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Delayed Loading of Views

This lesson teaches you to

  1. Define a ViewStub
  2. Load the ViewStub Layout

You should also read

Sometimes your layout might require 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 are needed.

Deferring loading resources is an important technique to use when you have complex views that your app might need in the future. You can implement this technique by defining a ViewStub for those 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's cheap to inflate and cheap to leave in a view hierarchy. Each ViewStub simply needs to include the android:layout attribute to specify the layout to inflate.

The following ViewStub is for a translucent progress bar overlay. It should be visible only when new items are being imported into the app.

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

Load the ViewStub Layout

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

findViewById(R.id.stub_import)).setVisibility(View.VISIBLE);
// or
View importPanel = ((ViewStub) findViewById(R.id.stub_import)).inflate();

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

Once visible/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 the one 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/inflated.)

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

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