导入了FacebookSDK，但缺少类运行时异常(imported FacebookSDK and yet missing class runtime exception)

 我有以下android布局xml： 
 
   <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:tools="http://schemas.android.com/tools"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">

       <com.facebook.widget.LoginButton
            android:id="@+id/authButton"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center_horizontal"
            android:layout_marginTop="30dp"
            />
    </LinearLayout>
 
 但是我得到运行时错误： 
 
 甚至可以在图形编辑器中看到： 
 
 
 但我不明白为什么，因为我已正确导入了FacebookSDK（我认为） 
 
 当我做cmd +右键单击XML时，我得到了正确的类实现 
 
 `package com.facebook.widget; 
 
 公共类LoginButton扩展Button {` 
 
 
 我错过了什么？ 

I have the following android layout xml:
 
   <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:tools="http://schemas.android.com/tools"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">

       <com.facebook.widget.LoginButton
            android:id="@+id/authButton"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center_horizontal"
            android:layout_marginTop="30dp"
            />
    </LinearLayout>
 
However I get runtime error: 
 
which can even be seen in the Graphical editor:
 
 
But I don't get why, as I have imported the FacebookSDK properly (i think) 
 
and when I do cmd + right click on the XML I get to the right class implementation 
 
`package com.facebook.widget;
 
public class LoginButton extends Button {`
 
 
What am I missing?

原文：https://stackoverflow.com/questions/21202121