我刚从 ActionBarSherlock 切换到 ActionBarCompat 并且一切正常，直到我遇到这个小错误。我有一个扩展 ActionBarActivity 的基本 Activity ，并有 3 个 fragment 与之关联。在 fragment onAttach 方法之一中，我尝试执行此代码:

@Override
public void onAttach(Activity activity) {
    super.onAttach(activity);
    final ActionBar actionBar = ((ActionBarActivity)activity).getSupportActionBar();

    actionBar.setHomeButtonEnabled(true);
    actionBar.setDisplayHomeAsUpEnabled(true);
    actionBar.setDisplayShowTitleEnabled(false);
    actionBar.setTitle("");
}

在我尝试更改屏幕方向之前它工作正常。然后我得到了这行代码的 NullPointerException:actionBar.setHomeButtonEnabled(true);。

我该如何解决这个问题？

编辑:

这是完整的错误日志:

06-10 18:53:31.440: E/AndroidRuntime(18586): FATAL EXCEPTION: main
06-10 18:53:31.440: E/AndroidRuntime(18586): Process: com.packagename.appname, PID: 18586
06-10 18:53:31.440: E/AndroidRuntime(18586): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.packagename.appname/com.packagename.appname.activities.NewActivity}: java.lang.NullPointerException
06-10 18:53:31.440: E/AndroidRuntime(18586):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2328)
06-10 18:53:31.440: E/AndroidRuntime(18586):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2386)
06-10 18:53:31.440: E/AndroidRuntime(18586):    at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:3947)
06-10 18:53:31.440: E/AndroidRuntime(18586):    at android.app.ActivityThread.access$1000(ActivityThread.java:169)
06-10 18:53:31.440: E/AndroidRuntime(18586):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1283)
06-10 18:53:31.440: E/AndroidRuntime(18586):    at android.os.Handler.dispatchMessage(Handler.java:102)
06-10 18:53:31.440: E/AndroidRuntime(18586):    at android.os.Looper.loop(Looper.java:136)
06-10 18:53:31.440: E/AndroidRuntime(18586):    at android.app.ActivityThread.main(ActivityThread.java:5476)
06-10 18:53:31.440: E/AndroidRuntime(18586):    at java.lang.reflect.Method.invokeNative(Native Method)
06-10 18:53:31.440: E/AndroidRuntime(18586):    at java.lang.reflect.Method.invoke(Method.java:515)
06-10 18:53:31.440: E/AndroidRuntime(18586):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1268)
06-10 18:53:31.440: E/AndroidRuntime(18586):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1084)
06-10 18:53:31.440: E/AndroidRuntime(18586):    at dalvik.system.NativeStart.main(Native Method)
06-10 18:53:31.440: E/AndroidRuntime(18586): Caused by: java.lang.NullPointerException
06-10 18:53:31.440: E/AndroidRuntime(18586):    at com.packagename.appname.fragments.NewFragment.onAttach(NewFragment.java:144)
06-10 18:53:31.440: E/AndroidRuntime(18586):    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:883)
06-10 18:53:31.440: E/AndroidRuntime(18586):    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1104)
06-10 18:53:31.440: E/AndroidRuntime(18586):    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1086)
06-10 18:53:31.440: E/AndroidRuntime(18586):    at android.support.v4.app.FragmentManagerImpl.dispatchCreate(FragmentManager.java:1879)
06-10 18:53:31.440: E/AndroidRuntime(18586):    at android.support.v4.app.FragmentActivity.onCreate(FragmentActivity.java:215)
06-10 18:53:31.440: E/AndroidRuntime(18586):    at android.support.v7.app.ActionBarActivity.onCreate(ActionBarActivity.java:97)
06-10 18:53:31.440: E/AndroidRuntime(18586):    at com.packagename.appname.activities.NewActivity.onCreate(NewActivity.java:73)
06-10 18:53:31.440: E/AndroidRuntime(18586):    at android.app.Activity.performCreate(Activity.java:5451)
06-10 18:53:31.440: E/AndroidRuntime(18586):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1093)
06-10 18:53:31.440: E/AndroidRuntime(18586):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2292)
06-10 18:53:31.440: E/AndroidRuntime(18586):    ... 12 more

仅供引用:NewFragment 第 144 行是:actionBar.setHomeButtonEnabled。

最佳答案

onAttach() 可能太早了。当 Activity 在旋转后重新创建时，它是最先调用的方法之一，甚至在 Activity 完全创建之前(即它的 onCreate() 尚未完成)。因此 ActionBar 还没有准备好也就不足为奇了。

检查 relationship between Activity and Fragment's lifecycles在文档中。

简而言之:将此代码移动到 onActivityCreated() 或 onStart() 中。

进一步说明

查看ActionBarActivity的源代码:

@Override
protected void onCreate(Bundle savedInstanceState) {
    mImpl = ActionBarActivityDelegate.createDelegate(this);
    super.onCreate(savedInstanceState);
    mImpl.onCreate(savedInstanceState);
}

• super.onCreate() 是 FragmentActivity.onCreate()，它在旋转后恢复之前的任何 fragment 。
• mImpl.onCreate(savedInstanceState)是ActionBarActivityDelegate.onCreate()，从窗口样式中读取mHasActionBar变量。
• 在 mHasActionBar 为真之前，getSupportActionBar() 将始终返回 null。

关于java - 从 Fragment (ActionBarCompat) 内部调用 getSupportActionBar() 时出现 NullPointerException，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/24152038/