Figure 2. Layout hierarchy for the layout in figure 1, using nested instances of {@link android.widget.LinearLayout}.
It is a common misconception that using the basic layout structures leads to the most efficient layouts. However, each widget and layout you add to your application requires initialization, layout, and drawing. For example, using nested instances of {@link android.widget.LinearLayout} can lead to an excessively deep view hierarchy. Furthermore, nesting several instances of {@link android.widget.LinearLayout} that use the {@code layout_weight} parameter can be especially expensive as each child needs to be measured twice. This is particularly important when the layout is inflated repeatedly, such as when used in a {@link android.widget.ListView} or {@link android.widget.GridView}.
In this lesson you'll learn to use Heirachy Viewer and Layoutopt to examine and optimize your layout.
The Android SDK tools include a tool called Heirachy Viewer that allows you to analyze your layout while your application is running. Using this tool helps you discover bottlenecks in the layout performance.
Hierarchy Viewer works by allowing you to select running processes on a connected device or emulator, then display the layout tree. The traffic lights on each block represent its Measure, Layout and Draw performance, helping you identify potential issues.
For example, figure 1 shows a layout that's used as an item in a {@link android.widget.ListView}. This layout shows a small bitmap image on the left and two stacked items of text on the right. It is especially important that layouts that will be inflated multiple times—such as this one—are optimized as the performance benefits will be multiplied.
Figure 1. Conceptual layout for an item in a {@link android.widget.ListView}.
The {@code hierarchyviewer} tool is available in {@code <sdk>/tools/}. When opened, the Hierarchy Viewer shows a list of available devices and its running components. Click Load View Hierarchy to view the layout hierarchy of the selected component. For example, figure 2 shows the layout for the list item illustrated by figure 1.
Figure 2. Layout hierarchy for the layout in figure 1, using nested instances of {@link android.widget.LinearLayout}.
Figure 3. Clicking a hierarchy node shows its performance times.
In figure 2, you can see there is a 3-level hierarchy with some problems laying out the text items. Clicking on the items shows the time taken for each stage of the process (figure 3). It becomes clear which items are taking the longest to measure, layout, and render, and where you should spend time optimizing.
The timings for rendering a complete list item using this layout are:
Because the layout performance above slows down due to a nested {@link android.widget.LinearLayout}, the performance might improve by flattening the layout—make the layout shallow and wide, rather than narrow and deep. A {@link android.widget.RelativeLayout} as the root node allows for such layouts. So, when this design is converted to use {@link android.widget.RelativeLayout}, you can see that the layout becomes a 2-level hierarchy. Inspection of the new layout looks like this:
Figure 4. Layout hierarchy for the layout in figure 1, using {@link android.widget.RelativeLayout}.
Now rendering a list item takes:
Might seem like a small improvement, but this time is multiplied several times because this layout is used for every item in a list.
Most of this time difference is due to the use of {@code layout_weight} in the {@link android.widget.LinearLayout} design, which can slow down the speed of measurement. It is just one example of how each layout has appropriate uses and you should carefully consider whether using layout weight is necessary.
It is always good practice to also run the layoutopt tool on your final layout files to search for places in your view hierarchy that may be optimized. Layoutopt is also in your SDK {@code tools/} directory and takes a layout directory name or a space-separated list of layout files that you'd like to inspect.
When you run {@code layoutopt} on a layout file, it prints a line number for each issue found, a description of the issue, and for some types of issues it also suggests a resolution. For example:
$ layoutopt samples/ samples/compound.xml 7:23 The root-level <FrameLayout/> can be replaced with <merge/> 11:21 This LinearLayout layout or its FrameLayout parent is useless samples/simple.xml 7:7 The root-level <FrameLayout/> can be replaced with <merge/>
After you apply the suggested layout optimizations, run Hierarchy Viewer again to inspect the performance changes.