24 août 2017
Since announcing ConstraintLayout
at Google I/O last year, we've continued to improve the layout's stability and
layout editor support. We've also added new features specific to
ConstraintLayout
that help you build various type of layouts, such
as introducing
chains and setting
size as a ratio. In addition to these features, there is a notable
performance benefit by using ConstraintLayout
. In this post, we'll
walk through how you can benefit from these performance improvements.
To better understand the performance of ConstraintLayout
, let's
take a step back and see how Android draws views.
When a user brings an Android view into focus, the Android framework directs the view to draw itself. This drawing process comprises 3 phases:
The system completes a top-down traversal of the view tree to determine how
large each ViewGroup
and View element should be. When a
ViewGroup
is measured, it also measures its children.
Another top-down traversal occurs, with each ViewGroup
determining
the positions of its children using the sizes determined in the measure phase.
The system performs yet another top-down traversal. For each object in the view
tree, a Canvas
object is created to send a list of drawing commands
to the GPU. These commands include the ViewGroup
and
View
objects' sizes and positions, which the system determined
during the previous 2 phases.
Each phase within the drawing process requires a top-down traversal of the view tree. Therefore, the more views you embed within each other (or nest) into the view hierarchy, the more time and computation power it takes for the device to draw the views. By keeping a flat hierarchy in your Android app layouts, you can create a fast and responsive user interface for your app.
With that explanation in mind, let's create a traditional layout hierarchy that
uses LinearLayout
and RelativeLayout
objects.
Let's say we want to build a layout like the image above. If you build it with traditional layouts, the XML file contains an element hierarchy similar to the following (for this example, we've omitted the attributes):
<RelativeLayout> <ImageView /> <ImageView /> <RelativeLayout> <TextView /> <LinearLayout> <TextView /> <RelativeLayout> <EditText /> </RelativeLayout> </LinearLayout> <LinearLayout> <TextView /> <RelativeLayout> <EditText /> </RelativeLayout> </LinearLayout> <TextView /> </RelativeLayout> <LinearLayout > <Button /> <Button /> </LinearLayout> </RelativeLayout>
Although there's usually room for improvement in this type of view hierarchy, you'll almost certainly still need to create a hierarchy with some nested views.
As discussed before, nested hierarchies can adversely affect performance. Let's
take a look at how the nested views actually affect the UI performance using
Android Studio's Systrace
tool. We called the measure and layout phases for each ViewGroup
(ConstraintLayout
and RelativeLayout
) programmatically
and triggered Systrace while the measure and layout calls are executing. The
following command generates an overview file that contains key events, such as
expensive measure/layout passes, that occur during a 20-second interval:
python $ANDROID_HOME/platform-tools/systrace/systrace.py --time=20 -o ~/trace.html gfx view res
For more details about how you can use Systrace, see the Analyzing UI Performance with Systrace guide.
Systrace automatically highlights the (numerous) performance problems with this layout, as well as suggestions for fixing them. By clicking the "Alerts" tab, you will find that drawing this view hierarchy requires 80 expensive passes through the measure and layout phases!
Triggering that many expensive measure and layout phases is far from ideal; such
a large amount of drawing activity could result in skipped frames that users
notice. We can conclude that the layout has poor performance due to the nested
hierarchy as well as the characteristic of RelativeLayout
, which
measures each of its children twice.
RelativeLayout
You can check the entire code on how we performed these measurements in our GitHub repository.
If you create the same layout using ConstraintLayout
, the XML file
contains an element hierarchy similar to the following (attributes again
omitted):
<android.support.constraint.ConstraintLayout> <ImageView /> <ImageView /> <TextView /> <EditText /> <TextView /> <TextView /> <EditText /> <Button /> <Button /> <TextView /> </android.support.constraint.ConstraintLayout>
As this example shows, the layout now has a completely flat hierarchy. This is
because ConstraintLayout
allows you to build complex layouts
without having to nest View
and ViewGroup
elements.
For example, let's look at the TextView
and EditText
in the middle of the layout:
RelativeLayout
, you need to create a new
ViewGroup
to align the EditText
vertically with the
TextView:
<LinearLayout android:id="@+id/camera_area" android:layout_width="match_parent" android:layout_height="wrap_content" android:orientation="horizontal" android:layout_below="@id/title" > <TextView android:text="@string/camera" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_gravity="center_vertical" android:id="@+id/cameraLabel" android:labelFor="@+id/cameraType" android:layout_marginStart="16dp" /> <RelativeLayout android:layout_width="match_parent" android:layout_height="wrap_content"> <EditText android:id="@+id/cameraType" android:ems="10" android:inputType="textPersonName" android:text="@string/camera_value" android:layout_width="match_parent" android:layout_height="wrap_content" android:layout_centerVertical="true" android:layout_marginTop="8dp" android:layout_marginStart="8dp" android:layout_marginEnd="8dp" /> </RelativeLayout> </LinearLayout>
By using ConstraintLayout
instead, you can achieve the same effect
just by adding a constraint from the baseline of the TextView
to
the baseline of the EditText
without creating another
ViewGroup
:
<TextView android:layout_width="wrap_content" android:layout_height="wrap_content" app:layout_constraintLeft_creator="1" app:layout_constraintBaseline_creator="1" app:layout_constraintLeft_toLeftOf="@+id/activity_main_done" app:layout_constraintBaseline_toBaselineOf="@+id/cameraType" />
When running the Systrace tool for the version of our layout that uses
ConstraintLayout
, you see far fewer expensive measure/layout passes
during the same 20-second interval. This improvement in performance makes sense,
now that we're keeping the view hierarchy flat!
ConstraintLayout
On a related note, we built the ConstraintLayout
variant of our
layout using just the layout
editor instead of editing the XML by hand. To achieve the same visual effect
using RelativeLayout
, we probably would have needed to edit the XML
by hand.
We analyzed how long every measure and layout pass took for two type of layouts,
ConstraintLayout
and RelativeLayout
, by using
OnFrameMetricsAvailableListener
,
which was introduced in Android 7.0 (API level 24). This class allows you to
collect frame-by-frame timing information about your app's UI
rendering.
By calling the following code, you can start recording per-frame UI actions:
window.addOnFrameMetricsAvailableListener( frameMetricsAvailableListener, frameMetricsHandler);
After timing information becomes available, the app triggers the
frameMetricsAvailableListener()
callback. We are interested in the
measure/layout performance, so we call FrameMetrics.LAYOUT_MEASURE_DURATION
when retrieving the actual frame duration.
Window.OnFrameMetricsAvailableListener { _, frameMetrics, _ -> val frameMetricsCopy = FrameMetrics(frameMetrics); // Layout measure duration in nanoseconds val layoutMeasureDurationNs = frameMetricsCopy.getMetric(FrameMetrics.LAYOUT_MEASURE_DURATION);
To learn more about the other types of duration information that
FrameMetrics
can receive, see the FrameMetrics
API reference.
Our performance comparison shows that ConstraintLayout
performs
about 40% better in the measure/layout phase than RelativeLayout
:
As these results show, ConstraintLayout
is likely to be more
performant than traditional layouts. Moreover, ConstraintLayout
has
other features that help you build complex and performant layouts, as discussed
in the benefits of a ConstraintLayout
object section. For details, see the Build
a Responsive UI with ConstraintLayout guide. We recommend that you use
ConstraintLayout
when designing your app's layouts. In almost all
cases when you would have previously need a deeply-nested layout,
ConstraintLayout
should be your go-to layout for optimal
performance and ease of use.
All the measurements above were performed in the following environment.
Device | Nexus 5X |
Android Version | 8.0 |
ConstraintLayout version | 1.0.2 |
Check out the developer
guide, the API
reference documentation, and the article
on Medium to fully understand what ConstraintLayout
can provide
for you. And once again, thank you to all who submitted feedback and issues over
the months since our alpha release of ConstraintLayout
. We're truly
grateful that we were able to release the production-ready 1.0
version of ConstraintLayout
earlier this year.
As we continue to improve ConstraintLayout
, please continue to send
us feedback using the Android issue tracker.