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.

用ViewStub引用的嵌入的layout可推迟加载

Define a ViewStub

　　ViewStub is a lightweight view with no dimension and 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 application.

viewstub示例

 <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().

加载ViewStub对应的layout的2方法

1，((ViewStub) findViewById(R.id.stub_import)).setVisibility(View.VISIBLE);

// or

2，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.

注意 作用第2种方法时，如果该ViewStub对应的lahout是临时的，那么可以不用 findViewById()，

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.)

一旦ViewStub被加载，那么ViewStub就被其指定的layout替换。

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