Unable to add window -- token null is not for an application

在MainActivity设置闹钟，

Intent intent=new Intent(MainActivity.this,TimeReceive.class);

PendingIntent sender=PendingIntent.getBroadcast(MainActivity.this, 0, intent, 0);

AlarmManager am=(AlarmManager) getSystemService(ALARM_SERVICE);

am.set(AlarmManager.RTC, calendar.getTimeInMillis(), sender);

String time="你设置的闹钟是####"+hour+":"+minute;

调用,TimeReceive类

public class TimeReceive extends BroadcastReceiver{

@Override

public void onReceive(Context context, Intent arg1) {

AlertDialog.Builder build=new Builder(mainActivity).

setTitle("闹钟提醒");

build.setIcon(R.drawable.ic_launcher);

build.setMessage("对不起 词库找不到相关解释" );

build.setPositiveButton("是" , null );

build.show();

}

错误提示

Unable to add window -- token null is not for an application

将 context进行强转后

MainActivity mainActivity=(MainActivity)context;

提示

android.app.ReceiverRestrictedContext cannot be cast to com.click369.alarmdemo.MainActivity

结论 广播接收者中不能使用弹出提示框