Load views on demand

Sometimes your layout requires complex views that are rarely used. Whether they are item details, progress indicators, or undo messages, you can reduce memory usage and speed up rendering by loading the views only when they're needed.

You can defer loading resources when you have complex views that your app needs in the future by defining a ViewStub for complex and rarely used views.

Define a ViewStub

ViewStub is a lightweight view with no dimension that doesn't draw anything or participate in the layout. As such, it requires few resources to inflate and leave in a view hierarchy. Each ViewStub includes the android:layout attribute to specify the layout to inflate.

Suppose you have a layout you want to load later in the user journey of your app:

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <ImageView
        android:src="@drawable/logo"
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>
</FrameLayout>

You can postpone loading using the following ViewStub. To make it show or load anything, you must make it show the referred layout:

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/root"
android:layout_width="match_parent"
android:layout_height="match_parent">

<ViewStub
    android:id="@+id/stub_import"
    android:inflatedId="@+id/panel_import"
    android:layout="@layout/heavy_layout_we_want_to_postpone"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:layout_gravity="bottom" />
</FrameLayout>

Load the ViewStub layout

The code snippets in the previous section produce something like figure 1:

Figure 1. Initial state of the screen: the ViewStub is hiding the heavy layout.

When you want to load the layout specified by the ViewStub, either set it to visible by calling setVisibility(View.VISIBLE) or call inflate().

The following code snippet simulates a postponed load. The screen loads as usual in the Activity and onCreate(), then it shows the heavy_layout_we_want_to_postpone layout:

Kotlin

override fun onCreate(savedInstanceState: Bundle?) {
  super.onCreate(savedInstanceState)
  setContentView(R.layout.activity_old_xml)

  Handler(Looper.getMainLooper())
      .postDelayed({
          findViewById<View>(R.id.stub_import).visibility = View.VISIBLE
          
          // Or val importPanel: View = findViewById<ViewStub>(R.id.stub_import).inflate()
      }, 2000)
}

Java

@Override
void onCreate(savedInstanceState: Bundle?) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_old_xml);

  Handler(Looper.getMainLooper())
      .postDelayed({
          findViewById<View>(R.id.stub_import).visibility = View.VISIBLE
          
          // Or val importPanel: View = findViewById<ViewStub>(R.id.stub_import).inflate()
      }, 2000);
}

Figure 2. The heavy layout is visible.

Note: The inflate() method returns the inflated View after it's complete, so you don't need to call findViewById() if you need to interact with the layout.

Once visible or inflated, the ViewStub element is no longer part of the view hierarchy. It is replaced by the inflated layout, and the ID for the root view of that layout is specified by the android:inflatedId attribute of the ViewStub. The ID android:id specified for the ViewStub is valid only until the ViewStub layout is visible or inflated.

Note: A drawback of ViewStub is that it doesn't support the <merge> tag in the layouts to be inflated.

For more information about this topic, see the blog post Optimize with stubs.