Android 使用带有接口侦听器的DialogFragment将结果返回给父活动时发生IllegalStateException

我正在尝试使用support Library中定义的DialogFragment添加登录对话框（我必须使用它来支持旧设备）。我可以使用Google页面上的例子：创建这样一个带有自定义布局的登录对话框。当我将所有内容放在一起时，Eclipse中的所有内容看起来都太匹配了，但是当我尝试运行它时，我立即得到一个非法状态异常。它甚至没有给我一个要看的行号。有人能帮我找到什么原因吗？谢谢

从源文件夹：

public class LoginDialogFragment extends DialogFragment {


public interface LoginDialogListener {
    public void onDialogPositiveClick(DialogFragment dialog);
    public void onDialogNegativeClick(DialogFragment dialog);
}

LoginDialogListener mListener;

public void onAttach(Activity activity) {
    super.onAttach(activity);
    // Verify that the host activity implements the callback interface
    try {
        // Instantiate the NoticeDialogListener so we can send events to the host
        mListener = (LoginDialogListener) activity;
    } catch (ClassCastException e) {
        // The activity doesn't implement the interface, throw exception
        throw new ClassCastException(activity.toString()
                + " must implement NoticeDialogListener");
    }
}

public Dialog onCreateDialog(Bundle savedInstanceState) {
    AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());
    // Get the layout inflater
    LayoutInflater inflater = getActivity().getLayoutInflater();

    // Inflate and set the layout for the dialog
    // Pass null as the parent view because its going in the dialog layout
    builder.setView(inflater.inflate(R.layout.login_dialog, null))
    // Add action buttons
           .setPositiveButton("Login", new DialogInterface.OnClickListener() {
               @Override
               public void onClick(DialogInterface dialog, int id) {
                // Send the positive button event back to the host activity
                   mListener.onDialogPositiveClick(LoginDialogFragment.this);
               }
           })
           .setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
               public void onClick(DialogInterface dialog, int id) {
                // LoginDialogFragment.this.getDialog().cancel();
                // Send the negative button event back to the host activity
                   mListener.onDialogNegativeClick(LoginDialogFragment.this);
               }
           });      
    return builder.create();
}

}



public class UsingPreferencesActivity
extends FragmentActivity
    implements LoginDialogFragment.LoginDialogListener{

protected void onCreate(Bundle savedInstanceState){

    this.showLoginDialog();

}

public void showLoginDialog() {
    // Create an instance of the dialog fragment and show it
    DialogFragment dialog = new LoginDialogFragment();
    dialog.show(getSupportFragmentManager(), "LoginDialogFragment");
}

public void onDialogPositiveClick(DialogFragment dialog) {
    Toast toast = Toast.makeText(getBaseContext(), "OK", Toast.LENGTH_LONG);
    toast.show();

}

public void onDialogNegativeClick(DialogFragment dialog) {
    Toast toast = Toast.makeText(getBaseContext(), "Cancel", 
Toast.LENGTH_LONG);
    toast.show();
}

public void onClickLoad(View view){
    Intent i = new Intent("com.example.homework09.AppPreferenceActivity");
    startActivity(i);
}


public void onClickDisplay(View view){
    SharedPreferences appPrefs = 
getSharedPreferences("com.example.homework09_preferences",MODE_PRIVATE);
    DisplayText(appPrefs.getString("editTextPref", ""));
}


public void onClickModify(View view){
    SharedPreferences appPrefs = 
getSharedPreferences("com.example.homework09_preferences", MODE_PRIVATE);
    SharedPreferences.Editor prefsEditor = appPrefs.edit();
    prefsEditor.putString("editTextPref", 
((EditText)findViewById(R.id.etNewUsername)).getText().toString());
    prefsEditor.commit();
    DisplayText(appPrefs.getString("editTextPref", ""));
}


public void onClickReset(View view){

    SharedPreferences appPrefs =  
getSharedPreferences("com.example.homework09_preferences", MODE_PRIVATE);
    SharedPreferences.Editor prefsEditor = appPrefs.edit();
    prefsEditor.putString("editTextPref", " ");
    prefsEditor.commit();
    DisplayText(appPrefs.getString("editTextPref", ""));
}


private void DisplayText(String str){
    Toast.makeText(getBaseContext(), str, Toast.LENGTH_LONG).show();
    TextView textView = (TextView)findViewById(R.id.tvUsername);
    textView.setText(str);
}

}



public class AppPreferenceActivity extends PreferenceActivity {

public void onCreate(Bundle savedInstanceState){
    super.onCreate(savedInstanceState);
    addPreferencesFromResource(R.xml.myapppreferences);
}

}

从res/layout/login_dialog.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >


<EditText 
    android:id="@+id/input"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:hint="[Enter username here]"/>

<CheckBox 
    android:id="@+id/input_chkbox"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="Save Username?"
    android:defaultValue="false"
    />

</LinearLayout>

从res/layout/main.xml

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical">

<Button 
    android:id="@+id/btnPreferences"
    android:text="Show Preference Screen"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:onClick="onClickLoad"
    />

<Button 
    android:id="@+id/btnDisplayValues"
    android:text="Display Username Preference"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:onClick="onClickDisplay"
    />



<Button 
    android:id="@+id/btnResetValues"
    android:text="Reset Username Preference"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:onClick="onClickReset"
    />


<EditText 
    android:id="@+id/etNewUsername"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:hint="Enter New Username Here."
    />

<CheckBox
    android:id="@+id/chkBox"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="Save Username as Preference?"
    ></CheckBox>

<Button 
    android:id="@+id/btnModifyValues"
    android:text="Update Username Preference"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:onClick="onClickModify"
    />


<TextView 
    android:id="@+id/tvUsername"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:textAlignment="center"
    android:text=" "
    android:hint="[See Username Preference Here]"
    />

</LinearLayout>

从xml/myapppreferences.xml

<?xml version="1.0" encoding="utf-8"?>
<PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android" >


    <CheckBoxPreference 
        android:title="Save Username?"
        android:defaultValue="false"
        android:summary="Yes or No"
        android:key="checkboxPref"
        />


    <EditTextPreference 
        android:summary="Enter Username"
        android:defaultValue="[Enter Username]"
        android:title="Username"
        android:key="editTextPref"
        />



</PreferenceScreen>

---

我发现了问题。在清理一些旧代码的过程中，我意外地从活动的onCreate方法中删除了setContentView语句，因此我的应用程序的主活动没有指定布局/视图。这就是导致错误的原因。因此，它与对话框片段等无关。一旦我将语句添加回onCreate（），如下所示，它就可以正常工作了：

protected void onCreate(Bundle savedInstanceState){
    super.onCreate(savedInstanceState);  // added - fixed problem
    this.setContentView(R.layout.main);  // added - fixed problem
    this.showLoginDialog();

}