Docs: API Overview page for Android N, Dev Preview 3

        <li><a href="#default_trusted_ca">Default Trusted CA</a></li>

        <li><a href="#apk_signature_v2">APK signature scheme v2</a></li>

        <li><a href="#scoped_directory_access">Scoped directory access</a></li>

        <li><a href="#keyboard_shortcuts_helper">Keyboard Shortcuts Helper</a></li>

        <li><a href="#sustained_performance_api">Sustained Performance API</a></li>

        <li><a href="#print_svc">Print service enhancements</a></li>

        <li><a href="#virtual_files">Virtual Files</a></li>

        <li><a href="#framemetrics_api">FrameMetricsListener API</a></li>

      </ol>

</div>

</div>

<p>

  Vulkan is only available to apps on devices with Vulkan-capable hardware,

  such as Nexus 5X and Nexus 6P. We're working closely with our

  such as Nexus 5X, Nexus 6P, and Nexus Player. We're working closely with our

  partners to bring Vulkan to more devices as soon as possible.

</p>

<p>Direct boot improves device startup times and lets registered

apps have limited functionality even after an unexpected reboot.

For example, if an encrypted device reboots while the user is sleeping,

registered alarms, messages and incoming calls can now continue notify

registered alarms, messages and incoming calls can now continue to notify

the user as normal. This also means accessibility services can also be

  available immediately after a restart.</p>

<a href="{@docRoot}preview/features/scoped-folder-access.html">Scoped

Directory Access</a> developer documentation.</p>

<h2 id="keyboard_shortcuts_helper">Keyboard Shortcuts Helper</h2>

<p>

In Android N, the user can press Meta+/ to trigger a Keyboard Shortcuts

screen that displays all shortcuts available both from the system and from

the app in focus. These are retrieved automatically from the app’s menu if

available, but developers can provide their own fine-tuned shortcuts lists

for the screen. This is done simply by overriding the following method

in {@code Activity.java}:

</p>

<pre>

public void onProvideKeyboardShortcuts(

     List<KeyboardShortcutGroup> data, Menu menu, int deviceId)

</pre>

<p>

To trigger the Keyboard Shortcuts Helper from anywhere in your app,

call {@code requestKeyboardShortcutsHelper} for the relevant activity.

</p>

<h2 id="sustained_performance_api">Sustained Performance API</h2>

<p>

Performance can fluctuate dramatically for long-running apps, because the

system throttles system-on-chip engines as device components reach their

temperature limits. This fluctuation presents a moving target for app

developers creating high-performance, long-running apps.

</p>

<p>

To address these limitations, Android N includes support for

<em>sustained performance mode</em>, enabling OEMs to provide hints on

device-performance capabilities for long-running applications. App developers

can use these hints to tune applications for a predictable,

consistent level of device performance over long periods of time.

</p>

<p>

App developers can try out this new API in the N Developer Preview on

Nexus 6P devices only. To use this feature,

set the sustained performance window flag for the window

you want to run in sustained performance mode. Set this flag using the

{@code setSustainedPerformanceMode(boolean)} method. The system automatically

disables this mode when the window is no longer in focus.

</p>

<h2 id="print_svc">Print Service Enhancements</h2>

<p>

  "{@docRoot}preview/setup-sdk.html#docs-dl">API Reference</a>.

</p>

<h2 id="framemetrics_api">FrameMetricsListener API</h2>

<p>

The FrameMetricsListener API allows an app to monitor its UI rendering

performance by exposing a streaming pubsub API to transfer frame

timing info for the app's current window. The data returned is

equivalent to that displayed by {@code adb shell dumpsys gfxinfo framestats},

but is not limited to the past 120 frames.

</p>

<p>

You can use FrameMetricsListener to measure interaction-level UI

performance in production, without a USB connection. This API

allows collection of data at a much higher granularity than does

{@code adb shell dumpsys gfxinfo}. This higher granularity is possible because

the system can collect data for particular interactions in the app; the system

need not capture a global summary of the entire app’s

performance, or clear any global state. You can use this

capability to gather performance data and catch regressions in UI performance

for real use cases within an app.

</p>

<p>

The API provides a callback interface to be implemented and

registered on the window that you wish to monitor:

</p>

<pre>

 public interface FrameMetricsListener {

        void onMetricsAvailable(Window window, FrameMetrics frameMetrics,

                int dropCountSinceLastInvocation);

    }

</pre>

<p>

The API returns a FrameMetrics object, which contains timing data that

the rendering subsystem reports for various milestones in a frame lifecycle.

The supported metrics are: {@code UNKNOWN_DELAY_DURATION},

{@code INPUT_HANDLING_DURATION}, {@code ANIMATION_DURATION},

{@code LAYOUT_MEASURE_DURATION}, {@code DRAW_DURATION}, {@code SYNC_DURATION},

{@code COMMAND_ISSUE_DURATION}, {@code SWAP_BUFFERS_DURATION},

{@code TOTAL_DURATION}, and {@code FIRST_DRAW_FRAME}

</p>

<h2 id="virtual_files">Virtual Files</h2>

<p>