Android 小发现：xml里定义的组件取出始终为null

先看下列xml代码：

<?xml
version="1.0"
encoding="utf-8"?>
<RelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/wordBgRL"
android:layout_width="match_parent"
android:layout_height="match_parent"
>
<Button
android:id="@+id/dictionaryButton"
android:background="@drawable/dictionary_btn_bg"
android:layout_alignParentTop="true"
android:layout_alignParentLeft="true"
android:layout_marginTop="10dp"
android:layout_marginLeft="10dp"
style="@style/WrapContentWidgetStyle"
/>
</RelativeLayout>

在java代码里取出wordBgRL 始终为null；

wordBgRL = (RelativeLayout)findViewById(R.id.wordBgRL); // wordBgRL alwasys == null

dictionaryButton = (Button)findViewById(R.id.dictionaryButton); // dictionaryButton != null

后来发现你必须自己另写一个RelativeLayout，可能因为 xmlns:android="http://schemas.android.com/apk/res/android" 它存在的原因吧。
<?xml
version="1.0"
encoding="utf-8"?>
<RelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
>
<RelativeLayout
android:id="@+id/wordBgRL"
style="@style/FillParentWidgetStyle"
>
<Button
android:id="@+id/dictionaryButton"
android:background="@drawable/dictionary_btn_bg"
android:layout_alignParentTop="true"
android:layout_alignParentLeft="true"
android:layout_marginTop="10dp"
android:layout_marginLeft="10dp"
style="@style/WrapContentWidgetStyle"
/>

</RelativeLayout>

</RelativeLayout>