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对不起，我相信这会很简单，但我只是看不出我在哪里出错了。

我有一个片段（工具列表），可以在两个窗格中启动许多其他片段（工具）。上面有一个活动，我正在使用它来管理按下“关闭按钮”时工具碎片的移除。然后它会从后台堆栈中弹出一个工具（如果存在）。

奇怪的是，我在一个片段中进行了此操作，但是将“剪切并粘贴”到另一个片段中的代码不起作用并引发了 NPE。任何想法为什么非常感谢。

活动片段：

import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentTransaction;    

public class MyActivity extends FragmentActivity implements OnFragmentButtonClickedListener {

private final static int CLOSE_BUTTON = 0;
...


public void onFragmentButtonClicked(int button, Fragment fragId) {
    Log.i(TAG, "onFragmentButton");
    // process closing of fragment
    if (button == CLOSE_BUTTON) {
        FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
        ft.remove(fragId);
        ft.commit();
    }
    // then restore most recent fragment from backstack
    if (getSupportFragmentManager().getBackStackEntryCount() > 0) {
        getSupportFragmentManager().popBackStack();
    }
}
}

此片段有效：

import android.support.v4.app.Fragment;

public class FirstFragment extends Fragment implements OnClickListener{

OnFragmentButtonClickedListener fragmentButtonListener;
private final static int CLOSE_BUTTON = 0;


// empty constructor
public FirstFragment() {};

// Activity must implement this interface
public interface OnFragmentButtonClickedListener {
    public void onFragmentButtonClicked (int button, Fragment fragId);
}

@Override
public void onAttach(Activity activity) {
    Log.i(TAG, "onAttach");
    super.onAttach(activity);
    try {
        fragmentButtonListener = (OnFragmentButtonClickedListener) activity;
    } catch (ClassCastException e) {
        Log.w(TAG, (activity.toString() + " must implement OnFragmentButtonClickedListener"));
    }
}

...

public void onClick(View v) {
    Log.i(TAG, "onClick");
    switch (v.getId()) {
    case R.id.btn_st_close:
        //close activity
        eraseData(prefs);
        Log.i(TAG, "Frag Id = " + String.valueOf(getFragmentManager().findFragmentById(getId())));
        fragmentButtonListener.onFragmentButtonClicked(CLOSE_BUTTON, getFragmentManager().findFragmentById(getId()));
        break;
    }
}

但是，当在 fragmentButtonListener 上调用 onFragmentButtonClicked() 方法时，这个会在 onClick 方法中抛出 NPE。

import android.support.v4.app.Fragment;

public class SecondFragment extends Fragment implements OnClickListener {

OnFragmentButtonClickedListener fragmentButtonListener;
private final static int CLOSE_BUTTON = 0;

// empty constructor
public SecondFragment() {};

// Activity must implement this interface
public interface OnFragmentButtonClickedListener {
    public void onFragmentButtonClicked (int button, Fragment fragId);
}

@Override
public void onAttach(Activity activity) {
    Log.i(TAG, "onAttach");
    super.onAttach(activity);
    try {
        fragmentButtonListener = (OnFragmentButtonClickedListener) activity;
    } catch (ClassCastException e) {
        Log.w(TAG, (activity.toString() + " must implement OnFragmentButtonClickedListener"));
    }
}

...

 public void onClick(View v) {
     Log.i(TAG, "OnClick");
    switch (v.getId()) {
    case R.id.btn_wc_close:
        //close activity
        eraseData(prefs);
        Log.i(TAG, "Frag Id = " + String.valueOf(getFragmentManager().findFragmentById(getId())));
        try {
            fragmentButtonListener.onFragmentButtonClicked(CLOSE_BUTTON, getFragmentManager().findFragmentById(getId()));
        }
        catch (Exception e) {
            Log.i(TAG, "Exception: " + e);
        }
        break;
        }
    }

从 logcat 中，我可以看到片段 Id 不为空，所以我很困惑。

08-20 09:13:49.096: I/Second Fragment(1691): onCreate
08-20 09:13:49.096: I/Second Fragment(1691): Frag Id = SecondFragment{41355d68 #1 id=0x7f090093}
08-20 09:13:49.106: I/Second Fragment(1691): onCreateView
08-20 09:13:49.406: I/Second Fragment(1691): onActivityCreated
08-20 09:13:49.560: I/Second Fragment(1691): onClick
0 09:13:51.766: I/Second Fragment(1691): Frag Id = SecondFragment{41355d68 #1 id=0x7f090093}
08-20 09:13:51.766: W/dalvikvm(1691): threadid=1: thread exiting with uncaught exception (group=0x40a13300)
08-20 09:13:51.826: E/AndroidRuntime(1691): FATAL EXCEPTION: main
08-20 09:13:51.826: E/AndroidRuntime(1691): java.lang.NullPointerException
08-20 09:13:51.826: E/AndroidRuntime(1691):     at SecondFragment.onClick(IceTurbFragment.java:136)
08-20 09:13:51.826: E/AndroidRuntime(1691):     at android.view.View.performClick(View.java:4084)
08-20 09:13:51.826: E/AndroidRuntime(1691):     at android.view.View$PerformClick.run(View.java:16966)
08-20 09:13:51.826: E/AndroidRuntime(1691):     at android.os.Handler.handleCallback(Handler.java:615)
08-20 09:13:51.826: E/AndroidRuntime(1691):     at android.os.Handler.dispatchMessage(Handler.java:92)
08-20 09:13:51.826: E/AndroidRuntime(1691):     at android.os.Looper.loop(Looper.java:137)
08-20 09:13:51.826: E/AndroidRuntime(1691):     at android.app.ActivityThread.main(ActivityThread.java:4745)
08-20 09:13:51.826: E/AndroidRuntime(1691):     at java.lang.reflect.Method.invokeNative(Native Method)
08-20 09:13:51.826: E/AndroidRuntime(1691):     at java.lang.reflect.Method.invoke(Method.java:511)
08-20 09:13:51.826: E/AndroidRuntime(1691):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
08-20 09:13:51.826: E/AndroidRuntime(1691):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
08-20 09:13:51.826: E/AndroidRuntime(1691):     at dalvik.system.NativeStart.main(Native Method)

谢谢你的帮助。

4

1 回答 1

1

我可以看到您在每个片段中定义了 2 个具有相同名称的接口。

public interface OnFragmentButtonClickedListener {
     public void onFragmentButtonClicked (int button, Fragment fragId);
}

但是该活动只能实现一个具有此名称的接口（在这种情况下，我相信您的活动实现了第一个片段的接口），这可能会导致第二个片段的错误空指针异常。

//where is OnFragmentButtonClickedListener come from?
public class MyActivity extends FragmentActivity implements FirstFragment OnFragmentButtonClickedListener

尝试在 2 个片段之外编写 OnFragmentButtonClickedListener（可能在另一个文件中）

于 2012-08-20T09:58:23.477 回答