DatabaseRegionObservation

DatabaseRegionObservation tracks changes in the results of database requests, and notifies each database transaction whenever the database changes.

For example:

let observation = DatabaseRegionObservation(tracking: Player.all)
let cancellable = try observation.start(
    in: dbQueue,
    onError: { error in ... },
    onChange: { (db: Database) in
        print("A modification of the player table has just been committed.")
    })


                    
                    
                    init(tracking:)

Creates a DatabaseRegionObservation which observes regions, and notifies whenever one of the observed regions is modified by a database transaction.

For example, this sample code counts the number of a times the player table is modified:

let observation = DatabaseRegionObservation(tracking: Player.all())

var count = 0
let cancellable = observation.start(
    in: dbQueue,
    onError: { error in ... },
    onChange: { _ in
        count += 1
        print("Players have been modified \(count) times.")
    }à

The observation lasts until the cancellable returned by start is cancelled or deallocated.

start(in:onError:onChange:)
Starts the observation in the provided database writer (such as a database queue or database pool), and returns a transaction observer.

For example:
```
let observation = DatabaseRegionObservation.tracking(Player.all())
let cancellable = observation.start(
    in: dbQueue,
    onError: { error in ... },
    onChange: { (db: Database) in
        print("A modification of the player table has just been committed.")
    })
```
If the start method is called from a writing database access method, the observation of impactful transactions starts immediately. Otherwise, it blocks the current thread until a write access can be established.