Control @Query auto-updates

[groue]

By default, @Query automatically updates the database values for the whole duration of the presence of the view in the SwiftUI engine.

You can now spare resources by stopping auto-updates when the view disappears, and restarting them when the view appears. To do so, use the $players.isAutoupdating SwiftUI binding, as in the example below:

import GRDBQuery
import SwiftUI

struct PlayerList: View {
    @Query(PlayerRequest(), in: \.dbQueue)
    var players: [Player]
    
    var body: some View {
        List(players) { player in
            HStack {
                Text(player.name)
                Spacer()
                Text("\(player.score) points")
            }
        }
        // Stop observing the database when the view disappears,
        // and start again when the view appears.
        .mirrorAppearanceState(to: $players.isAutoupdating)
    }
}

This great idea was suggested by @davedelong. Thank you!