Using the Version-Aware Component 使用版本识别组件

Now that you have two implementations of

TabHelper

and

CompatTab

—one for Android 3.0 and later and one for earlier versions of the platform—it's time to do something with these implementations. This lesson discusses creating the
logic for switching between these implementations, creating version-aware layouts, and finally using the backward-compatible UI component. http://blog.csdn.net/sergeycao

Add the Switching Logic

The

TabHelper

abstract class acts as a
factory for creating version-appropriate

TabHelper

and

CompatTab

instances, based on the current device's platform version:

public abstract class TabHelper {
    ...
    // Usage is TabHelper.createInstance(activity)
    public static TabHelper createInstance(FragmentActivity activity) {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
            return new TabHelperHoneycomb(activity);
        } else {
            return new TabHelperEclair(activity);
        }
    }

    // Usage is mTabHelper.newTab("tag")
    public CompatTab newTab(String tag) {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
            return new CompatTabHoneycomb(mActivity, tag);
        } else {
            return new CompatTabEclair(mActivity, tag);
        }
    }
    ...
}

Create a Version-Aware Activity Layout

The next step is to provide layouts for your activity that can support the two tab implementations. For the older implementation (

TabHelperEclair

), you need to ensure that your activity layout contains a

TabWidget

and

TabHost

, along with a container for tab contents:

res/layout/main.xml:

<!-- This layout is for API level 5-10 only. -->
<TabHost xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@android:id/tabhost"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <LinearLayout
        android:orientation="vertical"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:padding="5dp">

        <TabWidget
            android:id="@android:id/tabs"
            android:layout_width="match_parent"
            android:layout_height="wrap_content" />

        <FrameLayout
            android:id="@android:id/tabcontent"
            android:layout_width="match_parent"
            android:layout_height="0dp"
            android:layout_weight="1" />

    </LinearLayout>
</TabHost>

For the

TabHelperHoneycomb

implementation, all you need is a

FrameLayout

to contain the tab contents, since the tab indicators are provided by the

ActionBar

:

res/layout-v11/main.xml:

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

At runtime, Android will decide which version of the

main.xml

layout to inflate depending on the platform version. This is the same logic shown in the previous section to determine which

TabHelper

implementation to use.

Use TabHelper in Your Activity

In your activity's

onCreate()

method, you can obtain a

TabHelper

object and add tabs with the following code:

@Override
public void onCreate(Bundle savedInstanceState) {
    setContentView(R.layout.main);

    TabHelper tabHelper = TabHelper.createInstance(this);
    tabHelper.setUp();

    CompatTab photosTab = tabHelper
            .newTab("photos")
            .setText(R.string.tab_photos);
    tabHelper.addTab(photosTab);

    CompatTab videosTab = tabHelper
            .newTab("videos")
            .setText(R.string.tab_videos);
    tabHelper.addTab(videosTab);
}

When running the application, this code inflates the correct activity layout and instantiates either a

TabHelperHoneycomb

or

TabHelperEclair

object. The concrete class that's actually used is opaque to the activity, since they share the common

TabHelper

interface.

Below are two screenshots of this implementation running on an Android 2.3 and Android 4.0 device.



Figure 1. Example screenshots of backward-compatible tabs running on an Android 2.3 device (using

TabHelperEclair

) and an Android 4.0 device (using

TabHelperHoneycomb

).