從其他類（它擴展廣播接收器）調用MainActivity方法

Question

我想從其他類（PhoneStateReceiver）調用MainActivity的方法display_notification。

但得到這個錯誤：

java.lang.RuntimeException: Unable to start receiver com.weekendproject.roshu.spamblocker.PhoneStateReceiver: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference

錯誤是專門指出，這條線在MainActivity

意向resultIntent =新意圖（getApplicationContext（），MainActivity.class）;

但是我試圖改變getApplicationContext()成"this"和"getActivity()"，但其沒有工作

這裏是完整代碼

MainActivity.java

public class MainActivity extends ActionBarActivity { 

    public void display_notification(String incoming_number) { 

    NotificationCompat.Builder mBuilder = 
      new NotificationCompat.Builder(getApplicationContext()) 
        .setSmallIcon(R.drawable.ic_action) 
        .setContentTitle("SpamBlocker alert !!") 
        .setContentText("SpamBlocker blocked number : " + incoming_number); 

    // **Error occured in following line** 
    Intent resultIntent = new Intent(getApplicationContext(), MainActivity.class); 

    TaskStackBuilder stackBuilder = TaskStackBuilder.create(getApplicationContext()); 
    stackBuilder.addParentStack(MainActivity.class); 
    stackBuilder.addNextIntent(resultIntent); 
    PendingIntent resultPendingIntent = 
      stackBuilder.getPendingIntent(
        0, 
        PendingIntent.FLAG_UPDATE_CURRENT 
      ); 
    mBuilder.setContentIntent(resultPendingIntent); 
    NotificationManager mNotificationManager = 
      (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE); 
    Random r = new Random(); 
    int rand = r.nextInt(1000); 
    mNotificationManager.notify(rand, mBuilder.build()); 
    } 
} 

PhoneStateReceiver。 java

public class PhoneStateReceiver extends BroadcastReceiver { 

    MainActivity mActivity; 

    @Override 
    public void onReceive(Context context, Intent intent) { 
     mActivity = new MainActivity(); 
     mActivity.display_notification(incomingNumber);   // call main activity methods 

    } 
} 

Answer 1

您沒有使用的onReceive提供的上下文。嘗試將該上下文傳遞給display_notification，而不是使用在您接收廣播時可能未運行的活動上下文。

所以更改方法簽名這樣：

public void display_notification(String incoming_number, Context context) 

而且，通過從的onReceive背景是這樣的：

mActivity.display_notification(incomingNumber, context); 

，並使用此背景下，當您將它傳遞給意向像這樣：

Intent resultIntent = new Intent(context, MainActivity.class); 

EDITED

要麼把整個方法接收機，並使用該行這個由的onReceive

OR，

變化提供的上下文：

NotificationManager mNotificationManager = 
     (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE); 

重要組成部分是，context.getSystemService，其中上下文來自onReceive。

Answer 2

使用此：

Intent i = new Intent(); 
i.setClassName("com.test", "com.test.MainActivity"); 
i.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK); 
    context.startActivity(i); 

Answer 3

我認爲這不是一個好主意，新的活動對象，否則生命週期將無法正確維護，並且它的字段可以爲空，這可能導致NPE。
我建議你將你的Notification構建器移動到util類中，詳細代碼可能類似於@dhun的答案。
順便說一句，使這個util類的util方法不是一個好習慣，因爲它會讓你的代碼很難進行單元測試。在Google Testing Blog更多詳細信息。

Answer 4

您正在向後執行您的Activity和BroadcastReceiver。接收者應該是你的活動的一員，反之亦然。此外，你永遠不應該這樣明確地打電話給new MainActivity()。 Android在內部執行此操作，並在調用中包含一些與系統和應用程序相關的信息（如包名！）。試試這個：

MainActivity.java

public class MainActivity extends ActionBarActivity { 
    PhoneStateReceiver mReceiver 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     // create the new receiver, passing MainActivity as an argument 
     mReceiver = new PhoneStateReceiver(this); 

     // initialize the filter. this is like a letterbox, it lets android 
     // know where to send your broadcast 
     IntentFilter filter = new IntentFilter(); 
     filter.addAction("NUMBER_BLOCKED"); // P.O. box NUMBER_BLOCKED 
     registerReceiver(mReceiver, filter); 
    } 

    public void display_notification(String incoming_number) { 

     NotificationCompat.Builder mBuilder = 
       new NotificationCompat.Builder(getApplicationContext()) 
         .setSmallIcon(R.drawable.ic_action) 
         .setContentTitle("SpamBlocker alert !!") 
         .setContentText("SpamBlocker blocked number : " + incoming_number); 

     // **Error occured in following line** 
     Intent resultIntent = new Intent(getApplicationContext(), MainActivity.class); 

     TaskStackBuilder stackBuilder = TaskStackBuilder.create(getApplicationContext()); 
     stackBuilder.addParentStack(MainActivity.class); 
     stackBuilder.addNextIntent(resultIntent); 
     PendingIntent resultPendingIntent = 
       stackBuilder.getPendingIntent(
         0, 
         PendingIntent.FLAG_UPDATE_CURRENT 
       ); 
     mBuilder.setContentIntent(resultPendingIntent); 
     NotificationManager mNotificationManager = 
       (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE); 
     Random r = new Random(); 
     int rand = r.nextInt(1000); 
     mNotificationManager.notify(rand, mBuilder.build()); 
    } 
} 

PhoneStateReceiver.java

public class PhoneStateReceiver extends BroadcastReceiver { 
    MainActivity mActivity; 

    public PhoneStateReceiver(MainActivity activity) { 
     super(); 

     // store the activity from MainActivity.onCreate() 
     mActivity = activity; 
    } 

    @Override 
    public void onReceive(Context context, Intent intent) { 
     // retrieve INCOMING_NUMBER from the broadcast 
     String number = intent.getStringExtra("INCOMING_NUMBER"); 
     mActivity.display_notification(number); 
    } 
} 

而且，我不知道發生了什麼事的app 發送播放，但我感覺到這可能仍然不起作用。如果您仍然收到錯誤，請發佈兩個應用的完整代碼。