# Subscribe to changes

## Subscribing to collections

You can call the `onSnapshot` method on a collection reference (including a collection reference with queries, orderings, or limits) to be notified when documents within that collection change.

```javascript
const ref =
  adamite()
    .database()
    .collection("proejcts");

ref.onSnapshot((snap) => {
  console.log(snap.docs.length);
});
```

## Subscribing to documents

You can call the `onSnapshot` method on a document reference to be notified when that document changes.

```javascript
const ref =
  adamite()
    .database()
    .collection("proejcts")
    .ref("documentId");

ref.onSnapshot((snap) => {
  console.log(snap.data);
});
```

## Unsubscribing from notifications

On both a collection and document reference, the `onSnapshot` method returns a function which can be called to unsubscribe from updates.

```javascript
const unsubscribe = ref.onSnapshot((snap) => {
  // ...
});

unsubscribe();
```

## Getting detailed info about what changed

When the callback passed to `onSnapshot` is called, is is provided an up to date snapshot of the data. It is also provided detailed information about the change that happened on the document.

```javascript
ref.onSnapshot((snap, change) => {
  // inspect the previous value, will be undefined for creations
  const oldValue = change.oldSnapshot;
  
  // inspect the new value, will be undefined for deletions
  const newValue = change.newSnapshot;
  
  // determine change type (can be "create", "update", or "delete")
  const changeType = change.changeType;
});
```

## Stream changes directly

The `onSnapshot` method fetches an initial snapshot of the data before subscribing and reconstructs snapshots of the data (including the entire collection in some cases) whenever a document changes. 

In most cases, this is fine, but when dealing with large or rapidly updating sets of data, you may want to stream changes directly and handle them more efficiently. You can use the `stream` method on a collection or document reference to do this.

```javascript
const unsubscribe = ref.stream((change) => {
  // inspect the previous value, will be undefined for creations
  const oldValue = change.oldSnapshot;
  
  // inspect the new value, will be undefined for deletions
  const newValue = change.newSnapshot;
  
  // determine change type (can be "create", "update", or "delete")
  const changeType = change.changeType;
});
```