server/world: Add a cancellable neighbour update event

server/world/handler.go

@@ -66,6 +66,11 @@ type Handler interface {
 	// HandleRedstoneUpdate handles a redstone update at a position. ctx.Cancel() may be called
 	// to cancel the redstone update.
 	HandleRedstoneUpdate(ctx *Context, pos cube.Pos)
+	// HandleNeighbourUpdate handles a block at pos being updated because of a change to a
+	// neighbouring block at changedNeighbour. It is only called for a block or liquid that
+	// reacts to the change, that is, one implementing NeighbourUpdateTicker. ctx.Cancel() may
+	// be called to prevent both the block and any liquid at the position from being updated.
+	HandleNeighbourUpdate(ctx *Context, pos, changedNeighbour cube.Pos)
 	// HandleClose handles the World being closed. HandleClose may be used as a
 	// moment to finish code running on other goroutines that operates on the
 	// World specifically. HandleClose is called directly before the World stops
@@ -93,4 +98,5 @@ func (NopHandler) HandleEntitySpawn(*Tx, Entity)
 func (NopHandler) HandleEntityDespawn(*Tx, Entity)                                               {}
 func (NopHandler) HandleExplosion(*Context, mgl64.Vec3, *[]Entity, *[]cube.Pos, *float64, *bool) {}
 func (NopHandler) HandleRedstoneUpdate(*Context, cube.Pos)                                       {}
+func (NopHandler) HandleNeighbourUpdate(*Context, cube.Pos, cube.Pos)                            {}
 func (NopHandler) HandleClose(*Tx)                                                               {}

server/world/tick.go

@@ -7,6 +7,7 @@ import (
 	"time"
 
 	"github.com/df-mc/dragonfly/server/block/cube"
+	"github.com/df-mc/dragonfly/server/event"
 	"github.com/df-mc/dragonfly/server/internal/sliceutil"
 )
 
@@ -102,13 +103,21 @@ func (t ticker) performNeighbourUpdates(tx *Tx) {
 
 	for _, update := range updates {
 		pos, changedNeighbour := update.pos, update.neighbour
-		if ticker, ok := tx.Block(pos).(NeighbourUpdateTicker); ok {
-			ticker.NeighbourUpdateTick(pos, changedNeighbour, tx)
+		blockTicker, blockOk := tx.Block(pos).(NeighbourUpdateTicker)
+		liquid, _ := tx.World().additionalLiquid(pos)
+		liquidTicker, liquidOk := liquid.(NeighbourUpdateTicker)
+		if !blockOk && !liquidOk {
+			continue
 		}
-		if liquid, ok := tx.World().additionalLiquid(pos); ok {
-			if ticker, ok := liquid.(NeighbourUpdateTicker); ok {
-				ticker.NeighbourUpdateTick(pos, changedNeighbour, tx)
-			}
+		ctx := event.C(tx)
+		if tx.World().Handler().HandleNeighbourUpdate(ctx, pos, changedNeighbour); ctx.Cancelled() {
+			continue
+		}
+		if blockOk {
+			blockTicker.NeighbourUpdateTick(pos, changedNeighbour, tx)
+		}
+		if liquidOk {
+			liquidTicker.NeighbourUpdateTick(pos, changedNeighbour, tx)
 		}
 	}
 }