android - 从广播接收器调用电话时出现上下文错误

Question

我的应用检查 onResume 中的日期，以查看是否需要调用 MainActivity 中名为 changeReminder 的方法。也就是说，如果日期与存储的日期不同，则调用 changeReminder(也从其他方法调用 changeReminder，例如刷新按钮)。 changeReminder 依次调用 Reminder 类，该类将在创建实例时创建新提醒。

现在，我要向应用程序添加闹钟、广播接收器和通知。因为更改提醒是从 MainActivity 调用的，所以我在 BroadcastReceiver 类中创建了该 Activity 的一个实例，但是因为提醒类需要上下文(它始终来自 MainActivity)，所以它会引发错误。在这种情况下，有没有办法从广播接收器成功传递上下文？我想我在下面包含了相关代码，但如果需要我可以添加更多...

触发警报后的日志:

05-02 13:23:00.712  15511-15511/com.mycompany.dudesmyreminders I/System.out﹕ onReceive
05-02 13:23:00.722  15511-15511/com.mycompany.dudesmyreminders I/System.out﹕ start changeReminder
05-02 13:23:00.722  15511-15511/com.mycompany.dudesmyreminders I/System.out﹕ start Reminder
05-02 13:23:00.752  15511-15511/com.mycompany.dudesmyreminders E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.mycompany.dudesmyreminders, PID: 15511
    java.lang.RuntimeException: Unable to start receiver com.mycompany.dudesmyreminders.ReminderReceiver: java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.sqlite.SQLiteDatabase android.content.Context.openOrCreateDatabase(java.lang.String, int, android.database.sqlite.SQLiteDatabase$CursorFactory, android.database.DatabaseErrorHandler)' on a null object reference
            at android.app.ActivityThread.handleReceiver(ActivityThread.java:2726)
            at android.app.ActivityThread.access$1700(ActivityThread.java:144)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1449)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:155)
            at android.app.ActivityThread.main(ActivityThread.java:5696)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1028)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:823)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.sqlite.SQLiteDatabase android.content.Context.openOrCreateDatabase(java.lang.String, int, android.database.sqlite.SQLiteDatabase$CursorFactory, android.database.DatabaseErrorHandler)' on a null object reference
            at android.content.ContextWrapper.openOrCreateDatabase(ContextWrapper.java:267)
            at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:223)
            at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:163)
            at com.mycompany.dudesmyreminders.dbDataSource.open(dbDataSource.java:39)
            at com.mycompany.dudesmyreminders.Reminder.<init>(Reminder.java:37)
            at com.mycompany.dudesmyreminders.MainActivity.changeReminder(MainActivity.java:283)
            at com.mycompany.dudesmyreminders.ReminderReceiver.onReceive(ReminderReceiver.java:23)
            at android.app.ActivityThread.handleReceiver(ActivityThread.java:2712)
            at android.app.ActivityThread.access$1700(ActivityThread.java:144)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1449)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:155)
            at android.app.ActivityThread.main(ActivityThread.java:5696)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1028)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:823)

MainActivity onCreate 中的警报管理器:

//Alarm manager for notifications....
Calendar calendar = Calendar.getInstance();
calendar.set(Calendar.HOUR_OF_DAY, Calendar.getInstance().get(Calendar.HOUR_OF_DAY));
System.out.println("Hour = " + Calendar.getInstance().get(Calendar.HOUR_OF_DAY));
calendar.set(Calendar.MINUTE, Calendar.getInstance().get(Calendar.MINUTE) + 2);
System.out.println("Minute = " + Calendar.getInstance().get(Calendar.MINUTE));
calendar.set(Calendar.SECOND, 0);
Intent intent1 = new Intent(MainActivity.this, ReminderReceiver.class);
PendingIntent pendingIntent = PendingIntent.getBroadcast(MainActivity.this, 0,intent1, PendingIntent.FLAG_UPDATE_CURRENT);
AlarmManager am = (AlarmManager) MainActivity.this.getSystemService(MainActivity.ALARM_SERVICE);
am.setRepeating(AlarmManager.RTC_WAKEUP, calendar.getTimeInMillis(), AlarmManager.INTERVAL_DAY, pendingIntent);

广播接收器:

public class ReminderReceiver extends BroadcastReceiver {


    @Override
    public void onReceive(Context context, Intent intent) {
        // TODO: This method is called when the BroadcastReceiver is receiving
        // an Intent broadcast.
        //throw new UnsupportedOperationException("Not yet implemented");
        System.out.println("onReceive");

        MainActivity ma = new MainActivity();
        ma.changeReminder(context);
        String message = ma.getStoredPreference("CurrentMessage");

        NewReminderNotification.notify(context,message,1);


    }
}

主 Activity 中的 changeReminder:

public void changeReminder(Context context) {
    System.out.println("start changeReminder");

    Reminder reminder = new Reminder(this);

    // Save the message to the Shared Preferences file
    int altitude = getStoredPreferenceInt(SEEKBAR_CURRENT_INT);
    savePreference("CurrentMessage", reminder.determineReminder(altitude));
    savePreferenceInt(DBDATA_ID, (int) reminder.dbData.getId());
    //populates the reminder dbData for current session
    //DBDATA_ID is only changed in the line above
    dbDataCurrentReminder = datasource.getDB_Row_FromID(dbRTable, (long) getStoredPreferenceInt(DBDATA_ID));

    //display the message just saved
    DisplaySavedMessage();

    // save the current date to Preferences
    saveDate();
    setToggles(0);

}

提醒构造函数:

public Reminder(Context context) {
    System.out.println("start Reminder");
    ds = new dbDataSource(context);
    ds.open();
    DatabaseVersion = ds.DatabaseVersion;

}

Answer 1

您的代码存在一些问题。

MainActivity ma = new MainActivity();

首先，绝不自己创建Activity、Service 或ContentProvider 的实例。这些只能由框架实例化，而不是你。这就是您的实例不起作用的原因。

其次，不要在主应用程序线程上执行磁盘 I/O 或网络 I/O，因为在 I/O 进行时您的 UI 将被卡住。 BroadcastReceiver 的 onReceive() 在主应用程序线程上被调用，而您正在尝试(徒劳地)在该线程上执行数据库 I/O。

Is there a way to successfully pass the context from the Broadcast Receiver in this case?

欢迎您将该方法设为静态方法。

但是，更好的解决方案是将此代码移动到 IntentService 中。首先，它为您提供了一个安全的、受管理的后台线程来完成这项工作。其次，它与您应该使用的 WakefulBroadcastReceiver 一起工作，以确保设备可以保持唤醒状态足够长的时间，以便您执行此数据库 I/O。始终将 WakefulBroadcastReceiver、我的 WakefulIntentService 或您自己的 WakeLock 与 RTC_WAKEUP 或 ELAPSED_REALTIME_WAKEUP 一起使用警报。