从 Fragment 扩展 Activity 获取上下文答案

【问题标题】：Getting Context from Fragment extended Activity从 Fragment 扩展 Activity 获取上下文
【发布时间】：2015-02-25 19:58:48
【问题描述】：

我有一个“应用程序”，它的选项卡布局包含三个选项卡：
首先，我试图从 SQLite 数据库中检索一些行并将它们显示在 ListView 中。

处理标签布局的类称为PageFragment：

public class PageFragment extends Fragment {
    public static final String ARG_PAGE = "ARG_PAGE";

    private int mPage;
    public MainActivity Main;

    public static PageFragment newInstance(int page) {
        Bundle args = new Bundle();
        args.putInt(ARG_PAGE, page);
        PageFragment fragment = new PageFragment();
        fragment.setArguments(args);
        return fragment;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mPage = getArguments().getInt(ARG_PAGE);
    }

    ListView ItemsLst;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View view = null;
        switch(mPage)
        {
            case 1:
                view = inflater.inflate(R.layout.layout_schedule, container, false);

                DatabaseHandler db = new DatabaseHandler(getActivity().getApplicationContext());

                Medicine med = new Medicine("Cardioaspirina", "Pillola", 1, "2015-02-25", "12:00:00");

                db.AddMedicine(med);

                ScheduleList(view, db.GetAllMedicines());

                return view;
            case 2:
                view = inflater.inflate(R.layout.layout_settings, container, false);
                return view;
            case 3:
                view = inflater.inflate(R.layout.layout_info, container, false);
                return view;
        }
        return view;
    }

    public void ScheduleList(View v, List<Medicine> medicines) {
        ItemsLst = (ListView) v.findViewById(R.id.listview);
        ArrayAdapter<Medicine> adapter = new ArrayAdapter<Medicine>(getActivity(), android.R.layout.simple_list_item_1, medicines);
        ItemsLst.setAdapter(adapter);
    }
}

如您所见，在onCreateView() 方法中，我管理选项卡布局。为了从数据库中检索数据，我创建了一个名为 DatabaseHandler 的类，它具有 AddRecord、GetRecord、GetAllRecords 等方法。问题是DatabaseHandler 构造函数需要一个Context 变量，我尝试使用以下方法为其提供上下文：

DatabaseHandler db = new DatabaseHandler(getActivity().getApplicationContext());

我也尝试只使用getActivity()，但它会抛出 NullPointerException。

如果有用，这里是 DatabaseHandler 构造函数：

public DatabaseHandler(Context context) {
    super(context, DATABASE_NAME, null, DATABASE_VERSION);
}

那么有人知道如何从PageFragment 类中获取应用程序上下文吗？提前谢谢！

附：这是堆栈跟踪：

02-25 21:04:52.588    1947-1947/com.mms.dailypill I/art﹕ Not late-enabling -Xcheck:jni (already on)
02-25 21:04:53.092    1947-1959/com.mms.dailypill I/art﹕ Background sticky concurrent mark sweep GC freed 2943(255KB) AllocSpace objects, 0(0B) LOS objects, 29% free, 796KB/1135KB, paused 30.424ms total 49.618ms
02-25 21:04:53.093    1947-1972/com.mms.dailypill D/OpenGLRenderer﹕ Render dirty regions requested: true
02-25 21:04:53.094    1947-1947/com.mms.dailypill D/﹕ HostConnection::get() New Host Connection established 0xa6840570, tid 1947
02-25 21:04:53.106    1947-1947/com.mms.dailypill D/Atlas﹕ Validating map...
02-25 21:04:53.125    1947-1947/com.mms.dailypill E/SQLiteLog﹕ (1) near "TABLEDailyPill": syntax error
02-25 21:04:53.125    1947-1947/com.mms.dailypill D/AndroidRuntime﹕ Shutting down VM
02-25 21:04:53.183    1947-1947/com.mms.dailypill E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.mms.dailypill, PID: 1947
    android.database.sqlite.SQLiteException: near "TABLEDailyPill": syntax error (code 1): , while compiling: CREATE TABLEDailyPill(id INTEGER AUTOINCREMENT, name VARCHAR(50), format VARCHAR(15), amount INTEGER, exp_dateDATE, time HOUR, PRIMARY KEY(id));
            at android.database.sqlite.SQLiteConnection.nativePrepareStatement(Native Method)
            at android.database.sqlite.SQLiteConnection.acquirePreparedStatement(SQLiteConnection.java:889)
            at android.database.sqlite.SQLiteConnection.prepare(SQLiteConnection.java:500)
            at android.database.sqlite.SQLiteSession.prepare(SQLiteSession.java:588)
            at android.database.sqlite.SQLiteProgram.<init>(SQLiteProgram.java:58)
            at android.database.sqlite.SQLiteStatement.<init>(SQLiteStatement.java:31)
            at android.database.sqlite.SQLiteDatabase.executeSql(SQLiteDatabase.java:1674)
            at android.database.sqlite.SQLiteDatabase.execSQL(SQLiteDatabase.java:1605)
            at com.mms.dailypill.DatabaseHandler.onCreate(DatabaseHandler.java:63)
            at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:251)
            at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:163)
            at com.mms.dailypill.DatabaseHandler.AddMedicine(DatabaseHandler.java:84)
            at com.mms.dailypill.PageFragment.onCreateView(PageFragment.java:48)
            at android.support.v4.app.Fragment.performCreateView(Fragment.java:1786)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:947)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1126)
            at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:739)
            at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1489)
            at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:486)
            at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:141)
            at android.support.v4.view.ViewPager.populate(ViewPager.java:1073)
            at android.support.v4.view.ViewPager.populate(ViewPager.java:919)
            at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1441)
            at android.view.View.measure(View.java:17430)
            at android.widget.LinearLayout.measureVertical(LinearLayout.java:875)
            at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
            at android.view.View.measure(View.java:17430)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
            at android.view.View.measure(View.java:17430)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
            at android.support.v7.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:453)
            at android.view.View.measure(View.java:17430)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
            at android.view.View.measure(View.java:17430)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
            at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436)
            at android.widget.LinearLayout.measureVertical(LinearLayout.java:722)
            at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
            at android.view.View.measure(View.java:17430)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
            at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2560)
            at android.view.View.measure(View.java:17430)
            at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2001)
            at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1166)
            at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1372)
            at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1054)
            at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5779)
            at android.view.Choreographer$CallbackRecord.run(Choreographer.java:767)
            at android.view.Choreographer.doCallbacks(Chor
02-25 21:04:55.795    1947-1947/com.mms.dailypill I/Process﹕ Sending signal. PID: 1947 SIG: 9

【问题讨论】：

上下文似乎不再出现问题......但我刚刚读到错误是由 SQL 给出的。我将数据库类添加到问题中。

标签： android database sqlite android-fragments android-context

【解决方案1】：

首先不要使用getApplicationContext()，原因在此线程中解释：

When to call activity context OR application context?

关于如何在片段中获取上下文创建全局变量的问题：

private Context context;

//onCreate method 
context = this;

在创建视图方法时使用此上下文。

最后一件事是，我认为您的问题不在于上下文，而在于您的数据库创建，因为日志清除状态附近存在问题 “TABLEDailyPill”： 我认为您的查询需要待修复。

【讨论】：

【解决方案2】：

android.database.sqlite.SQLiteException: near "TABLEDailyPill": syntax error (code 1): , while compiling: CREATE TABLEDailyPill(id INTEGER AUTOINCREMENT, name VARCHAR(50), format VARCHAR(15), amount INTEGER, exp_dateDATE, time HOUR, PRIMARY KEY(id));

您在TABLE 和DailyPill 之间缺少一个空格，假设DailyPill 应该是表名。

【讨论】：

我在将 TABLE 名称与 DATABASE 名称以及 TABLE 后面的空格交换时犯了一个巨大的错误，但这不是问题...
@FedericoBelotti：如果您阅读错误消息，很明显TABLE 和DailyPill 之间缺少空格。因此，这肯定是这个堆栈跟踪的“问题”。
我编辑了查询并添加了空格，现在堆栈显示：02-25 21:42:24.721 1895-1895/com.mms.dailypill E/SQLiteLog﹕ (1) no such table: Medicines 02-25 21:42:24.722 1895-1895/com.mms.dailypill E/SQLiteDatabase﹕ Error inserting name=Cardioaspirina amount=1 time=12:00:00 format=Pillola exp_date=2015-02-25 android.database.sqlite.SQLiteException: no such table: Medicines (code 1): , while compiling: INSERT INTO Medicines(name,amount,time,format,exp_date) VALUES (?,?,?,?,?)
@FedericoBelotti：显然，您还没有创建该表。
我认为最好重新投影数据库类，这东西让我发疯！无论如何，非常感谢您有空！