Facebook SDK 3.0示例

Question

所以，我用这段代码练习，我从来不知道哪里出错了。我遵循了本教程中的确切内容

这是我的代码：应用程序意外停止。请帮忙

public class MainActivity extends FragmentActivity {
private static final int SPLASH = 0;
private static final int SELECTION = 1;
private static final int SETTINGS = 2;
private static final int FRAGMENT_COUNT = SETTINGS +1;
private Fragment[] fragments = new Fragment[FRAGMENT_COUNT];
private MenuItem settings; 
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    uiHelper = new UiLifecycleHelper(this, callback);
    setContentView(R.layout.main);

    FragmentManager fm = getSupportFragmentManager();
    fragments[SELECTION] = fm.findFragmentById(R.id.selectionFragment);
    fragments[SETTINGS] = fm.findFragmentById(R.id.userSettingsFragment);


    FragmentTransaction transaction = fm.beginTransaction();
    for(int i = 0; i < fragments.length; i++) {
        transaction.hide(fragments[i]);
    }
    transaction.commit();
}

private void showFragment(int fragmentIndex, boolean addToBackStack) {
    FragmentManager fm = getSupportFragmentManager();
    FragmentTransaction transaction = fm.beginTransaction();
    for (int i = 0; i < fragments.length; i++) {
        if (i == fragmentIndex) {
            transaction.show(fragments[i]);
        } else {
            transaction.hide(fragments[i]);
        }
    }
    if (addToBackStack) {
        transaction.addToBackStack(null);
    }
    transaction.commit();
}


private boolean isResumed = false;
 @Override
 public void onResume() {
super.onResume();
uiHelper.onResume();
isResumed = true;
  }
@Override
  public void onPause() {
super.onPause();
uiHelper.onPause();
isResumed = false;
  }
   @Override
   public void onActivityResult(int requestCode, int resultCode, Intent data) {
super.onActivityResult(requestCode, resultCode, data);
uiHelper.onActivityResult(requestCode, resultCode, data);
   }

  @Override
  public void onDestroy() {
super.onDestroy();
uiHelper.onDestroy();
 }

   @Override
  protected void onSaveInstanceState(Bundle outState) {
super.onSaveInstanceState(outState);
uiHelper.onSaveInstanceState(outState);
 }


private void onSessionStateChange(Session session, SessionState state, Exception   exception)   {
// Only make changes if the activity is visible
if (isResumed) {
    FragmentManager manager = getSupportFragmentManager();
    // Get the number of entries in the back stack
    int backStackSize = manager.getBackStackEntryCount();
    // Clear the back stack
    for (int i = 0; i < backStackSize; i++) {
        manager.popBackStack();
    }
    if (state.isOpened()) {
        // If the session state is open:
        // Show the authenticated fragment
        showFragment(SELECTION, false);
    } else if (state.isClosed()) {
        // If the session state is closed:
        // Show the login fragment
        showFragment(SPLASH, false);
    }
}
}
@Override
protected void onResumeFragments() {
super.onResumeFragments();
Session session = Session.getActiveSession();

if (session != null && session.isOpened()) {
    // if the session is already open,
    // try to show the selection fragment
    showFragment(SELECTION, false);
} else {
    // otherwise present the splash screen
    // and ask the person to login.
    showFragment(SPLASH, false);
}
}

  private UiLifecycleHelper uiHelper;
    private Session.StatusCallback callback = 
new Session.StatusCallback() {
@Override
public void call(Session session, 
        SessionState state, Exception exception) {
    onSessionStateChange(session, state, exception);
}
 };

 @Override
 public boolean onPrepareOptionsMenu(Menu menu) {
// only add the menu when the selection fragment is showing
if (fragments[SELECTION].isVisible()) {
    if (menu.size() == 0) {
        settings = menu.add(R.string.settings);
    }
    return true;
} else {
    menu.clear();
    settings = null;
}
return false;
 }

 @Override
 public boolean onOptionsItemSelected(MenuItem item) {
if (item.equals(settings)) {
    showFragment(SETTINGS, true);
    return true;
}
return false;
 }
}


    05-21 05:46:22.031: D/dalvikvm(527): GC_EXTERNAL_ALLOC freed 89K, 52% free 2603K/5379K, external 1808K/2137K, paused 89ms
    05-21 05:46:22.221: D/AndroidRuntime(527): Shutting down VM
    05-21 05:46:22.221: W/dalvikvm(527): threadid=1: thread exiting with uncaught exception (group=0x40015560)
    05-21 05:46:22.241: E/AndroidRuntime(527): FATAL EXCEPTION: main
    05-21 05:46:22.241: E/AndroidRuntime(527): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.firstandroidapp/com.firstandroidapp.MainActivity}: java.lang.NullPointerException
    05-21 05:46:22.241: E/AndroidRuntime(527):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)
    05-21 05:46:22.241: E/AndroidRuntime(527):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
    05-21 05:46:22.241: E/AndroidRuntime(527):  at android.app.ActivityThread.access$1500(ActivityThread.java:117)
    05-21 05:46:22.241: E/AndroidRuntime(527):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
    05-21 05:46:22.241: E/AndroidRuntime(527):  at android.os.Handler.dispatchMessage(Handler.java:99)
    05-21 05:46:22.241: E/AndroidRuntime(527):  at android.os.Looper.loop(Looper.java:123)
    05-21 05:46:22.241: E/AndroidRuntime(527):  at android.app.ActivityThread.main(ActivityThread.java:3683)
    05-21 05:46:22.241: E/AndroidRuntime(527):  at java.lang.reflect.Method.invokeNative(Native Method)
    05-21 05:46:22.241: E/AndroidRuntime(527):  at java.lang.reflect.Method.invoke(Method.java:507)
    05-21 05:46:22.241: E/AndroidRuntime(527):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
    05-21 05:46:22.241: E/AndroidRuntime(527):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
    05-21 05:46:22.241: E/AndroidRuntime(527):  at dalvik.system.NativeStart.main(Native Method)
    05-21 05:46:22.241: E/AndroidRuntime(527): Caused by: java.lang.NullPointerException
    05-21 05:46:22.241: E/AndroidRuntime(527):  at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:609)
    05-21 05:46:22.241: E/AndroidRuntime(527):  at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1431)
    05-21 05:46:22.241: E/AndroidRuntime(527):  at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:523)
    05-21 05:46:22.241: E/AndroidRuntime(527):  at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1129)
    05-21 05:46:22.241: E/AndroidRuntime(527):  at android.app.Activity.performStart(Activity.java:3791)
    05-21 05:46:22.241: E/AndroidRuntime(527):  at    android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1620)
     05-21 05:46:22.241: E/AndroidRuntime(527):     ... 11 more
     05-21 05:47:01.391: I/Process(527): Sending signal. PID: 527 SIG: 9

Answer 1

我也有同样的问题。

我解决问题的方法是首先进行调试，看看我的任何片段，“飞溅”或“选择”是否为空。

并且它们是空的。

原因是，在我的Fragments类中，当我应该导入“android.support.v4.app.Fragment”时，我导入了“android.app.Fragment”

堆栈溢出的两个答案都帮助我解决了我的问题：

Android fragment manager backstackrecord . run抛出NullPointerException

findFragmentById返回null