- Designing for Performance diff --git a/docs/html/guide/practices/screens_support.jd b/docs/html/guide/practices/screens_support.jd new file mode 100644 index 0000000000000..b2f814a87fdbf --- /dev/null +++ b/docs/html/guide/practices/screens_support.jd @@ -0,0 +1,829 @@ +page.title=Supporting Multiple Screens + +@jd:body + + +
- Android runs on devices that have different screen sizes and resolutions. +
- The screen on which your application is displayed 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 for precise control of your UI, if needed. +
- Older applications run in a compatibility mode that provides best-effort rendering on the current screen. +
- Its important to follow the best practices described in this document and to test your application in all supported screens. +
- Overview of Screen Support +
- Range 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 + +
- Screen size +
- Actual physical size, measured as the screen's diagonal.
+
+
For simplicity, Android collapses all actual screen sizes into three generalized sizes: large, normal, and small. Applications can provide custom layouts for each of these three sizes — the platform transparently handles the rendering of the layouts at the actual screen size.
+
+ - Aspect ratio +
- 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
longandnotlong.
+
+ - 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 three generalized densities: high, medium, and low. Applications can provide custom resources for each of these three densities — the platform handles the scaling of the resources up or down to meet the actual screen density.
+
+ - Density independent pixel (dip) +
- A virtual pixel unit that applications can use in defining their UI, to express layout dimensions or position in a density-independent way.
+
The dpi 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 dip units needed, based on the actual density of the screen in use. The conversion of dip units to screen pixels is simple:
+ +pixels = dips * (density / 160). For example, on 240 dpi screen, 1 dip would equal 1.5 physical pixels. Using dip units to define your application's UI is highly recommended, as a way of ensuring proper display of your UI on different screens. - A set of three generalized sizes: large, normal, and small, and +
- A set of three generalized densities: high (hdpi), medium (mdpi), and low (ldpi) +
- QVGA (240x320),
2.8"-3.2" diagonal
+ - WQVGA (240x400),
3.5"-3.8" diagonal
+ - FWQVGA (240x432),
2.8"-3.2" diagonal
+ - HVGA (320x480),
3.0"-3.5" diagonal
+ - WVGA (480x800),
3.3"-4.0" diagonal
+ - FWVGA (480x854),
3.5"-4.0" diagonal
+ - WVGA (480x800),
4.8"-5.5" diagonal
+ - FWVGA (480x854),
5.0"-5.8" diagonal
+ - The platform provides a set of resource qualifiers that let you provide size- and density-specific resources, if needed. The qualifiers for
+size-specific resources are
large,normal, andsmall, and those for density-specific resources arehdpi(high),mdpi(medium), andldpi(low). The qualifiers correspond to the generalized densities given in Table 1, above.
+ - The platform also provides a
<supports-screens>element, whose attributesandroid:largeScreens,android:normalScreens, andandroid:smallScreenslet you specify what generalized screen sizes your application supports. A fourth attribute,android:anyDensity, lets you indicate whether or not your application includes built-in support for multiple densities.
+ - 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, or if the application is using resources + + +from the platform 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 qualifier
+ +hdpiand 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.
+ + - 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:anyDensityattribute in Manifest attributes for screens support.
+
+ - Compatibility-mode display on larger screen-sizes
+
+
If the current screen's size is larger than your application supports, as specified in the
+ +supports-screenselement, 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:anyDensityattribute in Manifest elements for screens support and the Screen-Compatibility Examples section.
+ - Through pre-scaling of drawable resources (scaled at resource loading time) +
- Through auto-scaling of device-independent pixel (dip) 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)
+ - If set to "
true", the platform disables its density-compatibility features for all screen densities — specifically, the auto-scaling of absolute pixel units and math — and relies on the application to use density-independent pixel units and/or to manage the adaptation of pixel values according to density of the current screen.
+
+ - If set to "
false", the platform enables its density-compatibility features for all screen densities. In this case, the platform provides a scaled, virtual 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 units and math as needed to match the actual device screen density.
+
Multiple screens quickview:
+-
+
In this document
+-
+
-
+
See Also
+ + +Android is designed to run on a variety of devices that offer a 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 all supported 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 or earlier, 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, however you should make sure to test your application on all of the new screens supported in Android 1.6.
+ +In particular, if you have an existing application that you would like to make available for users of devices with small screens (such as QVGA), 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 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
+-
+
Range of Screens Supported
+ +Android 1.5 and earlier versions of the platform were designed to support a single screen configuration — HVGA (320x480) resolution on a 3.2" screen. Because the platform targeted just one screen, application developers could write their applications specifically for that screen, without needing to worry about how their applications would be displayed on other screens.
+ +Starting from Android 1.6, the platform adds support for multiple screen sizes and resolutions, reflecting the many new types and sizes of devices on which the platform will run. This means that developers must design their applications for proper display on a range of devices and screens.
+ +To simplify the way application developers design their user interfaces for multiple devices, and to allow more devices to participate without impacting applications, the platform divides the range of actual supported screen sizes and resolutions into:
+ +-
+
Applications can provide custom resources (primarily layouts) for any of the three generalized sizes, if needed, and they can also provide resources (primarily drawables such as images) for any of the three 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 range of screens supported by Android and the generalized screen configurations that the platform maps them to are shown in the table below.
+ +| + | + Low density (120), ldpi + | ++ Medium density (160), mdpi + | ++ High density (240), ldpi + | +
| + Small screen + | +
+
|
+ + | + |
| + Normal screen + | +
+
|
+
+
|
+
+
|
+
| + Large screen + | ++ |
+
|
+ + |
Table 1. Summary of device screens supported by Android.
+ +As shown above, the various screen configurations are arranged around a baseline screen — HVGA (320x480) resolution on a 3.2" screen — which is assigned a size of "normal" and a density of "medium". The HVGA screen is used as the baseline because all applications written against Android 1.5 or earlier are (by definition) written for the HVGA screen used on the T-Mobile G1 and similar devices.
+ + + +Although the platform currently supports the nine possible size-density configurations listed in the table, you do not necessarily need to custom resources for each one of them. The platform provides robust compatibility features, described in the sections below, that can handle most of the work of rendering your application on the current device screen, provided that the UI is properly implemented. For more information, see Best Practices for Screen Independence.
+ + +How Android supports multiple screens
+ +The foundation of Android's support for multiple screens is a set of built-in compatibility features that together manage the 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 with two key ways to control the way your application is displayed, if you need or want to use them:
+ +Using the alternate resources framework
+ +The platform's support for loading screen size- and density-specific resources at run time is based on the alternate 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 Alternate Resources. +
-
+
At run time, the platform provides three types of support to your application, to ensure the best possible display on the current device screen:
+ + +-
+
+ 
+
+
+ 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.
+ +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 HVGA 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.
+ + +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:
+ +-
+
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 1. 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 (dip or dp) or scale-independent pixels (sip or sp). 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 from dips to pixels for more information.
Manifest attributes for screens support
+ + Android 1.6 introduces a new manifest element, <supports-screens>, whose attributes you can use to control the display of your application on different classes of device screens.
| + Attribute + | ++ Description + | +
|---|---|
+ android:smallScreens
+ |
+
+ Whether or not the application UI is designed for use on small screens — "true" if it is, and "false" if not. The default value is "true".
+ |
+
+ android:normalScreens
+ |
+
+ Whether or not the application UI is designed for use on normal screens — "true" if it is, and "false" if not. Default value is "true".
+ |
+
+ android:largeScreens
+ |
+
+ Whether or not the application UI is designed for use on large screens — "true" if it is, and "false" if not.
+ |
+
+ android:anyDensity
+ |
+
+ Whether or not the application is designed to manage its UI properly in different density environments — "
|
+
In general, when you declare a screen-size attribute as "true", you are signaling to the platform that your application wants to manage its UI by itself, for all screen sizes, without the platform applying any size-compatibility behaviors (such as a virtual HVGA display area). + +
If you declare the android:anyDensity attribute as "true", you are signaling to the platform that your application wants to manage its UI by itself, for all screen densities, using the actual screen dimensions and pixels. In this case, the application must ensure that it declares its UI dimensions using device-independent pixels and scales any actual pixel values or math by the scaling factor available from {@link android.util.DisplayMetrics#density android.util.DisplayMetrics.density}.
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 large, normal, and small screens in any densities.
+ +<manifest xmlns:android="http://schemas.android.com/apk/res/android">+ +
+ <supports-screen + android:largeScreens="true" + android:normalScreens="true" + android:smallScreens="true" + android:resizable="true" + android:anyDensity="true" /> + </manifestt> +
+ Note for different default value: +
+ +The default value for these parameters differs, depending on the the value of the android:minSdkVersion attribute in the application's manifest, as well as the value of android:targetSdkVersion, if declared:
-
+
-
+ If
android:minSdkVersionorandroid:targetSdkVersionis "3" (Android 1.5) or smaller, the default value for everything except android:normalScreens isfalse. If you are primarily targeting pre-Android 1.6 platforms but also want to support other densities/screen sizes, you need to set the appropriate attributes totrue. +
+ -
+ If
android:minSdkVersionorandroid:targetSdkVersionis "4" (Android 1.6) or larger, the default value for everything istrue. If your application requires Android 1.6 features, but does not support these densities and/or screen sizes, you need to set the appropriate attributes tofalse. +
+ -
+ Note that
android:normalScreensalways defaults totrue. +
+
Resource directory qualifiers for screen size and density
+ +Android 1.6 supports new resource directory qualifiers for selecting 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 Table 1, earlier in this document.
+ +| Screen characteristic | +Qualifier | +Description | +
|---|---|---|
| Size | +small |
+ Smaller screen size than T-Mobile G1, such as QVGA low density. | +
normal |
+ T-Mobile G1/HTC Magic screen size, or equivalent. | +|
large |
+Larger than T-Mobile G1/HTC Magic screen size. Typical example is a tablet like device. | +|
| Density | +ldpi |
+low density resources, for 100 to 140 dpi screens. | +
mdpi |
+medium density resources for 140 to 180 dpi screens. | +|
hdpi |
+High density resources for 190 to 250 dpi screens. | +|
nodpi |
+For density-independent resources. The platform does not auto-scale resources tagged with this qualifier, based on screen density. | +|
| Aspect ratio | +long |
+for screen that has taller/wider aspect ratio than normal aspect ratio. | +
notlong |
+normal aspect ratio. | +
+ 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 supports low and high density, and employs different layout schemes. +
+res/layout/my_layout.xml // layout for normal screen size +res/layout-small/my_layout.xml // layout for small screen size +res/layout-large/my_layout.xml // layout for large screen size +res/layout-large-land/my_layout.xml // layout for large screen size in landscape mode + +res/drawable-ldpi/my_icon.png // icon image for low density +res/drawable-mdpi/dpi/my_icon.png // icon for medium density +res/drawable-hdpi/my_icon.png // icon 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 Alternate 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 screen configurations listed in Table 1 earlier in this document.
+ +You can easily ensure that your application will display properly on different screens. Here is a quick checklist:
+ +-
+
- + Prefer wrap_content, fill_parent and the dip unit to px in XML layout files + +
- + Avoid AbsoluteLayout + +
- + Do not use hard coded pixel values in your code + +
- + Use density and/or resolution specific resources + +
1. Prefer wrap_content, fill_parent and the dip unit to absolute pixels
+
+When defining the layout_width and layout_height of views in an XML layout file, using wrap_content, fill_parent or the dip will guarantee that the view is given an appropriate size on the current device screen. For instance, a view with a layout_width="100dip" 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 (scaled point, the scale factor depends on a user setting) or dip (if you don't want to allow the user to scale the text) to define font sizes.
2. Avoid AbsoluteLayout
+ +{@link android.widget.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 {@link android.widget.FrameLayout 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 APIs use pixels are 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, if myView.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 from dips to pixels +
+
+ In some cases, you will need to express dimensions in dip 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 HVGA 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 is recognized. 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 dip and then converted to actual pixels.
+
// The gesture threshold expressed in dip +private static final float GESTURE_THRESHOLD_DIP = 16.0f; + +// Convert the dips to pixels +final float scale = getContext().getResources().getDisplayMetrics().density; +mGestureThreshold = (int) (GESTURE_THRESHOLD_DIP * scale + 0.5f); + +// Use mGestureThreshold as a distance in pixels ++ +
The {@link android.util.DisplayMetrics#density android.util.DisplayMetrics.density} field specifies the the scale factor you must use to convert dips 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 {@link android.util.DisplayMetrics DisplayMetrics} class for details.
Use pre-scaled configuration values
+ +The {@link android.view.ViewConfiguration 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
+ +
+Figure 2. Comparison of pre-scaled and auto-scaled bitmaps.
+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 can also take control over the scaling of images assets. The sections below provide more 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 create your 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 the resources 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 of android: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 {@link android.graphics.BitmapFactory.Options 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.
+
+
Using auto-scaling instead of pre-scaling is more CPU expensive than pre-scaling but uses less memory. You can refer to the documentation of {@link android.graphics.BitmapFactory BitmapFactory}, {@link android.graphics.Bitmap Bitmap}, and {@link android.graphics.Canvas Canvas} for more information on auto-scaling.
+ +Figure 1, 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 earlier platform version, as well as to +
- Newer devices that are running Android 1.6 and offering various screen sizes and resolutions +
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 and existing 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 any and all devices.
+ +The recommended strategy is to develop against the most recent version of the platform you are targeting, and test on the minimum one you want to run on. Here's how to do that:
+ +-
+
- Maintain compatability with existing devices by leaving your application's
android:minSdkVersionattribute as it is. You do not need to increment the value of the attribute to support new devices and multiple screens. + - Extend compatibility for Android 1.6 devices by adding a new attribute —
android:targetSdkVersion— to theuses-sdkelement. Set the value of the attribute to "4". This allows your application to "inherit" the platform's multiple screens support, even though it is technically using an earlier version of the API.
+ - Add an empty
<screens-support>element as a child of<manifest>. If you need to enable size or density attributes later, this is where you will add them.
+ - Change your application's build properties, such that it compiles against the Android 1.6 (API Level 4) library, 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. For more information about how to set the build target for your application, see +
- Set up AVDs for testing your application on Android 1.6. 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 platform as the system image to run. For more information, see How to Test Your Application on Multiple Screens, below. +
- Set up AVDs for testing your application on Android 1.5 (or earlier platform). 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. +
- Compile your application against the Android 1.6 library and run it on the AVDs you created. Observe the way your application looks and runs, and test all of the user interactions. +
- Debug any display or functional issues. For issues that you resolve in your application code, make certain not to use any APIs introduced in API Level 4 or later. 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 an API introduced in API Level 4 or later will mean that your application will no longer be compatible with devices running Android 1.5 or earlier. +
- For resource-related issues, you can try resolving them by:
+
-
+
- Adding a
anyDensity="false"attribute to<screens-support>, to enable density-compatibility scaling.
+ - Creating any size- or density-specific resources you need and placing them in directories tagged with the correct qualifiers. Qualifiers must be arranged in a proscribed order. See Alternate Resources for more information). +
- If you need to add resources that are to be used only only when the application is run on Android 1.6 or later platforms, you can tag the directories containing those resources with the
-v4qualifier.
+
+ - Adding a
- If you want the platform to create a virtual HVGA screen area when displaying it on larger screens, add a
largeScreens="false"attribute to the to<screens-support>element in the manifest.
+ - Continue testing and debugging until your application performs as expected on all of the platforms and screen sizes your application will support. +
- Sign your application using the same private key you used when publishing the previous version and publish the application as an update. +
In particular, remember to test your application on an AVD that emulates a small-screen device. Devices that offer a screen 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 many cases, the reduced screen area and density mean that you may need to make tradeoffs in design, content, and function on those devices.
+ +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, you set up a series of AVDs that emulate the screen sizes and densities you want to support. The Android SDK includes four 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
+
+The objective of your testing is to ensure Table 1 of this document. For general information about working with AVDs, see Android Virtual Devices
The Android 1.6 SDK includes these emulator 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) + +
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 setup 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 <name> -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.
+
+Using the default densities, the emulator skins included in the Android 1.6 SDK will emulate the following screen sizes:
-
+
- + QVGA, low density: 3.3" + +
- + HVGA, medium density: 3.6" + +
- + WVGA800, high density: 3.9" + +
- + WVGA854, high density: 4.1" + +
You should also make sure to test your application on different physical screen sizes within a single size-density configuration. For example, according to Table 1, the minimum supported diagonal of QVGA is 2.8". To display this is 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 <name> -scale 0.6+ +
If you would like to test your application on a screen not supported by the built-in skins, you can either adjust an existing skin, or create a custom resolution.
+ +For example, to test on a large WVGA800 screen with medium density:
+ +-
+
-
+ Create an AVD based on the WVGA800 skin (using the
androidtool's command line.)
+
+ - + Answer "yes" when asked about using custom hardware + +
- enter "160" when asked about the value for hw.lcd.density (120-low, 160-medium, 240-high). +
When running this AVD, the emulator 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+ +
If you would like to test your application with a resolution not supported by +the provided skins, you can use the desired resolution in place of the skin +name. For instance, for FWQVGA you would use:
+ +android create avd ... --skin 240x432+ +
Next, you would need to set the proper density for the screen. When asked by +the tool whether you want to create a custom hardware profile for the new AVD, +enter "yes". Continue through the various profile settings until the tools asks +you to specify "Abstracted LCD density". Consult Table 1, +earlier in this document, and enter the appropriate value. For the FWQVGA +screen, the density should be "160", or medium.
+ +
+ Figure 3. A typical set of AVDs for testing screens support.
+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. The platform's screen-compatibility +features are designed to provide applications 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 on the baseline screen size +and resolution.
+ +The platform displays older applications (such as those built against Android +1.5) as shown in these examples. However, note that older applications can +declare that they are targeted to the Android 1.6 (or later) platform, while +maintaining the same minimum supported API Level.
+ +Internally, the platform provides these screen-compatibility features, +depending 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). + +
Here is an example of how an application running under compatibility mode looks mode in various screen configurations.
+ +|
+ HVGA, normal size, normal density + [ emulator -skin HVGA ] + 
+ |
+
+ WVGA, normal size, high density + [emulator -skin WVGA854 -dpi-device 240] +  + 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 graph is chopped. | +The lap time area is chopped. |
+
 |
+  |
+
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 just does not have enough pixels.)
+ +
+
+
diff --git a/docs/html/guide/topics/manifest/supports-screens-element.jd b/docs/html/guide/topics/manifest/supports-screens-element.jd
index 00797edd55668..3fb0172addea6 100644
--- a/docs/html/guide/topics/manifest/supports-screens-element.jd
+++ b/docs/html/guide/topics/manifest/supports-screens-element.jd
@@ -30,40 +30,8 @@ framework will scale down assets by a factor of 0.75 (low dpi screens)
or scale them up by a factor of 1.5 (high dpi screens).
The screen density is expressed as dots-per-inch (dpi).
-Currently supported densities:
--
-
- Low density: 120 dpi -
- Medium density: 160 dpi -
- High density: 240 dpi -
Table of display types:
- -| Low Density | Medium Density | High Density | -|
|---|---|---|---|
| Small Screen | -QVGA | -n/a | -VGA | -
| Normal Screen | -WQVGA | -HVGA | -WVGA, FWVGA | -
| Large Screen | -n/a | -VGA, WVGA, FWVGA | -n/a | -
For more information, see +Multiple Screens Support.
-
+
- Multiple Screens Support
- {@link android.util.DisplayMetrics}