# Data source

Provides the collection of messages that will be rendered (actually the [[Decorator]] has the last word), and receives callbacks from the view controller to load more messages:

```swift
public protocol ChatDataSourceProtocol: class {
    var hasMoreNext: Bool { get }
    var hasMorePrevious: Bool { get }
    var chatItems: [ChatItemProtocol] { get }
    weak var delegate: ChatDataSourceDelegateProtocol? { get set }

    func loadNext() // Should trigger chatDataSourceDidUpdate with UpdateType.Pagination
    func loadPrevious() // Should trigger chatDataSourceDidUpdate with UpdateType.Pagination
    func adjustNumberOfMessages(preferredMaxCount preferredMaxCount: Int?, focusPosition: Double, completion:(didAdjust: Bool) -> Void) // If you want, implement message count contention for performance, otherwise just call completion(false)
}
```

When there's a change in the collection of messages it must notify the view controller:

```swift
public protocol ChatDataSourceDelegateProtocol: class {
    func chatDataSourceDidUpdate(chatDataSource: ChatDataSourceProtocol, updateType: UpdateType)
}
```

The updateType parameter tells how the update should be performed:

```
public enum UpdateType {
    case normal
    case firstLoad
    case pagination
    case reload
    case messageCountReduction
}
```
As of today, `pagination`, `reload` and `messageCountReduction` behave exactly the same: they will trigger a `reloadData` of the UICollectionView.

`firstLoad` will do a `reloadData` as well, but it will force the layout to be calculated in the main thread. This is useful when presenting the view controller to avoid showing an empty screen for a short time (especially on slow devices)

Lastly, `normal` will trigger a `performBatchUpdates` in the UICollectionView. All the insertions and deletions will be calculated by Chatto in a background thread using the `uid`s of the ChatItemProtocol

Learn more about the [[Update flow]]