如何使用firebase在通知奥利奥及以上的背景中显示图像

Question

我已经可以使用以下代码在前台显示通知。仅当应用程序在前台运行时才有效。当应用程序在后台运行时，仅默认通知显示，自定义字段不起作用！我使用标题发送标题，使用消息发送消息，使用img_url发送图像。

public class generatePictureStyleNotification extends AsyncTask<String, Void, Bitmap> {

    private Context mContext;
    private String title, message, imageUrl;

    public generatePictureStyleNotification(Context context, String title, String message, String imageUrl) {
        super();
        this.mContext = context;
        this.title = title;
        this.message = message;
        this.imageUrl = imageUrl;
    }

    @Override
    protected Bitmap doInBackground(String... params) {

        InputStream in;
        try {
            URL url = new URL(this.imageUrl);
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();
            connection.setDoInput(true);
            connection.connect();
            in = connection.getInputStream();
            Bitmap myBitmap = BitmapFactory.decodeStream(in);
            return myBitmap;
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }


    protected void onPostExecute(Bitmap result) {
        super.onPostExecute(result);
        intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
        NotificationManager notificationManager =
                (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);

        int notificationId = 1;
        String channelId = "channel-01";
        String channelName = "Channel Name";
        int importance = NotificationManager.IMPORTANCE_HIGH;

        if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.O) {
            NotificationChannel mChannel = new NotificationChannel(
                    channelId, channelName, importance);

            assert notificationManager != null;
            notificationManager.createNotificationChannel(mChannel);
        }

        PendingIntent pendingIntent = PendingIntent.getActivity(mContext, 0, intent,
                PendingIntent.FLAG_UPDATE_CURRENT);
        Uri defaultSoundUri = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION);

        NotificationCompat.Builder mBuilder = new NotificationCompat.Builder(mContext, channelId)
                .setSmallIcon(R.drawable.icon)
                .setContentTitle(title)
                .setContentText(message)
                .setAutoCancel(true)
                .setLargeIcon(result)
                .setSound(defaultSoundUri)
                .setDefaults(Notification.DEFAULT_LIGHTS | Notification.DEFAULT_SOUND)
                .setContentIntent(pendingIntent);

        TaskStackBuilder stackBuilder = TaskStackBuilder.create(mContext);
        stackBuilder.addNextIntent(intent);
        PendingIntent resultPendingIntent = stackBuilder.getPendingIntent(
                0,
                PendingIntent.FLAG_UPDATE_CURRENT
        );
        mBuilder.setContentIntent(resultPendingIntent);

        notificationManager.notify((int) System.currentTimeMillis(), mBuilder.build());


    }
}