The way you manage the hierarchy of your
View objects can
have a substantial impact on your app’s performance. This page describes how to
assess whether your view hierarchy is slowing your app down, and offers some
strategies for addressing issues that may arise.
Layout and Measure Performance
The rendering pipeline includes a layout-and-measure
stage, during which the system appropriately positions the relevant items in
your view hierarchy. The measure part of this stage determines the sizes and
View objects. The layout part determines where on the screen to
Both of these pipeline stages incur some small cost per view or layout that they
process. Most of the time, this cost is minimal and doesn’t noticeably affect
performance. However, it can be greater when an app adds or removes View
objects, such as when a
object recycles them or reuses them. The
cost can also be higher if a
View object needs to consider
resizing to main its constraints: For example, if your app calls
SetText() on a
View object that wraps text, the
View may need to resize.
If cases like these take too long, they can prevent a frame from rendering within the allowed 16ms, so that frames are dropped, and animation becomes janky.
Because you cannot move these operations to a worker thread—your app must process them on the main thread—your best bet is to optimize them so that they can take as little time as possible.
Managing complexity: layouts matter
allow you to nest UI objects in the view hierarchy. This nesting can also impose
a layout cost. When your app processes an object for layout, the app performs
the same process on all children of the layout as well. For a complicated
layout, sometimes a cost only arises the first time the system computes the
layout. For instance, when your app recycles a complex list item in a
RecyclerView object, the
system needs to lay out all of the objects. In another example, trivial changes
can propagate up the chain toward the parent
until they reach an object that doesn’t affect the size of the parent.
The most common case in which layout takes an especially long time is when
View objects are nested within one another. Each nested layout
object adds cost to the layout stage. The flatter your hierarchy, the less
time that it takes for the layout stage to complete.
If you are using the
RelativeLayout class, you may be able to achieve the same
effect, at lower cost, by using nested, unweighted
LinearLayout views instead. Additionally, if your app
targets Android 7.0 (API level 24), it is likely that
you can use a special layout editor to create a
object instead of
RelativeLayout. Doing so allows you
to avoid many of the issues this section
class offers similar layout control, but
with much-improved performance. This class uses its own constraint-solving
system to resolve relationships between views in a very different way from
Typically, the framework executes the layout or measure stage in a single pass and quite quickly. However, with some more complicated layout cases, the framework may have to iterate multiple times on parts of the hierarchy that require multiple passes to resolve before ultimately positioning the elements. Having to perform more than one layout-and-measure iteration is referred to as double taxation.
- Executes a layout-and-measure pass, during which the framework calculates each child object’s position and size, based on each child’s request.
- Uses this data, also taking object weights into account, to figure out the proper position of correlated views.
- Performs a second layout pass to finalize the objects’ positions.
- Goes on to the next stage of the rendering process.
The more levels your view hierarchy has, the greater the potential performance penalty.
Containers other than
RelativeLayout may also give rise to double taxation. For
LinearLayoutview could result in a double layout-and-measure pass if you make it horizontal. A double layout-and-measure pass may also occur in a vertical orientation if you add measureWithLargestChild, in which case the framework may need to do a second pass to resolve the proper sizes of objects.
GridLayouthas a similar issue. While this container also allows relative positioning, it normally avoids double taxation by pre-processing the positional relationships among child views. However, if the layout uses weights or fill with the
Gravityclass, the benefit of that preprocessing is lost, and the framework may have to perform multiple passes if it the container were a
Multiple layout-and-measure passes are not, in themselves, a performance burden. But they can become so if they’re in the wrong spot. You should be wary of situations where one of the following conditions applies to your container:
- It is a root element in your view hierarchy.
- It has a deep view hierarchy beneath it.
- There are many instances of it populating the screen, similar to children
Diagnosing View Hierarchy Issues
Layout performance is a complex problem with many facets. There are a couple of tools that can give you solid indications about where performance bottlenecks are occurring. A few other tools provide less definitive information, but can also provide helpful hints.
One tool that provides excellent data about performance is Systrace, which is built into the Android SDK. The Systrace tool allows you to collect and inspect timing information across an entire Android device, allowing you to see when layout performance problems cause performance problems. For more information about Systrace, see Analyze UI Performance with Systrace.
Profile GPU rendering
The other tool most likely to provide you with concrete information about performance bottlenecks is the on-device Profile GPU rendering tool, available on devices powered by Android 6.0 (API level 23) and later. This tool allows you to see how long the layout-and-measurestage is taking for each frame of rendering. This data can help you diagnose runtime performance issues, and help you determine what, if any layout-and-measure issues you need to address.
In its graphical representation of the data it captures, Profile GPU rendering uses the color blue to represent layout time. For more information about how to use this tool, see Profile GPU Rendering Walkthrough.
Android Studio’s Lint tool can help you gain a sense of inefficiencies in the view hierarchy. To use this tool, select Analyze > Inspect Code, as shown in Figure 1.
Information about various layout items appears under Android > Lint > Performance. To see more detail, you can click on each item to expand it, and see more information in the pane on the right side of the screen. Figure 2 shows an example of such a display.
Clicking on one of these items reveals, in the pane to the right, the problem associated with that item.
To understand more about specific topics and issues in this area, see the Lint documentation.
Android Studio’s Layout Inspector tool provides a visual representation of your app’s view hierarchy. It is a good way to navigate the hierarchy of your app, providing a clear visual representation of a particular view’s parent chain, and allowing you to inspect the layouts that your app constructs.
The views that Layout Inspector presents can also help identify performance problems arising from double taxation. It can also provide an easy way for you to identify deep chains of nested layouts, or layout areas with a large amount of nested children, another potential source of performance costs. In these scenarios, the layout-and-measure stages can be particularly costly, resulting in performance issues.
For more information, see Debug Your Layout with Layout Inspector.
Solving View Hierarchy Issues
The fundamental concept behind solving performance problems that arise from view hierarchies is simple in concept, but more difficult in practice. Preventing view hierarchies from imposing performance penalties encompasses the dual goals of flattening your view hierarchy and reducing double taxation. This section discusses some strategies for pursuing these goals.
Removing redundant nested layouts
Developers often use more nested layouts than necessary. For example, a
RelativeLayout container might contain a single child that is also a
RelativeLayout container. This nesting amounts to redundancy, and adds
unnecessary cost to the view hierarchy.
Lint can often flag this problem for you, reducing debugging time.
One frequent cause of redundant nested layouts is the <include> tag. For example, you may define a re-usable layout as follows:
<LinearLayout> <!-- some stuff here --> </LinearLayout> </pre>
And then an include tag to add this item to the parent container:
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:orientation="vertical" android:layout_width="match_parent" android:layout_height="match_parent" android:background="@color/app_bg" android:gravity="center_horizontal"> <include layout="@layout/titlebar"/> <TextView android:layout_width="match_parent" android:layout_height="wrap_content" android:text="@string/hello" android:padding="10dp" /> ... </LinearLayout>
The include unnecessarily nests the first layout within the second layout.
Adopting a cheaper layout
You may not be able to adjust your existing layout scheme so that it doesn’t contain redundant layouts. In certain cases, the only solution may be to flatten your hierarchy by switching over to an entirely different layout type.
For example, you may find that a
provides the same functionality as a more complex layout with many
positional dependencies. In the N release of Android, the
ConstraintLayout class provides similar functionality to
RelativeLayout, but at a significantly lower cost.