Ionic Firestore Chat-unable to constantly check for updates - firebase

Question

Currently, I am newbie at Ionic and I am trying to implement a chat service for an app I am working on. I wanted to try and use firestore and this is what I have so far.
I have managed to read and receive messages from firestore but only after I send my message.
In other words, the function to retrieve the messages only activates after I have clicked the send button.
I was wondering if there was a way for me to constantly check firestore for updates, so that the chat can be updated in "real time".
This is the code that I have for receiving the chat messages. I have placed them in the ionviewwilload() to receive all messages when entering the chatroom and when I click send on my messages.
retrieveCollection() : void
{
this._DB.getChatMessages(this._COLL,this._COLL2)
.then((data) =>
{
console.log(data);
// IF we don't have any documents then the collection doesn't exist
// so we create it!
if(data.length === 0)
{
// this.generateCollectionAndDocument();
}
// Otherwise the collection does exist and we assign the returned
// documents to the public property of locations so this can be
// iterated through in the component template
else
{
this.chats = data;
}
})
.catch();
}
The getchatmessages function is then linked to my provider to then retrieve my messages from firestore before returning it as a promise.
getChatMessages(collectionObj: string, collectionObj2) : Promise<any>{
let user : string = firebase.auth().currentUser.uid;
return new Promise((resolve, reject) => {
this._DB.collection(collectionObj).doc(collectionObj2).collection("messages")
.orderBy("sendDate", "asc")
.onSnapshot
((querySnapshot) => {
let obj : any = [];
querySnapshot
.forEach(function(doc) {
console.log(typeof doc);
console.log(doc);
obj.push({
id : doc.id,
message : doc.data().message,
type : doc.data().type,
user :doc.data().user,
image: doc.data().image
});
});
resolve(obj);
})
});
}
So my question is to ask if there is a special method or function that I have missed in the documentation that allows me to constantly check for updates like that in a chat app.
Many thanks to those who reply.