ViewStub和include布局使用(官方文档)
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Loading Views On Demand
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.
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 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()
.
((ViewStub) findViewById(R.id.stub_import)).setVisibility(View.VISIBLE);// orView 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 theViewStub
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.
Re-using Layouts with <include/>
Reusing layouts is particularly powerful as it allows you create reusable complex layouts. For example, a yes/no button panel, or custom progress bar with description text. It also means that any elements of your application that are common across multiple layouts can be extracted, managed separately, then included in each layout. So while you can create individual UI components by writing a custom View
, you can do it even more easily by re-using a layout file.Although Android offers a variety of widgets to provide small and re-usable interactive elements, you might also need to re-use larger components that require a special layout. To efficiently re-use complete layouts, you can use the <include/>
and <merge/>
tags to embed another layout inside the current layout.
Create a Re-usable Layout
If you already know the layout that you want to re-use, create a new XML file and define the layout. For example, here's a layout from the G-Kenya codelab that defines a title bar to be included in each activity (titlebar.xml
):
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="wrap_content" android:background="@color/titlebar_bg"> <ImageView android:layout_width="wrap_content" android:layout_height="wrap_content" android:src="@drawable/gafricalogo" /></FrameLayout>
The root View
should be exactly how you'd like it to appear in each layout to which you add this layout.
Use the <include> Tag
Inside the layout to which you want to add the re-usable component, add the <include/>
tag. For example, here's a layout from the G-Kenya codelab that includes the title bar from above:
Here's the layout file:
<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>
You can also override all the layout parameters (any android:layout_*
attributes) of the included layout's root view by specifying them in the<include/>
tag. For example:
<include android:id="@+id/news_title" android:layout_width="match_parent" android:layout_height="match_parent" layout="@layout/title"/>
However, if you want to override layout attributes using the <include>
tag, you must override both android:layout_height
and android:layout_width
in order for other layout attributes to take effect.
Use the <merge> Tag
The <merge />
tag helps eliminate redundant view groups in your view hierarchy when including one layout within another. For example, if your main layout is a vertical LinearLayout
in which two consecutive views can be re-used in multiple layouts, then the re-usable layout in which you place the two views requires its own root view. However, using another LinearLayout
as the root for the re-usable layout would result in a vertical LinearLayout
inside a vertical LinearLayout
. The nested LinearLayout
serves no real purpose other than to slow down your UI performance.
To avoid including such a redundant view group, you can instead use the <merge>
element as the root view for the re-usable layout. For example:
<merge xmlns:android="http://schemas.android.com/apk/res/android"> <Button android:layout_width="fill_parent" android:layout_height="wrap_content" android:text="@string/add"/> <Button android:layout_width="fill_parent" android:layout_height="wrap_content" android:text="@string/delete"/></merge>
Now, when you include this layout in another layout (using the <include/>
tag), the system ignores the <merge>
element and places the two buttons directly in the layout, in place of the <include/>
tag.
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