The way you manage the hierarchy of your {@link android.view.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.
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 boundaries of {@link android.view.View} objects. The layout part determines where on the screen to position the {@link android.view.View} objects.
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 {@link android.support.v7.widget.RecyclerView} object recycles them or reuses them. The cost can also be higher if a {@link android.view.View} object needs to consider resizing to main its constraints: For example, if your app calls {@link android.widget.TextView#setText(char[], int, int) SetText()} on a {@link android.view.View} object that wraps text, the {@link android.view.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.
Android Layouts 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 {@link android.support.v7.widget.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 hierarchies of {@link android.view.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 {@link android.widget.RelativeLayout} class, you may be able to achieve the same effect, at lower cost, by using nested, unweighted {@link android.widget.LinearLayout} views instead. Additionally, if your app targets Android N (API level 24), it is likely that you can use a special layout editor to create a {@code ConstraintLayout} object instead of {@link android.widget.RelativeLayout}. Doing so allows you to avoid many of the issues this section describes. The {@code ConstraintLayout} 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 standard layouts.
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 the layout or measure stage before ultimately positioning the elements. Having to perform more than one layout-and-measure iteration is referred to as double taxation.
For example, when you use the {@link android.widget.RelativeLayout} container, which allows you to position {@link android.view.View} objects with respect to the positions of other {@link android.view.View} objects, the framework performs the following actions:
The more levels your view hierarchy has, the greater the potential performance penalty.
Containers other than {@link android.widget.RelativeLayout} may also give rise to double taxation. For example:
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:
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 Android Studio. The Systrace tool allows you to collect and inspect timing information across an entire Android device, allowing you to see specifically where performance bottlenecks arise. For more information about Systrace, see Analyze UI Performance with Systrace.
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.
Figure 1. Locating Inspect Code in the Android Studio.
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.
Figure 2. Viewing information about specific issues that the lint tool has identified.
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 Hierarchy Viewer 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 Hierarchy Viewer 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.
You can also can get a sense of relative time taken by layout-and-measure operations by clicking the “profile node” button.
For more information about Hierarchy Viewer, see Optimizing Your UI.
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.
Developers often use more nested layouts than necessary. For example, a {@link android.widget.RelativeLayout} container might contain a single child that is also a {@link android.widget.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.
The merge tag can help prevent this issue. For information about this tag, see Re-using Layouts with <include>.
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 {@link android.widget.TableLayout} provides the same functionality as a more complex layout with many positional dependencies. In the N release of Android, the {@code ConstraintLayout} class provides similar functionality to {@link android.widget.RelativeLayout}, but at a significantly lower cost.