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>