displaying the application on a screen of that size or any larger size, if also disallowed. Otherwise, the

platform gives the application a normal display area that can use the full device screen area, if appropriate.

Android Market also makes use of the attributes. It uses them to filter the application fromdevices whose screens are not supported by the application. Specifically, Android Market considers an application

compatible with a device if the application supports a screen that is the same or smaller than the current device

screen. Android Market filters the application if it disallows the device's screen size and does not support a smaller

size. In general, Android does not provide downward size-compatibility features for applications.

Here are some examples:

Assume that you declare smallScreens="false" normalScreens="true" largeScreens="false"

xlargeScreens="false" in your application's manifest.

Although the application is not designed for display on large or extra large screens, the platform can still run it

successfully in screen-compatibility mode. Android Market shows the application to devices with normal, large,

andxlarge size screens, but does filter it from smallsize screens, because the application provides no screen

support at smallsize. Android's screen-compatibility mode mode does not provide support for screens that are

smaller than those the application supportsit only provides support for screens that are larger. Thus, although

the application declares "false" for large andxlarge screens, the application still functions, but runs in compatibility

mode.
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Assume that you declare smallScreens="false" normalScreens="false" largeScreens="true"

xlargeScreens="true" in your application's manifest.

Android Market filters the application from users of devices with smalland normalsize screens. In effect, this

prevents such users from installing the application.

If you declare the android:anyDensity attribute as "true", you are signaling to the platform that your application

is designed to display properly on any screen density. In this case, the application must ensure that it declares its UI

dimensions using density-independent pixels (dp) and scales any absolute pixel values (px) or math by the scaling

factor available from android.util.DisplayMetrics.density . See Converting dp units to pixel units for anexample.

Note that the setting of the android:anyDensity attribute does not affect the platform's pre-scaling of drawable

resources, such as bitmaps and nine-patch images, which always takes place by default.

The following example shows a manifest that declares support for small, normal, large, and xlarge screens in any

density.

...

Default values for attributes

The default values for the attributes differ, depending on the the value of the

android:minSdkVersionattribute in the application's manifest, as well as on the value of

android:targetSdkVersion , if declared.

Above, table 2 indicates the default values for each attribute, based on the values you provide for the

android:minSdkVersionand android:targetSdkVersion , in the element.

Note: If your application uses APIs introduced in Android 1.6 or higher, but does not support specific screen

densities and/or screen sizes, you need to explicitly set the appropriate attributes to "false" (because most are

"true", by default).

Resource directory qualifiers for screen size and density

Android supports resource directory qualifiers for controlling the selection of resources based on the characteristics of

the screen on which your application is running. You can use these qualifiers to provide size- and density-specific

resources in your application. For more information about the generalized sizes and densities that correspond to the

qualifiers, see Range of Screens Supported, earlier in this document.

Screen

characteristic

Qualifier Description

Size small Resources designed forsmallsize screens.

normal Resources designed fornormalsize screens.

large Resources designed for large size screens.

xlarge Resources designed forextra large size screens.

Density ldpi Resources designed for low-density (ldpi) screens.

mdpi Resources designed for medium-density (mdpi) screens.

hdpi Resources designed for high-density (hdpi) screens.

xhdpi Resources designed for extra high-density (xhdpi) screens.

nodpi Density-independent resources. The platform does not auto-scale
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resources tagged with this qualifier, regardless of the current screen'sdensity.

Aspect ratio long Resources for screens of any size or density that have a significantlytaller (in portrait mode) and wider (in landscape mode) aspect ratio thanthe baseline screen configuration.

notlong Resources for use only on screens that have an aspect ratio that issimilar to the baseline screen configuration.

Platformversion

v Resources that are for use only on a specific API Level or higher. Forexample, if your application is designed to run on both Android 1.5 (APILevel 3) and Android 1.6 (API Level 4 and higher), you can use the -v4qualifier to tag any resources that should be excluded when yourapplication is running on Android 1.5 (API Level 3).

Note that the density and the screen size are independent parameters and are interpreted by the system individually.

For example, WVGA high density is considered a normal screen because its physical size is about the same as one of

T-Mobile G1. On the other hand, a WVGA medium density screen is considered a large screen it offers the same

resolution but at lower pixel density, meaning that it is both physically larger than the baseline screen and can display

significantly more information than a normal screen size.

Here is an example of the resource directory structure of an application that employs different layout schemes for

different screen sizes and supports low and high density screens.

res/layout/my_layout.xml // layout for normal screen sizeres/layout-small/my_layout.xml // layout for small screen sizeres/layout-large/my_layout.xml // layout for large screen sizeres/layout-large-land/my_layout.xml // layout for large screen size in landscapemode

res/layout-xlarge/my_layout.xml // layout for extra large screen size

res/drawable-lhdpi/my_icon.png // image for low densityres/drawable-mdpi/dpi/my_icon.png // image for medium densityres/drawable-hdpi/my_icon.png // image for high density

res/drawable-nodpi/composite.xml // density independent resource

For more information about how to use resource qualifiers or how the platform selects them, please read Providing

Alternative Resources.

Best practices for Screen Independence

The objective of supporting multiple screens is to create an application that can run properly on any display and

function properly on any of the generalized screen configurations supported by the platform.

You can easily ensure that your application will display properly on different screens. Here is a quick checklist:

1. Usewrap_content, fill_parent, or the dp unit (instead ofpx), when specifying dimensions in an XML

layout file

2. Do not useAbsoluteLayout

3. Do not use hard coded pixel values in your code

4. Use density and/or resolution specific resources

1. Use wrap_content, fill_parent, or the dp unit, instead of absolute pixels

When defining the layout_width and layout_height of views in an XML layout file, usingwrap_content ,

fill_parent or the dp will guarantee that the view is given an appropriate size on the current device screen. For

instance, a view with a layout_width="100dp" will measure 100 pixels wide on an HVGA@160 density display

and 150 pixels on a WVGA@240 density display, but the view will occupy approximately the same physical space.

Similarly, you should prefer the sp (scale-independent pixel, the scale factor depends on a user setting) ordp (if you
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don't want to allow the user to scale the text) to define font sizes.

2. Avoid AbsoluteLayout

AbsoluteLayout is one of the layout containers offered by the Android UI toolkit. Unlike the other layouts however,

AbsoluteLayout enforces the use of fixed positions which might easily lead to user interfaces that do not work well

on different displays. Because of this,AbsoluteLayout was deprecated in Android 1.5 (API Level 3).

You can achieve much the same layout by using a FrameLayout instead, and setting layout_margin attributes of

the children. This approach is more flexible and will yield better results on different screens.

3. Do not use hard-coded pixel values in your code

For performance reasons and to keep the code simpler, the Android framework API uses pixels as the standard unit

for expressing dimension or coordinate values. That means that the dimensions of a View are always expressed in the

code in pixels. For instance, ifmyView.getWidth() returns 10, the view is 10 pixels wide. In some cases, you may

need to scale the pixel values that you use in your code. The sections below provide more information.

Converting dp units to pixel units

In some cases, you will need to express dimensions in dp and then convert them to pixels. Imagine an application in

which a scroll gesture is recognized after the user's finger has moved by at least 16 pixels. On a baseline screen, the

user will have to move his finger by 16 pixels / 160 dpi = 1/10th of an inch (or 2.5 mm) before the gesture isrecognized. On a device with a high (240) density display, the user will move his finger by only 16 pixels / 240 dpi =

1/15th of an inch (or 1.7 mm.) The distance is much shorter and the application thus appears more sensitive to the

user. To fix this issue, the gesture threshold must be expressed in the code in dp and then converted to actual pixels.

// The gesture threshold expressed in dp

privatestaticfinalfloat GESTURE_THRESHOLD_DP =16.0f;

// Convert the dps to pixels

finalfloat scale = getContext().getResources().getDisplayMetrics().density;mGestureThreshold =(int)(GESTURE_THRESHOLD_DP * scale +0.5f);

// Use mGestureThreshold as a distance in pixels

The android.util.DisplayMetrics.density field specifies the the scale factor you must use to convert dps to

pixels according to the current screen density. You can access the current screen's metrics through a Context or

Activity. On a medium (160) density screen, DisplayMetrics.density equals "1.0", whereas on a high (240)

density screen it equals "1.5". You can refer to the documentation of the DisplayMetrics class for details.

Use pre-scaled configuration values

The ViewConfiguration class can be used to access the most common distances, speeds, and times used in the

Android framework. For instance, the distance in pixels used by the framework as the scroll threshold can be obtained

as follows:

ViewConfiguration.get(aContext).getScaledTouchSlop()

Methods starting with the getScaled prefix are guaranteed to return a value in pixels that will display properly

regardless of the current screen density.

4. Use density and/or size-specific resources

Even with the size- and density-compatibility

features that the platform provides, you may still

want to make adjustments to the UI of your

application when it displayed on certain screen

sizes or densities. You can do this by providing

size- or density-specific resources assets,

layouts, strings, and so on. If you want, you canalso take control over the scaling of images

assets. The sections below provide more
http://developer.android.com/reference/android/view/ViewConfiguration.htmlhttp://developer.android.com/reference/android/util/DisplayMetrics.htmlhttp://developer.android.com/reference/android/util/DisplayMetrics.html#densityhttp://developer.android.com/reference/android/widget/FrameLayout.htmlhttp://developer.android.com/reference/android/widget/AbsoluteLayout.html


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Figure 3. Comparison of pre-scaled and auto-scaled bitmaps.

information.

Custom resources and directories

If you need to control exactly how your

application will look on various displays, simply

adjust your assets and layouts in configuration-

specific resources directories. For example,

consider an icon that you want to display on

medium and high density screens. Simply createyour icon at two different sizes (for instance

100x100 for medium density and 150x150 for

high density) and put the two variations in the

appropriate directories, using the proper

qualifiers:

res/drawable-mdpi/icon.png //

for medium-density screens

res/drawable-hdpi/icon.png //

for high-density screens

If a density qualifier is not defined in a resource

directory name, the platform assumes that theresources in that directory are designed for the

baseline medium density. It is not recommended

that you put density-specific resources such as

images in the default directory.

For more information about valid resource

qualifiers, see Resource directory qualifiers, earlier in this document.

Pre-scaling and auto-scaling of bitmaps and nine-patches

When a bitmap or nine-patch image is loaded from the application's resources, the platform attempts to pre-scale it to

match the display's density. For instance, if you placed a 100x100 icon in the res/drawable/ directory and loaded

that icon as a bitmap on a high-density screen, Android would automatically scale up the icon and produce a 150x150

bitmap.

This pre-scaling mechanism works independently of the source. For instance, an application targeted for a high-

density screen may have bitmaps only in the res/drawable-hdpi/ directory. If one of the bitmaps is a 240x240

icon and is loaded on a medium-density screen, the resulting bitmap will measure 160x160.

The platform pre-scales resources as needed, whether the application is running with density-compatibility features

enabled or not (as specified by the value ofandroid:anyDensity ). However, when running with density-

compatibility enabled, the platform continues to report the size of pre-scaled bitmaps and other resources as if they

were loaded in a medium-density environment. For example, when density-compatibility is enabled, if you load a

76x76 image from the default resources for display on a high-density screen, the platform will pre-scale the image to

114x114 internally. However, the API still reports the size of the image as 76x76. This discrepancy may cause

unexpected behavior if your application somehow directly manipulates the scaled bitmap, but this was considered a

reasonable trade-off to keep the performance of existing applications as good as possible.

This does not apply for the case that an application creates an in-memory bitmap internally and draws something on

it, for later display on the screen. The platform auto-scales such bitmaps on the fly, at draw time. Other side effects of

such a case might be that fonts drawn in such a bitmap will be scaled at the bitmap level, when the off-screen bitmap

is finally rendered to the display, resulting in scaling artifacts.

There are situations in which you may not want Android to automatically scale a resource. The easiest way to

accomplish this is to put it in a "nodpi" resource directory:

res/drawable-nodpi/icon.png

You can also take complete control of the scaling mechanism by using the BitmapFactory.Options class, which

lets you define whether you want the bitmap to be pre-scaled and what the density of the bitmap should be. For

instance, if you are loading a bitmap from a web server, you may want to force the bitmap's density to be high density.

When pre-scaling is disabled, the resulting bitmap is in auto-scaling mode. The bitmap is associated with a density

(that you may or may not have specified through the BitmapFactory.Options) which will be used to scale the

bitmap on screen at drawing time.
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Using auto-scaling instead of pre-scaling is more CPU expensive than pre-scaling but uses less memory. You can

refer to the documentation ofBitmapFactory, Bitmap, and Canvas for more information on auto-scaling.

Figure 3, at right, demonstrates the results of the pre-scale and auto-scale mechanisms when loading low (120),

medium (160) and high (240) density bitmaps on a baseline screen. The differences are subtle, because all of the

bitmaps are being scaled to match the current screen density, however the scaled bitmaps have slightly different

appearances depending on whether they are pre-scaled or auto-scaled at draw time.

Strategies for Legacy Applications

If you have already developed and published an Android application based on Android 1.5 or earlier platform version,

you need to consider how you will adapt your application so that it is deployable to:

Existing devices, which may be running Android 1.5 (or lower) platform version, as well as to

Newer devices that are running Android 1.6 (or higher) and offering various screen sizes and resolutions

Note: Even if your application targets Android 1.6 already, you should follow the same strategies below in order

to supportxhdpiandxlarge screens on Android 2.3 (API Level 9), while maintaining compatibility with older

versions of the platform.

To support the newer devices and the different screens they use, you might need to make some changes in your app,

but at the same time your app may be very stable and so you want to minimize the changes. There are a variety of

ways that you can extend your existing application to support new devices with multiple screens andexisting devices

running older platform versions. You should be able to make these changes to your application such that you can

distribute a single .apk to all devices.

The recommended strategy is to develop against the most recent version of the platform you are targeting, and test on

the minimum platform version you want to run on. Here's how to do that:

1. Maintain compatibility with existing devices by leaving your application's android:minSdkVersion attribute as

it is. You do notneed to increment the value of the attribute to support new devices and multiple screens.

2. Extend compatibility for Android 1.6 (and higher) devices by adding a new attribute

android:targetSdkVersion to the uses-sdk element. Set the value of the attribute to "4". [To supportxhdpiandxlarge screens, set the value to "9".] This allows your application to "inherit" the platform's multiple

screens support, even though it is technically using an earlier version of the API.

3. Add an empty element as a child of. If you need to enable size or density

attributes later, this is where you will add them.

4. Change your application's build properties, such that it compiles against the Android 1.6 (API Level 4) library [or

against Android 2.3 (API Level 9) to supportxhdpiandxlarge screens], rather than against the Android 1.5 (or

earlier) library. You will not be able to compile your application against the older platform because of the new

manifest attribute.

5. Set up AVDs for testing your application on Android 1.6 [or Android 2.3] and higher releases. Create AVDs that

use the screen sizes and densities that you want to support. When you create the AVDs, make sure to select the

Android 1.6 [or Android 2.3] or higher platform as the system image to run. For more information, see How to Test

Your Application on Multiple Screens, below.

6. Set up AVDs for testing your application on older versions of the platform, as low as the version declared by your

android:minSdkVersion. You need AVDs running the older platforms you are targeting, so that you can test

for compatibility and ensure that there are no functional regressions.

7. Compile your application against the Android 1.6 [or Android 2.3] library and run it on the AVDs you created.

Observe the way your application looks and runs, and test all of the user interactions.

8. Debug any display or functional issues. For issues that you resolve in your application code, make certain not to

use any APIs introduced later than the version declared by yourandroid:minSdkVersion. If you are in doubt,

refer to SDK reference documentation and look for the API Level specifier for the API you want to use. Using

newer APIs not supported by your minimum version will mean that your application will no longer be compatible

with devices running on that version.

9. For resource-related issues, you can try resolving them by:

Adding a anyDensity="false" attribute to , to enable density-compatibilityscaling.

Creating any size- or density-specific resources you need and placing them in directories tagged with the
http://developer.android.com/guide/practices/screens_support.html#testinghttp://developer.android.com/reference/android/graphics/Canvas.htmlhttp://developer.android.com/reference/android/graphics/Bitmap.htmlhttp://developer.android.com/reference/android/graphics/BitmapFactory.html


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Figure 4. A typical set of AVDs for testing screens support.

correct qualifiers. Qualifiers must be arranged in a proscribed order. See Providing Alternative Resources for

more information.

Note that if you add size- or density-specific resource directories tagged with any of the resource qualifiers

listed in this document, you should make sure to also tag those directories with the v qualifier

(for example, -v4 to target API Level 4). This ensures that those resources will be ignored when the

application is run on Android 1.5 or lower platform versions.

10. If your application does not offer support (such as custom layouts) for large screens and you want the platform to

display your application in screen-compatibility mode on larger screens, add the largeScreens="false" and

xlargeScreens="false"attributes to the element in the manifest. See Screen-Compatibility Examples for illustrations of how the platform displays your application in this case.

11. If your application does not offer support (such as custom layouts) for small screens (such as on a QVGA low-

density screen) and you do not want Android Market to offer the application to users of small-screen devices, you

mustadd a smallScreens="false" attribute to the element.

12. Continue testing and debugging until your application performs as expected on all of the platforms and screen

sizes your application will support.

13. Export, zipalign, and sign your application using the same private key you used when publishing the previous

version, then publish the application to users as an update.

In particular, remember to test your application on an AVD that emulates a small-screen device. Devices that offer

screens with QVGA resolution at low density are available now. Users of those devices may want to download your

application, so you should understand how your application will look and function on a small-screen device. In manycases, the reduced screen area and density mean that you may need to make tradeoffs in design, content, and

function on those devices.

Also give extra attention to testing your application on an AVD that emulates an xlarge screen. Devices with extra large

screens are tablet-sized or larger, so you should pay close attention to how usable your application is on such

screens. You might want to design new layouts specifically for extra large screens, to address usability aspects such

as the location and size of buttons in your UI. To test your application on an extra large screen, create an AVD

targeted to Android 2.3 with a high resolution, such as 1280 x 800, and the default density of 160dpi. This AVD will

use any resources you've provided with the xlargeresouce qualifier.

How to Test Your Application on Multiple Screens

Before publishing an application that supports multiple screens, you should thoroughly test it in all of the targeted

screen sizes and densities. You can test how it displays with the platform's compatibility features enabled or with

screen-specific UI resources included in your application. The Android SDK includes all the tools you need to test your

application on any supported screen.

As a test environment

for your applications,

set up a series of

AVDs that emulate

the screen sizes and

densities you want to

support. The AndroidSDK includes several

emulator skins to get

you started. You can

use the Android AVD

Manager or the

android tool to

create AVDs that use

the various emulator

skins and you can

also set up custom

AVDs to test densities

other than the

defaults. For general

information about

working with AVDs,

see Creating and
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Managing Virtual

Devices.

The Android SDK provides a set of default emulator skins that you can use for testing. The skins are included as part

of each Android platform that you can install in your SDK. The Android 1.6 platform offers these default skins:

QVGA (240x320, low density, small screen)

HVGA (320x480, medium density, normal screen)

WVGA800 (480x800, high density, normal screen)

WVGA854 (480x854 high density, normal screen)

The Android 2.0 platform offers all of the Android 1.6 default skins, above, plus:

WQVGA400 (240x400, low density, normal screen)

WQVGA432 (240x432, low density, normal screen)

If you are using the android tool command line to create your AVDs, here's an example of how to specify the skin

you want to use:

android create avd ...--skin WVGA800

We also recommend that you test your application in an emulator that is set up to run at a physical size that closely

matches an actual device. This makes it a lot easier to compare the results at various resolutions and densities. To do

so you will need to know the approximate density, in dpi, of your computer monitor (a 30" Dell monitor has for instance

a density of about 96 dpi.). Use your monitor's dpi as the value of the -scale option, when launching the emulator,

for example:

emulator -avd -scale 96dpi

If you are working in Eclipse with ADT, you can specify the -scale 96dpi option in the Target tab of run and debug

configurations, under "Additional Emulator Command Line Options" field.

Note that starting the emulator with the -scale option will scale the entire emulator display, based on both the dpi of

the skin and of your monitor. The default emulator skins included in the Android SDK are listed in Table 1, earlier inthis document.

You should also make sure to test your application on

different physical screen sizes within a single size-

density configuration. For example, to display this screen

configuration on a 30" monitor you will need to adjust the

value passed to -scale to 96*2.8/3.3 = 81dpi. You can

also pass a float value to -scale to specify your own

scaling factor:

emulator -avd -scale 0.6

If you would like to test your application on a screen that

uses a resolution or density not supported by the built-in

skins, you can either adjust an existing skin, or create an

AVD that uses a custom resolution or density.

In the AVD Manager, you can specify a custom skin

resolution or density in the Create New AVD dialog, as

shown in Figure 5, at right.

In the android tool, follow these steps to create an

AVD with a custom resolution or density:

1. Use the create avd command to create a new

AVD, specifying the --skin option with a value thatreferences either a default skin name (such as

"WVGA800") or a custom skin resolution (such as
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Figure 5. Resolution and density options that you can

use, when creating an AVD using the AVD Manager.

240x432). Here's an example:

android create avd -n -t

--skin WVGA800

2. To specify a custom density for the skin, answer

"yes" when asked whether you want to create a custom hardware profile for the new AVD.

3. Continue through the various profile settings until the tool asks you to specify "Abstracted LCD density"

(hw.lcd.density). Enter an appropriate value, such as "120" for a low-density screen, "160" for a medium density

screen, or "240" for a high-density screen.

4. Set any other hardware options and complete the AVD creation.

In the example above (WVGA medium density), the new AVD will emulate a 5.8" WVGA screen.

As an alternative to adjusting the emulator skin configuration, you can use the emulator skin's default density and add

the -dpi-device option to the emulator command line when starting the AVD. For example,

emulator -avd WVGA800 -scale 96dpi-dpi-device 160

Screen-Compatibility Examples

This section provides examples of how the Android platform displays an application written for the baseline screen

configuration HVGA (320x480) resolution on a 3.2" screen with all of the platform's size- and density-

compatibility features enabled. That is, the examples show how the platform displays an application that doesn't

provide built-in support for the screen on which it is being rendered, but which instead relies completely on the

platform.

The platform's screen-compatibility features are designed to provide such an application with a virtual baseline screen

environment against which to run, while at the same time ensuring for the user a physical display that is approximately

the same as the baseline screen size and density.

Legacy applications that have not been modified to support multiple screens would be typical examples of such

applications. In most cases, you would want to add multiple-screens support to a legacy application and publish an

updated version, as described in Strategies for Legacy Applications. However, if you did not do so, the platform still

performs best-effort rendering of your application, as illustrated below.

Internally, these are the compatibility features that the platform provides, based on the current device screen:

If the device's screen density is not medium, the application's layout and drawing of its content is as if the screen

is medium density, but the framework scales the layout and images (if the image for the target density is not

available) to fit the target density. It scales 1.5 times if the target density is high density (160->240 virtual dpi), or

0.75 times if the target density is low density (160 -> 120 virtual dpi).

If the device's screen size is small, there are few options options for making Android 1.5 applications work well on

such a screen, so Android Market will filter applications that are not known to support these screens from the

device.

If the device's screen size is large, it limits the application's screen to the normal size and draws a black

background around the application. For example, if an application supports high density, but does not support

large screens, it only uses a 480x720 area of the screen and the rest will be filled with a black background (see

example below).

HVGA, normal size, normal density[ emulator -skin HVGA ]

WVGA, normal size, high density[emulator -skin WVGA854 -dpi-device 240]
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The application occupies full screen as its

considered to be normal size. (close to 480x720)

VGA, large size, medium density[ emulator -skin 640x480 ]

The application occupies 320x480 of VGA.

SVGA, large size, high density[ emulator -skin 800x600 -dpi-device 240]

The application occupies 480x720 (=1.5 x

[320x480]) of 800x600.

Screen-compatibility limitations on small, low-density screens

Because these device has smaller state/resolution, there are known limitations when application runs in compatibility

mode.

QVGA

Because QVGA (240x320) screens have less screen area available and lower density than normal, which is 240x360

in low density, some applications cannot render all their content properly on those screens. As a result, on a QVGA

device, Android Market will filter out all applications that do not declare they support small screens.

Examples:

The part of z value graphis chopped.

The lap time area ischopped.


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Images with 1 pixel height/width.

If an image has 1 pixel height or width, it may not be shown on the screen due to rounding issue. This is inevitable as

it just does not have enough pixels.

For example, in the screen below, the divider in the menu is invisible because the width of the image is trancated to

0. (This particular problem is solvable because menu is handled inside framework, but there is no generic solution as it

ust does not have enough pixels.)

Except as noted, this content is licensed under Apache 2.0. For details and restrictions, see the Content License.Android 3.0 r1 - 02 May 2011 14:59

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