title: Android/Java: how to send notifications to user, even when app is not "actively" being used?
body: <p>I want to be able to send a notification to a user IF something changes.
  For example,  my application is crime-related. So users can submit reports of crimes that have happened in their neighborhoods.</p>
  
  <p>When a new crime is reported, I want to be able to send ALL users in that specific neighbourhood a notification, even if they are not actively using the app.</p>
  
  <p>How can this be done? I'm quite new at this but to my understanding services like Firebase Messaging require you to type out a message manually and select users to send the message to manually. I'm wondering if there's a way this can be done without someone having to manually do work?</p>
  
  <p>Similar to how snapchat/instagram and stuff will send you notifications that someone has sent you a message even when you are not using your phone.</p>
  
  <p>In my case, I just want the same standard notification "New crime in your area" to be displayed...</p>
  
  <p>How can I do this? (Currently for notifications I'm just using Notification Channels), thank you so much!</p>