使用另一个按钮而不是后退按钮

Question

我在 android 中使用后退按钮时遇到问题。我知道我可以使用 onBackPress() 方法来处理它，但我需要一种方法来处理 onResume 生命周期内的后退按钮事件。我必须这样做的原因是我已经从 onResume 方法在另一个 class 中启动了一个计时器。当我按下后退按钮时，这个计时器应该停止。但是当我使用 onBackPress() 方法停止它时，我得到了一个 NullPointerException，因为计时器从未启动。当我使用按钮停止 onResume 方法内的计时器时，计时器会正确停止。所以问题是我可以使用另一个按钮而不是后退按钮吗？或者当用户按下后退按钮时，如何使用 onResume 中的后退按钮事件？

Answer 1

您可能正在将计时器创建为原始 Activity 的变量或子项。当 Activity 暂停（或屏幕外）时，变量可能会被销毁。这就是按钮工作的原因（相同 Activity）而不是 onBackPressed，后者在另一个 Activity.

中调用

您需要在另一个线程上使用静态变量或全局变量运行 - 例如 Service 或在 Application 对象中。 Application 对象是最后的手段，因此请先查看 Service。

Answer 2

尝试在调用 onBackPressed()

之前添加 FLAG_ACTIVITY_CLEAR_TOP

来自 http://developer.android.com/reference/android/content/Intent.html:

If set, and the activity being launched is already running in the current task, then instead of launching a new instance of that activity, all of the other activities on top of it will be closed and this Intent will be delivered to the (now on top) old activity as a new Intent.

Answer 3

不需要做这种额外的工作，如果你需要所有这些东西只是因为计时器 nullpointer 在停止计时器之前写下条件。

让你的计时器变量 public static 然后

if(YouActivity.timer!=null)
   YouActivity.timer.stop();

Answer 4

import android.content.SharedPreferences;
import android.util.Log;
import com.google.firebase.iid.FirebaseInstanceId;
import com.google.firebase.iid.FirebaseInstanceIdService;

public class MyFirebaseInstanceIDService extends FirebaseInstanceIdService
{
    private static final String TAG = MyFirebaseInstanceIDService.class.getSimpleName();

    @Override
    public void onTokenRefresh()
    {
        super.onTokenRefresh();
        String refreshedToken = FirebaseInstanceId.getInstance().getToken();
        storeRegIdInPref(refreshedToken);
        sendRegistrationToServer(refreshedToken);
    }

    private void sendRegistrationToServer(final String token)
    {
        Log.e(TAG, "sendRegistrationToServer: " + token);
    }

    private void storeRegIdInPref(String token) {
        SharedPreferences pref = getApplicationContext().getSharedPreferences(Constant.SHARED_PREF, 0);
        SharedPreferences.Editor editor = pref.edit();
        editor.putString("regId", token);
        editor.commit();
    }
}


import android.app.NotificationChannel;
import android.app.NotificationManager;
import android.app.PendingIntent;
import android.content.Context;
import android.content.Intent;
import android.graphics.BitmapFactory;
import android.graphics.Color;
import android.media.RingtoneManager;
import android.net.Uri;
import android.os.Build;
import android.support.annotation.RequiresApi;
import android.support.v4.app.NotificationCompat;
import android.support.v4.content.LocalBroadcastManager;
import android.util.Log;
import android.widget.Toast;
import com.google.firebase.messaging.FirebaseMessagingService;
import com.google.firebase.messaging.RemoteMessage;
import java.util.Random;

public class MyFirebaseMessagingService extends FirebaseMessagingService {

    public static final String TAG = MyFirebaseMessagingService.class.getSimpleName();
    public NotificationManager notificationManager;
    public static final String ADMIN_CHANNEL_ID = "admin_channel";

    @Override
    public void onCreate() {
        super.onCreate();
        notificationManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
    }

    @Override
    public void onMessageReceived(RemoteMessage remoteMessage)
    {
        Log.e(TAG, "From: " + remoteMessage.getData().toString());
        if (remoteMessage.getData().size() > 0)
        {
            Intent notificationIntent = new Intent(this, DashboardActivity.class);
            if (remoteMessage.getData().get("").equalsIgnoreCase("")) {
                int notificationId = new Random().nextInt(60000);
                FastSave.getInstance().saveString(Constant.FIREBASE_PUSH_MESSAGE_TYPE_FULLSCREEN, remoteMessage.getData().get("type"));
                FastSave.getInstance().saveString(Constant.FIREBASE_PUSH_MESSAGE, remoteMessage.getData().get("message"));
                FastSave.getInstance().saveString(Constant.FIREBASE_NOTIFICATION_TYPE, remoteMessage.getData().get("notification_type"));
                 notificationIntent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
                final PendingIntent pendingIntent = PendingIntent.getActivity(this,
                        notificationId, notificationIntent,
                        PendingIntent.FLAG_ONE_SHOT);
                Uri defaultSoundUri = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION);
                notificationManager =
                        (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);

                if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
                    setupChannels();
                }

                if (!NotificationUtils.isAppIsInBackground(getApplicationContext())) {
                    Intent pushNotification = new Intent(Constant.FIREBASE_PUSH_NOTIFICATION);
                    pushNotification.setAction(Constant.FIREBASE_PUSH_NOTIFICATION);
                    pushNotification.putExtra("notification_type", remoteMessage.getData().get("notification_type"));
                    pushNotification.putExtra("message", remoteMessage.getData().get("message"));
                    pushNotification.putExtra("message_id", remoteMessage.getData().get("message_id"));
                    pushNotification.putExtra("type", remoteMessage.getData().get("type"));
                    LocalBroadcastManager.getInstance(this).sendBroadcast(pushNotification);
                } else {
                    NotificationCompat.Builder notificationBuilder =
                            new NotificationCompat.Builder(this, ADMIN_CHANNEL_ID)
                                    .setLargeIcon(BitmapFactory.decodeResource(getResources(), R.mipmap.ic_sine_peasomy_launcher))
                                    .setSmallIcon()
                                    .setContentTitle("")
                                    .setContentText(remoteMessage.getData().get("message"))
                                    .setAutoCancel(true)
                                    .setSound(defaultSoundUri)
                                    .setContentIntent(pendingIntent);

                    notificationManager.notify(notificationId, notificationBuilder.build());
                }

            }
        }
    }

    @RequiresApi(api = Build.VERSION_CODES.O)
    private void setupChannels() {
        CharSequence adminChannelName = getString(R.string.notifications_admin_channel_name);
        String adminChannelDescription = getString(R.string.notifications_admin_channel_description);
        NotificationChannel adminChannel;
        adminChannel = new NotificationChannel(ADMIN_CHANNEL_ID, adminChannelName, NotificationManager.IMPORTANCE_LOW);
        adminChannel.setDescription(adminChannelDescription);
        adminChannel.enableLights(true);
        adminChannel.setLightColor(Color.RED);
        adminChannel.enableVibration(true);
        if (notificationManager != null) {
            notificationManager.createNotificationChannel(adminChannel);
        }
    }
}


import android.app.ActivityManager;
import android.app.Notification;
import android.app.NotificationManager;
import android.app.PendingIntent;
import android.content.ComponentName;
import android.content.ContentResolver;
import android.content.Context;
import android.content.Intent;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.media.Ringtone;
import android.media.RingtoneManager;
import android.net.Uri;
import android.os.Build;
import android.support.v4.app.NotificationCompat;
import android.text.Html;
import android.text.TextUtils;
import android.util.Patterns;
import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.List;

public class NotificationUtils {

    private static String TAG = NotificationUtils.class.getSimpleName();

    private Context mContext;

    public NotificationUtils(Context mContext) {
        this.mContext = mContext;
    }

    public void showNotificationMessage(String title, String message, String timeStamp, Intent intent) {
        showNotificationMessage(title, message, timeStamp, intent, null);
    }

    public void showNotificationMessage(final String title, final String message, final String timeStamp, Intent intent, String imageUrl) {
        if (TextUtils.isEmpty(message))
            return;


        final int icon ;
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
            icon = R.mipmap.ic_launcher;
        } else {
            icon = R.mipmap.ic_launcher;
        }

        intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP | Intent.FLAG_ACTIVITY_SINGLE_TOP);
        final PendingIntent resultPendingIntent =
                PendingIntent.getActivity(
                        mContext,
                        0,
                        intent,
                        PendingIntent.FLAG_UPDATE_CURRENT
                );

        final NotificationCompat.Builder mBuilder = new NotificationCompat.Builder(
                mContext);

        final Uri alarmSound = Uri.parse(ContentResolver.SCHEME_ANDROID_RESOURCE
                + "://" + mContext.getPackageName() + "/raw/notification");

        if (!TextUtils.isEmpty(imageUrl)) {

            if (imageUrl != null && imageUrl.length() > 4 && Patterns.WEB_URL.matcher(imageUrl).matches()) {

                Bitmap bitmap = getBitmapFromURL(imageUrl);

                if (bitmap != null) {
                    showBigNotification(bitmap, mBuilder, icon, title, message, timeStamp, resultPendingIntent, alarmSound);
                } else {
                    showSmallNotification(mBuilder, icon, title, message, timeStamp, resultPendingIntent, alarmSound);
                }
            }
        } else {
            showSmallNotification(mBuilder, icon, title, message, timeStamp, resultPendingIntent, alarmSound);
            playNotificationSound();
        }
    }


    private void showSmallNotification(NotificationCompat.Builder mBuilder, int icon, String title, String message, String timeStamp, PendingIntent resultPendingIntent, Uri alarmSound) {

        NotificationCompat.InboxStyle inboxStyle = new NotificationCompat.InboxStyle();

        inboxStyle.addLine(message);

        Notification notification;

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
            notification = mBuilder.setSmallIcon(icon).setTicker(title).setWhen(0)
                    .setAutoCancel(true)
                    .setContentTitle(title)
                    .setContentIntent(resultPendingIntent)
                    .setSound(alarmSound)
                    .setStyle(inboxStyle)
                    .setWhen(getTimeMilliSec(timeStamp))
                    .setSmallIcon(R.mipmap.ic_launcher)
                    .setLargeIcon(BitmapFactory.decodeResource(mContext.getResources(), icon))
                    .setContentText(message)
                    .build();
        } else {
            notification = mBuilder.setSmallIcon(icon).setTicker(title).setWhen(0)
                    .setAutoCancel(true)
                    .setContentTitle(title)
                    .setContentIntent(resultPendingIntent)
                    .setSound(alarmSound)
                    .setStyle(inboxStyle)
                    .setWhen(getTimeMilliSec(timeStamp))
                    .setSmallIcon(R.mipmap.ic_launcher)
                    .setLargeIcon(BitmapFactory.decodeResource(mContext.getResources(), icon))
                    .setContentText(message)
                    .build();
        }

        NotificationManager notificationManager = (NotificationManager) mContext.getSystemService(Context.NOTIFICATION_SERVICE);
        notificationManager.notify(Constant.NOTIFICATION_ID, notification);
    }

    private void showBigNotification(Bitmap bitmap, NotificationCompat.Builder mBuilder, int icon, String title, String message, String timeStamp, PendingIntent resultPendingIntent, Uri alarmSound) {
        NotificationCompat.BigPictureStyle bigPictureStyle = new NotificationCompat.BigPictureStyle();
        bigPictureStyle.setBigContentTitle(title);
        bigPictureStyle.setSummaryText(Html.fromHtml(message).toString());
        bigPictureStyle.bigPicture(bitmap);
        Notification notification;

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
            notification = mBuilder.setSmallIcon(icon).setTicker(title).setWhen(0)
                    .setAutoCancel(true)
                    .setContentTitle(title)
                    .setContentIntent(resultPendingIntent)
                    .setSound(alarmSound)
                    .setStyle(bigPictureStyle)
                    .setWhen(getTimeMilliSec(timeStamp))
                    .setSmallIcon(R.mipmap.ic_launcher)
                    .setLargeIcon(BitmapFactory.decodeResource(mContext.getResources(), icon))
                    .setContentText(message)
                    .build();
        } else {
            notification = mBuilder.setSmallIcon(icon).setTicker(title).setWhen(0)
                    .setAutoCancel(true)
                    .setContentTitle(title)
                    .setContentIntent(resultPendingIntent)
                    .setSound(alarmSound)
                    .setStyle(bigPictureStyle)
                    .setWhen(getTimeMilliSec(timeStamp))
                    .setSmallIcon(R.mipmap.ic_launcher)
                    .setLargeIcon(BitmapFactory.decodeResource(mContext.getResources(), icon))
                    .setContentText(message)
                    .build();
        }
        NotificationManager notificationManager = (NotificationManager) mContext.getSystemService(Context.NOTIFICATION_SERVICE);
        notificationManager.notify(Constant.NOTIFICATION_ID_BIG_IMAGE, notification);
    }

    public Bitmap getBitmapFromURL(String strURL) {
        try {
            URL url = new URL(strURL);
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();
            connection.setDoInput(true);
            connection.connect();
            InputStream input = connection.getInputStream();
            Bitmap myBitmap = BitmapFactory.decodeStream(input);
            return myBitmap;
        } catch (IOException e) {
            e.printStackTrace();
            return null;
        }
    }

    public void playNotificationSound() {
        try {
            Uri alarmSound = Uri.parse(ContentResolver.SCHEME_ANDROID_RESOURCE
                    + "://" + mContext.getPackageName() + "/raw/notification");
            Ringtone r = RingtoneManager.getRingtone(mContext, alarmSound);
            r.play();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public static boolean isAppIsInBackground(Context context)
    {
        boolean isInBackground = true;
        ActivityManager am = (ActivityManager) context.getSystemService(Context.ACTIVITY_SERVICE);
        if (Build.VERSION.SDK_INT > Build.VERSION_CODES.KITKAT_WATCH) {
            List<ActivityManager.RunningAppProcessInfo> runningProcesses = am.getRunningAppProcesses();
            for (ActivityManager.RunningAppProcessInfo processInfo : runningProcesses) {
                if (processInfo.importance == ActivityManager.RunningAppProcessInfo.IMPORTANCE_FOREGROUND) {
                    for (String activeProcess : processInfo.pkgList) {
                        if (activeProcess.equals(context.getPackageName())) {
                            isInBackground = false;
                        }
                    }
                }
            }
        } else {
            List<ActivityManager.RunningTaskInfo> taskInfo = am.getRunningTasks(1);
            ComponentName componentInfo = taskInfo.get(0).topActivity;
            if (componentInfo.getPackageName().equals(context.getPackageName())) {
                isInBackground = false;
            }
        }

        return isInBackground;
    }

    public static void clearNotifications(Context context) {
        NotificationManager notificationManager = (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);
        notificationManager.cancelAll();
    }

    public static long getTimeMilliSec(String timeStamp) {
        SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        try {
            Date date = format.parse(timeStamp);
            return date.getTime();
        } catch (ParseException e) {
            e.printStackTrace();
        }
        return 0;
    }
}


    private BroadcastReceiver mReceiveNotification;





    mReceiveNotification = new BroadcastReceiver() {
            @Override
            public void onReceive(Context context, final Intent intent)
            {
                Log.d("TAG", "onReceive:.............final... " + intent.getExtras().toString());
                nearby_site = new Dialog(mContext);

                if (intent.getAction().equals(Constant.FIREBASE_PUSH_NOTIFICATION))
                {
                    if (intent.getStringExtra("notification_type").equals("send_message"))
                    {
                        startActivity(new Intent(mContext, FullscreenNotificationActivity.class));
                    }

                    else if (intent.getStringExtra("notification_type").equals("create_invitation"))
                    {

                        final Dialog  create_invitation  = new Dialog(mContext);
                        create_invitation.requestWindowFeature(Window.FEATURE_NO_TITLE);
                        create_invitation.setContentView(R.layout.message_invitation);
                        create_invitation.getWindow().setBackgroundDrawable(new ColorDrawable(Color.TRANSPARENT));
                        create_invitation.setCancelable(false);
                        TextView txtOkay = create_invitation.findViewById(R.id.txtOkay);
                        TextView txt_message_invite=create_invitation.findViewById(R.id.txt_message_invite);

                        try {
                            create_invitation.show();
                        }
                        catch (Exception e)
                        { 
                    }
                }
            }
        }
    }


    @Override
    protected void onDestroy() {
        super.onDestroy();
        LocalBroadcastManager.getInstance(this).unregisterReceiver(mReceiveNotification);

    }

    @Override
    protected void onPause() {
        super.onPause();
        LocalBroadcastManager.getInstance(this).unregisterReceiver(mReceiveNotification);
    }

    @Override
    protected void onResume() {
        super.onResume();

        LocalBroadcastManager.getInstance(this).registerReceiver(mReceiveNotification,
                new IntentFilter(Constant.FIREBASE_PUSH_NOTIFICATION));}

使用另一个按钮而不是后退按钮

Use another button instead of back button

android

back-button