# detach.ludic — the structural attach/detach pair and its @OnAttach / @OnDetach
# hooks. `attach P on e` adds a property to a LIVE entity (seeding its fields and
# firing @OnAttach); `detach P on e` removes it (firing @OnDetach, which still
# reads the outgoing value before the has-flag clears). This is the structural
# counterpart to enable/disable — attach/detach create and destroy the property's
# presence, whereas disable/enable only pause it while keeping the data.
#
# Running it prints: 15 1 25 0
#   15   @OnAttach(Shield): amount seeded to 5, prints 5 + 10
#   1    one live Shield now matches the query
#   25   @OnDetach(Shield): reads the outgoing amount 5, prints 5 + 20
#   0    the Shield is gone — nothing matches
#
#   bin/ludic-dev game-build bin/ludicc examples/lang/detach.ludic /tmp/detach
#   /tmp/detach </dev/null
program Detach {
  property Tag    { v: int = 0 }
  property Shield { amount: int = 0 }
  model Unit { Tag }

  @OnAttach(Shield) handler Up   { print(Shield.amount + 10) }   # structural: property born
  @OnDetach(Shield) handler Down { print(Shield.amount + 20) }   # structural: property dies

  handler Seed phase Start { spawn Unit { Tag { v: 1 } } }

  handler Run phase Render {
    for (u) in query [Unit] { attach Shield on self() { amount: 5 } }   # @OnAttach -> 15
    var n = 0
    for (s) in query [Shield] { n += 1 }
    print(n)                                                           # 1

    for (s) in query [Shield] { detach Shield on self() }              # @OnDetach -> 25
    n = 0
    for (s) in query [Shield] { n += 1 }
    print(n)                                                           # 0
    quit()
  }
}