Display Support

Updates made to these display-specific areas are provided below:

Resizing activities and displays

To indicate that an app may not support multi-window mode or resizing, activities use the resizeableActivity=false attribute. Common issues encountered by apps when activities are resized include:

  • An activity can have a different configuration from the app or another non-visual component. A common mistake is to read display metrics from the app context. The returned values won't be adjusted to the visible area metrics in which an activity is displayed.
  • An activity may not handle resizing and crash, display a distorted UI, or lose state due to relaunch without saving the instance state.
  • An app may attempt to use absolute input coordinates (instead of those relative to the window position), which may break the input in multi-window.

In Android 7 (and higher), an app can be set resizeableActivity=false to always run in full screen mode. In this case, the platform prevents non-resizable activities from going into split screen. If the user tries to invoke a non-resizable activity from the launcher while already in a split-screen mode, the platform exits split-screen mode and launches the non-resizable activity in full-screen mode.

Apps that explicitly set this attribute to false in the manifest must not be launched in multi-window mode, unless the compatibility mode is applied:

  • Same configuration is applied to the process, which contains all activities and non-activity components.
  • Applied configuration meets the CDD requirements for app-compatible displays.

In Android 10, the platform still prevents non-resizable activities from going into split-screen mode, but they can be temporarily scaled if the activity has declared a fixed orientation or aspect ratio. If not, the activity resizes to fill up the entire screen as in Android 9 and lower.

The default implementation applies the following policy:

When an activity declared to be incompatible with multi-window through use of the android:resizeableActivity attribute and when that activity meets one of the conditions described below, then when the applied screen configuration must change, the activity and process are saved with the original configuration and the user is provided with an affordance to relaunch the app process to use the updated screen configuration.

  • Is fixed orientation via the application of android:screenOrientation
  • App has default maximum or minimum aspect ratio by targeting API level or declares the aspect ratio explicitly

This figure displays a non-resizable activity with a declared aspect ratio. When folding the device, the window is scaled down to fit the area while maintaining the aspect ratio using the appropriate letterboxing. In addition, a restart activity option is provided to the user each time the display area for the activity is changed.

When unfolding the device, the configuration, size, and aspect ratio of the activity don't change, but the option to restart the activity is displayed.

When resizeableActivity is not set (or it is set to true), the app fully supports resizing.

Implementation

A non-resizable activity with fixed orientation or aspect ratio is called size compatibility mode (SCM) in code. The condition is defined in ActivityRecord#shouldUseSizeCompatMode(). When an SCM activity is launched, the screen-related configuration (such as size or density) is fixed in the requested override configuration, so the activity is no longer dependent on the current display configuration.

If the SCM activity can't fill the entire screen, it is top aligned and horizontally centered. The activity bounds are computed by AppWindowToken#calculateCompatBoundsTransformation().

When an SCM activity uses a different screen configuration than its container (for example, the display is resized, or activity moved to another display), ActivityRecord#inSizeCompatMode() is true and SizeCompatModeActivityController (in System UI) receives the callback to show the process restart button.

Display sizes and aspect ratios

Android 10 provides support for new aspect ratios from high ratios of long and thin screens to 1:1 ratios. Apps can define ApplicationInfo#maxAspectRatio and the ApplicationInfo#minAspectRatio of the screen that they are able to handle.

Figure 1. Example app ratios supported in Android 10

Device implementations can have secondary displays with sizes and resolutions smaller than those required by Android 9, and lower (minimum of 2.5 inches width or height, minimum of 320 DP for smallestScreenWidth), but only activities that opt in to support these small displays can be placed there.

Apps can opt in by declaring a minimum supported size that is smaller than oe equal to the target display size. Use the android:minHeight and android:minWidth activity layout attributes in the AndroidManifest to do so.

Display policies

Android 10 separates and moves certain display policies from the default WindowManagerPolicy implementation in PhoneWindowManager to per-display classes, such as:

  • Display state and rotation
  • Some keys and motion event tracking
  • System UI and decoration windows

In Android 9 (and lower), the PhoneWindowManager class handled display policies, state and settings, rotation, decoration window frame tracking, and more. Android 10 moves most of this to the DisplayPolicy class, except for rotation tracking, which has been moved to DisplayRotation.

Display window settings

In Android 10, the configurable per-display windowing setting has been expanded to include:

  • Default display windowing mode
  • Overscan values
  • User rotation and rotation mode
  • Forced size, density, and scaling mode
  • Content removal mode (when display is removed)
  • Support for system decorations and IME

The DisplayWindowSettings class contains settings for these options. They're persisted to disc in /data partition in display_settings.xml every time a setting is changed. For details, see DisplayWindowSettings.AtomicFileStorage and DisplayWindowSettings#writeSettings(). Device manufacturers can provide default values in display_settings.xml for their device configuration. However, because the file is stored in /data, additional logic may be needed to restore the file if erased by a wipe.

By default, Android 10 uses DisplayInfo#uniqueId as an identifier for a display when persisting the settings. uniqueId should be populated for all displays. In addition, it's stable for physical and network displays. It's also possible to use the port of a physical display as the identifier, which can be set in DisplayWindowSettings#mIdentifier. Upon each write, all settings are written so it's safe to update the key that's used for a display entry in storage. For details, see "Static Display Identifiers" below.

Settings are persisted in the /data directory for historical reasons. Originally, they were used to persist user-set settings, such as display rotation.

Static display identifiers

Android 9 (and lower) did not provide stable identifiers for displays in the framework. When a display was added to the system, Display#mDisplayId or DisplayInfo#displayId was generated for that display by incrementing a static counter. If the system added and removed the same display, a different ID resulted.

If a device had multiple displays available from boot, the displays could be assigned different identifiers, depending on the timing. While Android 9 (and earlier) included DisplayInfo#uniqueId, it didn't contain enough information to differentiate between displays because physical displays were identified as either local:0 or local:1, to represent the built-in and external display.

Android 10 changes DisplayInfo#uniqueId to add a stable identifier and to differentiate between local, network, and virtual displays.

Display type Format
Local
local:<stable-id>
Network
network:<mac-address>
Virtual
virtual:<package-name-and-name>

In addition to updates to uniqueId, DisplayInfo.address contains DisplayAddress, a display identifier that is stable across reboots. In Android 10, DisplayAddress supports physical and network displays. DisplayAddress.Physical contains a stable display ID (same as in uniqueId) and can be created with DisplayAddress#fromPhysicalDisplayId().

Android 10 also provides a convenient method to get port information (Physical#getPort()). This method can be used in the framework to statically identify displays. For example, it's used in DisplayWindowSettings). DisplayAddress.Network contains the MAC address and can be created with DisplayAddress#fromMacAddress().

These additions allow device manufacturers to identify displays in static multi-display set-ups and to configure different system settings and features using static display identifiers, such as ports for physical displays. These methods are hidden and are intended only to be used within system_server.

Given a HWC display ID (which can be opaque and not always stable), this method returns the (platform-specific) 8-bit port number that identifies a physical connector for display output, as well as the display's EDID blob. SurfaceFlinger extracts manufacturer or model information from the EDID to generate the stable 64-bit display IDs exposed to the framework. If this method isn't supported or errors out, SurfaceFlinger falls back to the legacy MD mode, where DisplayInfo#address is null and DisplayInfo#uniqueId is hard-coded, as described above.

To verify that this feature is supported, run:

$ dumpsys SurfaceFlinger --display-id
# Example output.
Display 21691504607621632 (HWC display 0): port=0 pnpId=SHP displayName="LQ123P1JX32"
Display 9834494747159041 (HWC display 2): port=1 pnpId=HWP displayName="HP Z24i"
Display 1886279400700944 (HWC display 1): port=2 pnpId=AUS displayName="ASUS MB16AP"

Per-display focus

To support several input sources that target individual displays at the same time, Android 10 can be configured to support multiple focused windows, at most one per-display. This is intended only for special types of devices when multiple users interact with the same device at the same time and use different input methods or devices, such as Android Automotive.

It is strongly recommended that this feature not be enabled for regular devices, including multi-screen devices or those used for desktop-like experiences. This is due primarily to a security concern that may cause users to wonder which window has input focus.

Imagine the user who enters secure information into a text input field, perhaps logging in to a banking app or entering text that contains sensitive information. A malicious app could create a virtual off-screen display with which to execute an activity, also with a text input field. Legitimate and malicious activities have focus and both display an active input indicator (blinking cursor).

However, since input from a keyboard (hardware or software) is entered into the topmost activity only (that app that was most recently launched). By creating a hidden virtual display, a malicious app could grab user input, even when using a software keyboard on the primary device display.

Use com.android.internal.R.bool.config_perDisplayFocusEnabled to set per-display focus.

Compatibility

Issue: In Android 9 and lower, at most one window in the system has focus at a time.

Solution: In the rare case when two windows from the same process would be focused, the system provides focus only to the window that's higher in the Z-order. This restriction is removed for apps that target Android 10, at which point it's expected that they can support multiple windows being focused on simultaneously.

Implementation

WindowManagerService#mPerDisplayFocusEnabled controls the availability of this feature. In ActivityManager, ActivityDisplay#getFocusedStack() is now used instead of global tracking in a variable. ActivityDisplay#getFocusedStack() determines focus based on Z-order instead of caching the value. This is so that only one source, WindowManager, need track the Z-order of activities.

ActivityStackSupervisor#getTopDisplayFocusedStack() takes a similar approach for those cases when the topmost focused stack in the system must be identified. The stacks are traversed from top to bottom, searching for the first eligible stack.

InputDispatcher can now have multiple focused windows (one per display). If an input event is display-specific, then it's dispatched to the focused window in the corresponding display. Otherwise, it's dispatched to the focused window in the focused display, which is the display that the user most recently interacted with.

See InputDispatcher::mFocusedWindowHandlesByDisplay and InputDispatcher::setFocusedDisplay(). Focused apps are also updated separately in InputManagerService through NativeInputManager::setFocusedApplication().

In WindowManager, focused windows are also tracked separately. See DisplayContent#mCurrentFocus and DisplayContent#mFocusedApp and the respective uses. Related focus tracking and updating methods have been moved from WindowManagerService to DisplayContent.