所以我添加编译’com.facebook.android:facebook-android-sdk:4.0.0′.

然后添加FacebookSdk.sdkInitialize(getApplicationContext());
到MainActivity.java

但是当我补充

<com.facebook.login.widget.LoginButton
    android:id="@+id/login_button"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_gravity="center_horizontal"
    android:layout_marginTop="30dp"
    android:layout_marginBottom="30dp" />`

我有以下错误：

Rendering Problems The following classes could not be instantiated:
com.facebook.login.widget.LoginButton (Open Class,Show Exception)
Tip: Use View.isInEditMode() in your custom views to skip code or show sample data when shown in the IDE  
Exception Details java.lang.NoClassDefFoundError: Could not initialize class com.facebook.login.widget.LoginButton
at java.lang.reflect.Constructor.newInstance(Constructor.java:422)
at android.view.LayoutInflater.rInflate_Original(LayoutInflater.java:806)
at android.view.LayoutInflater_Delegate.rInflate(LayoutInflater_Delegate.java:64)
at android.view.LayoutInflater.rInflate(LayoutInflater.java:782)
at android.view.LayoutInflater.inflate(LayoutInflater.java:504)
at android.view.LayoutInflater.inflate(LayoutInflater.java:385) Copy stack to clipboard