android show notification with a popup on top of any application

If you want use Heads-up Notifications like this:

You must change Notification priority or NotificationChannel importance.

The notification priority, set by setPriority(). The priority determines how intrusive the notification should be on Android 7.1 and lower. (For Android 8.0 and higher, you must instead set the channel importance)

On Android 7.1 (API level 25) and lower:

• Set notification priority to NotificationCompat.PRIORITY_HIGH or NotificationCompat.PRIORITY_MAX.
• Set ringtone and vibrations – you can use setDefaults(Notification.DEFAULT_ALL)

Android 8.0 (API level 26) and higher:

• Set notification channel priority to NotificationManager.IMPORTANCE_HIGH

Notification:

NotificationCompat.Builder builder = new NotificationCompat.Builder(this);
            builder.setSmallIcon(R.drawable.ic_wald_poi)
                    .setLargeIcon(BitmapFactory.decodeResource(getResources(), R.drawable.ic_poi))
                    .setColor(getResources().getColor(R.color.primary))
                    .setContentTitle(getString(R.string.hello))
                    .setContentIntent(notificationPendingIntent)
                    .setContentText(String.format(getString(R.string.notification), viewObject.getTitle()))
                    .setDefaults(NotificationCompat.DEFAULT_ALL)
                    .setStyle(bigText)
                    .setPriority(NotificationCompat.PRIORITY_HIGH) // or NotificationCompat.PRIORITY_MAX

Notification channel:

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
    // Create the NotificationChannel
    val name = getString(R.string.notification_channel_name)
    val descriptionText = getString(R.string.notification_channel_description)
    val importance = NotificationManager.IMPORTANCE_HIGH
    val mChannel = NotificationChannel(NOTIFICATION_CHANNEL_ID, name, importance)
    mChannel.description = descriptionText
    // Register the channel with the system; you can't change the importance
    // or other notification behaviors after this
    val notificationManager = getSystemService(NOTIFICATION_SERVICE) as NotificationManager
    notificationManager.createNotificationChannel(mChannel)
}

Important

If you’d like to further customize your channel’s default notification behaviors, you can call methods such as enableLights(), setLightColor(), and setVibrationPattern() on the NotificationChannel. But remember that once you create the channel, you cannot change these settings and the user has final control of whether these behaviors are active. Other option is to uninstall and install application again.
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Examples of conditions that may trigger heads-up notifications include:

• The user’s activity is in fullscreen mode (the app uses
  fullScreenIntent).
• The notification has high priority and uses
  ringtones or vibrations on devices running Android 7.1 (API level 25)
  and lower.
• The notification channel has high importance on devices
  running Android 8.0 (API level 26) and higher.

Priority:

Notification.PRIORITY_HIGH and Notification.PRIORITY_MAX was deprecated in API level 26. use NotificationCompat instead.

Here is more info 🙂