en/devices/tech/perf/task-snapshots.html - platform/docs/source.android.com - Git at Google

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   <head>
     <title>Task Snapshots</title>
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 <h2 id="introduction">Introduction</h2>

 <p>
 <em>Task Snapshots</em> is infrastructure introduced in Android O that combines
 screenshots for <em>Recents Thumbnails</em> as well as <em>Saved Surfaces</em>
 from Window Manager. Recents Thumbnails represent the last state of a task in
 the Recents view.
 </p>

 <p>
 When an activity went into a stopped state, Window Manager didn't destroy the
 surfaces of the activity as long as that activity was on the top of the task. If
 this activity had to be shown again, Window Manager was able to start the
 animation without waiting for the activity to finish drawing its first frame, as
 it was able to use this Saved Surface.
 </p>
 <h2 id="architecture">Architecture</h2
 >

 <p>
 The two concepts of Recents Thumbnails and Saved Surfaces are unified with Task
 Snapshots. When a task goes into background, Window Manager places a screenshot
 of this task into a GraphicBuffer. As long as the application of the top
 activity of the task stays in memory, this GraphicBuffer will be retained in
 memory. Now, when the same activity is brought to the front again, Window
 Manager will create a starting window (TaskSnapshotSurface), and attach the
 GraphicBuffer without copying any memory to the buffer queue of the starting
 window. As soon as the activity has drawn its first frame, the Task Snapshot
 starting window will fade out smoothly like regular splash screens.
 </p>

 <p>
 The same GraphicBuffer is also sent over Binder to SystemUI to be used to draw
 the preview state of a task in the Recents view. Since this is just a reference
 to a buffer, sending it over binder expends few resources. When the
 GraphicBuffer arrives at SystemUI, it is wrapped into a hardware Bitmap and then
 drawn onto the screen without any memory uploading to the graphics memory.
 </p>

 <h2 id="benefits">Benefits</h2>

 <p>
 There are three main benefits to this new architecture:
 </p>

 <ul>
 <li>If the task snapshot is used as a starting window, there is a nice crossfade
 between the snapshot and the real content.</li>
 <li>When the task snapshot is drawn in SystemUI, it can be done so without any
 copying. Previously the bitmap had to be copied into Ashmem, then into graphics
 memory. Since this method stores the snapshot directly in graphics memory, no
 copying is needed.</li>
 <li>The state you see in Recents always matches the state you'll first see when
 reopening the app. Having the same buffer here also saves a lot of memory.
 That's why Recents is now able to show these images at full resolution.
 Previously, it was down sampled by 64% to save memory.</li>
 </ul>

 <h2 id="implementation">Implementation</h2>

 <p>
 This feature exists entirely in the Android platform. No integration is
 required, and customization is not supported. Device manufacturers may, however,
 disable the Task Snapshots feature entirely.
 </p>

 <p>
 To disable this feature, modify this function:
 </p>

 <pre class="prettyprint">
 frameworks/base/services/core/java/com/android/server/wm/TaskSnapshotController.java#215
 </pre>

 <p>
 Note that if the feature is disabled, the Recents view will not show any
 thumbnails whatsoever. This feature is automatically disabled on low-RAM
 devices.
 </p>

 <h2 id="examples-and-source">Examples and source</h2>

 <p>
 Find the rest of the code for this feature within the TaskSnapshot* files in:
 </p>

 <pre class="prettyprint">
 frameworks/base/+/master/services/core/java/com/android/server/wm/
 </pre>

 </body>
 </html>
	<html devsite>
	<head>
	<title>Task Snapshots</title>
	<meta name="project_path" value="/_project.yaml" />
	<meta name="book_path" value="/_book.yaml" />
	</head>
	<body>
	<!--
	Copyright 2017 The Android Open Source Project

	Licensed under the Apache License, Version 2.0 (the "License");
	you may not use this file except in compliance with the License.
	You may obtain a copy of the License at

	http://www.apache.org/licenses/LICENSE-2.0

	Unless required by applicable law or agreed to in writing, software
	distributed under the License is distributed on an "AS IS" BASIS,
	WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	See the License for the specific language governing permissions and
	limitations under the License.
	-->

	<h2 id="introduction">Introduction</h2>

	<p>
	<em>Task Snapshots</em> is infrastructure introduced in Android O that combines
	screenshots for <em>Recents Thumbnails</em> as well as <em>Saved Surfaces</em>
	from Window Manager. Recents Thumbnails represent the last state of a task in
	the Recents view.
	</p>

	<p>
	When an activity went into a stopped state, Window Manager didn't destroy the
	surfaces of the activity as long as that activity was on the top of the task. If
	this activity had to be shown again, Window Manager was able to start the
	animation without waiting for the activity to finish drawing its first frame, as
	it was able to use this Saved Surface.
	</p>
	<h2 id="architecture">Architecture</h2
	>

	<p>
	The two concepts of Recents Thumbnails and Saved Surfaces are unified with Task
	Snapshots. When a task goes into background, Window Manager places a screenshot
	of this task into a GraphicBuffer. As long as the application of the top
	activity of the task stays in memory, this GraphicBuffer will be retained in
	memory. Now, when the same activity is brought to the front again, Window
	Manager will create a starting window (TaskSnapshotSurface), and attach the
	GraphicBuffer without copying any memory to the buffer queue of the starting
	window. As soon as the activity has drawn its first frame, the Task Snapshot
	starting window will fade out smoothly like regular splash screens.
	</p>

	<p>
	The same GraphicBuffer is also sent over Binder to SystemUI to be used to draw
	the preview state of a task in the Recents view. Since this is just a reference
	to a buffer, sending it over binder expends few resources. When the
	GraphicBuffer arrives at SystemUI, it is wrapped into a hardware Bitmap and then
	drawn onto the screen without any memory uploading to the graphics memory.
	</p>

	<h2 id="benefits">Benefits</h2>

	<p>
	There are three main benefits to this new architecture:
	</p>

	<ul>
	<li>If the task snapshot is used as a starting window, there is a nice crossfade
	between the snapshot and the real content.</li>
	<li>When the task snapshot is drawn in SystemUI, it can be done so without any
	copying. Previously the bitmap had to be copied into Ashmem, then into graphics
	memory. Since this method stores the snapshot directly in graphics memory, no
	copying is needed.</li>
	<li>The state you see in Recents always matches the state you'll first see when
	reopening the app. Having the same buffer here also saves a lot of memory.
	That's why Recents is now able to show these images at full resolution.
	Previously, it was down sampled by 64% to save memory.</li>
	</ul>

	<h2 id="implementation">Implementation</h2>

	<p>
	This feature exists entirely in the Android platform. No integration is
	required, and customization is not supported. Device manufacturers may, however,
	disable the Task Snapshots feature entirely.
	</p>

	<p>
	To disable this feature, modify this function:
	</p>

	<pre class="prettyprint">
	frameworks/base/services/core/java/com/android/server/wm/TaskSnapshotController.java#215
	</pre>

	<p>
	Note that if the feature is disabled, the Recents view will not show any
	thumbnails whatsoever. This feature is automatically disabled on low-RAM
	devices.
	</p>

	<h2 id="examples-and-source">Examples and source</h2>

	<p>
	Find the rest of the code for this feature within the TaskSnapshot* files in:
	</p>

	<pre class="prettyprint">
	frameworks/base/+/master/services/core/java/com/android/server/wm/
	</pre>

	</body>
	</html>