getActivity 从片段返回 null

【问题标题】：getActivity returning null from fragmentgetActivity 从片段返回 null
【发布时间】：2011-09-29 19:45:16
【问题描述】：

为了学校，我正在制作一个 android 应用程序。对于这个应用程序，我有一个片段，它显示了一个来自数据库的仅包含字符串的网格视图。为此，我需要一个片段。当我调用 getActivity() 时，它返回 null。正如here 建议的那样，onAttach 方法在应用程序崩溃之前不会被调用。我应该如何解决这个问题？
weekFragmetn.xml：

package nl.siebeh.schoolmate;

import java.util.ArrayList;

import android.app.Activity;
import android.content.Context;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.util.Log;
import android.view.ContextMenu;
import android.view.ContextMenu.ContextMenuInfo;
import android.view.LayoutInflater;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.GridView;

public class weekFragment extends Fragment {
    dbLayer db;
    Context mContext;

    @Override
    public void onAttach(Activity activity) {
        mContext = getActivity();
        Log.i("Schoolmate","onAttach called");
        super.onAttach(activity);
    }

    public weekFragment newInstance(String title) {
        Log.i("Schoolmate","newInstance called");
        db = new dbLayer(mContext);
        weekFragment fragment = new weekFragment();
        ArrayList<ArrayList<Object>> result = null;

        if(title == "leraren"){
            result = db.getAllRowsAsArrays("weekTeacher", null);
        }else if(title == "locatie"){
            result = db.getAllRowsAsArrays("weekLocation", null);
        }else if(title == "vakken"){
            result = db.getAllRowsAsArrays("weekSubjects", null);
        }

        String[] array = new String[60];

        for(int i = 0; i < 60; i++){
            array[i] = "";
        }

        array[1] = getResources().getStringArray(R.array.days)[0];
        array[2] = getResources().getStringArray(R.array.days)[1];
        array[3] = getResources().getStringArray(R.array.days)[2];
        array[4] = getResources().getStringArray(R.array.days)[3];
        array[5] = getResources().getStringArray(R.array.days)[4];

        for(int i = 1; i < 10; i++){
            array[i] = Integer.toString(i);
        }

        for(int position = 0; position < result.size(); position++){
            ArrayList<Object> row = result.get(position);
            int hour = Integer.valueOf(row.get(1).toString()).intValue();
            int day =  Integer.valueOf(row.get(0).toString()).intValue();
            int pos = 6 * hour + day;
            array[pos] = row.get(position).toString();
        }
        fragment.mContent = array;

        return fragment;
    }

    private String[] mContent = {""};

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        if (container == null) {
            return null;
        }
        View v = inflater.inflate(R.layout.week_fragment, container, false);
        GridView gridview = (GridView) v.findViewById(R.id.gridview);
        gridview.setAdapter(new weekAdapter(mContext, mContent));
        return gridview;
    }

    @Override
    public boolean onContextItemSelected(MenuItem item) {
        return super.onContextItemSelected(item);
    }

    @Override
    public void onCreateContextMenu(ContextMenu menu, View v, ContextMenuInfo menuInfo) {
        super.onCreateContextMenu(menu, v, menuInfo);
    }



}

logcat：（由于单击项目列表中的项目启动活动，片段被启动/嵌入）

    09-29 19:10:56.611: INFO/ActivityManager(70): Starting: Intent { cmp=nl.siebeh.schoolmate/.overviewWeek } from pid 536
09-29 19:10:56.970: INFO/Schoolmate(536): newInstance called
09-29 19:11:06.631: WARN/ActivityManager(70): Launch timeout has expired, giving up wake lock!
09-29 19:11:06.671: WARN/ActivityManager(70): Activity idle timeout for HistoryRecord{40770188 nl.siebeh.schoolmate/.overviewWeek}
09-29 19:12:20.661: DEBUG/SntpClient(70): request time failed: java.net.SocketException: Address family not supported by protocol
09-29 19:14:11.811: DEBUG/dalvikvm(536): Debugger has detached; object registry had 413 entries
09-29 19:14:11.811: ERROR/SchoolMate(536): java.lang.NullPointerException
09-29 19:14:11.811: INFO/SchoolMate(536): Context is null
09-29 19:14:11.820: WARN/System.err(536): java.lang.NullPointerException
09-29 19:14:11.820: WARN/System.err(536):     at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:118)
09-29 19:14:11.820: WARN/System.err(536):     at nl.siebeh.schoolmate.dbLayer.<init>(dbLayer.java:50)
09-29 19:14:11.820: WARN/System.err(536):     at nl.siebeh.schoolmate.weekFragment.newInstance(weekFragment.java:31)
09-29 19:14:11.820: WARN/System.err(536):     at nl.siebeh.schoolmate.fragmentAdapter.getItem(fragmentAdaptar.java:21)
09-29 19:14:11.820: WARN/System.err(536):     at nl.siebeh.schoolmate.titleFragmentAdapter.getItem(titleFragmentAdapter.java:1)
09-29 19:14:11.820: WARN/System.err(536):     at android.support.v4.app.FragmentPagerAdapter.instantiateItem(FragmentPagerAdapter.java:62)
09-29 19:14:11.820: WARN/System.err(536):     at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:321)
09-29 19:14:11.820: WARN/System.err(536):     at android.support.v4.view.ViewPager.populate(ViewPager.java:441)
09-29 19:14:11.820: WARN/System.err(536):     at android.support.v4.view.ViewPager.onAttachedToWindow(ViewPager.java:563)
09-29 19:14:11.820: WARN/System.err(536):     at android.view.View.dispatchAttachedToWindow(View.java:6156)
09-29 19:14:11.820: WARN/System.err(536):     at android.view.ViewGroup.dispatchAttachedToWindow(ViewGroup.java:1122)
09-29 19:14:11.820: WARN/System.err(536):     at android.view.ViewGroup.dispatchAttachedToWindow(ViewGroup.java:1127)
09-29 19:14:11.820: WARN/System.err(536):     at android.view.ViewGroup.dispatchAttachedToWindow(ViewGroup.java:1127)
09-29 19:14:11.820: WARN/System.err(536):     at android.view.ViewGroup.dispatchAttachedToWindow(ViewGroup.java:1127)
09-29 19:14:11.820: WARN/System.err(536):     at android.view.ViewGroup.dispatchAttachedToWindow(ViewGroup.java:1127)
09-29 19:14:11.820: WARN/System.err(536):     at android.view.ViewRoot.performTraversals(ViewRoot.java:765)
09-29 19:14:11.830: WARN/System.err(536):     at android.view.ViewRoot.handleMessage(ViewRoot.java:1859)
09-29 19:14:11.840: WARN/System.err(536):     at android.os.Handler.dispatchMessage(Handler.java:99)
09-29 19:14:11.840: WARN/System.err(536):     at android.os.Looper.loop(Looper.java:123)
09-29 19:14:11.840: WARN/System.err(536):     at android.app.ActivityThread.main(ActivityThread.java:3683)
09-29 19:14:11.840: WARN/System.err(536):     at java.lang.reflect.Method.invokeNative(Native Method)
09-29 19:14:11.840: WARN/System.err(536):     at java.lang.reflect.Method.invoke(Method.java:507)
09-29 19:14:11.850: WARN/System.err(536):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
09-29 19:14:11.850: WARN/System.err(536):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
09-29 19:14:11.850: WARN/System.err(536):     at dalvik.system.NativeStart.main(Native Method)
09-29 19:14:11.850: DEBUG/AndroidRuntime(536): Shutting down VM
09-29 19:14:11.850: WARN/dalvikvm(536): threadid=1: thread exiting with uncaught exception (group=0x40015560)
09-29 19:14:11.870: ERROR/AndroidRuntime(536): FATAL EXCEPTION: main
09-29 19:14:11.870: ERROR/AndroidRuntime(536): java.lang.NullPointerException
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at nl.siebeh.schoolmate.weekFragment.newInstance(weekFragment.java:49)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at nl.siebeh.schoolmate.fragmentAdapter.getItem(fragmentAdaptar.java:21)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at nl.siebeh.schoolmate.titleFragmentAdapter.getItem(titleFragmentAdapter.java:1)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.support.v4.app.FragmentPagerAdapter.instantiateItem(FragmentPagerAdapter.java:62)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:321)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.support.v4.view.ViewPager.populate(ViewPager.java:441)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.support.v4.view.ViewPager.onAttachedToWindow(ViewPager.java:563)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.view.View.dispatchAttachedToWindow(View.java:6156)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.view.ViewGroup.dispatchAttachedToWindow(ViewGroup.java:1122)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.view.ViewGroup.dispatchAttachedToWindow(ViewGroup.java:1127)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.view.ViewGroup.dispatchAttachedToWindow(ViewGroup.java:1127)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.view.ViewGroup.dispatchAttachedToWindow(ViewGroup.java:1127)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.view.ViewGroup.dispatchAttachedToWindow(ViewGroup.java:1127)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.view.ViewRoot.performTraversals(ViewRoot.java:765)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.view.ViewRoot.handleMessage(ViewRoot.java:1859)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.os.Handler.dispatchMessage(Handler.java:99)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.os.Looper.loop(Looper.java:123)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.app.ActivityThread.main(ActivityThread.java:3683)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at java.lang.reflect.Method.invokeNative(Native Method)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at java.lang.reflect.Method.invoke(Method.java:507)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at dalvik.system.NativeStart.main(Native Method)
09-29 19:14:11.890: WARN/ActivityManager(70):   Force finishing activity nl.siebeh.schoolmate/.overviewWeek
09-29 19:14:12.413: WARN/ActivityManager(70): Activity pause timeout for HistoryRecord{40770188 nl.siebeh.schoolmate/.overviewWeek}
09-29 19:14:21.910: WARN/ActivityManager(70): Launch timeout has expired, giving up wake lock!
09-29 19:14:22.441: WARN/ActivityManager(70): Activity idle timeout for HistoryRecord{4070c7a0 nl.siebeh.schoolmate/.schoolmate}
09-29 19:14:32.451: WARN/ActivityManager(70): Activity destroy timeout for HistoryRecord{40770188 nl.siebeh.schoolmate/.overviewWeek}

【问题讨论】：

    标签： android android-fragments android-viewpager


    【解决方案1】：

    您没有getActivity() 电话，因此很难回答问题。

    您将获得NullPointerException，因为mContext 是null。我的猜测是mContext 是null，因为在fragmentAdapter 的代码中，你调用new weekFragment()，然后调用newInstance()。您大概认为您正在遵循Fragment JavaDocs 中概述的一些模式，但在其中，newInstance() 是一个static 方法，而不是您在此处使用的实例方法。

    从newInstance() 中取出几乎所有代码并将其移动到onActivityCreated() 或片段附加到活动的某个时间，以便getActivity() 工作。在此过程中，删除您的 onAttach() 实现和 mContext 数据成员，这两个都不需要（并且 mContext 以非常可怕的方式填充）。

    【讨论】：

    • 感谢您的快速回复。在 onAttach 中是否应该将 mContext 设置为 getActivity()，它现在是我在另一个站点上找到的。我马上试试看。
    • @Swilzard：“在 onAttach 中，mContext 是否应该设置为 getActivity()”——不，只要在需要 Activity 时调用 getActivity()。
    • @CommonsWare ：在我的情况下，我在单击按钮时启动片段..所以这里假设活动已经创建。但现在如果从片段中调用 getActivity().finish() ......它给出了 NULL POINTER......任何建议。？
    • @Shubh：我的建议是打开一个新的 StackOverflow 问题。
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