diff --git a/packages/ludic.base/ecs_hooks.ludic b/packages/ludic.base/ecs_hooks.ludic
new file mode 100644
index 00000000..b79f8656
--- /dev/null
+++ b/packages/ludic.base/ecs_hooks.ludic
@@ -0,0 +1,18 @@
+# ludic.base/ecs_hooks.ludic - observers on a table: told each handle as its row is made, and before
+# it is removed, so what depends on a table (a drawing, a message) follows it without a scan
+# an observer: a function handed an entity's handle (its states are supplied, like a system's)
+export property TbHook {
+  run: fn(int) -> void = null
+}
+
+# f is told every handle added from here on, and every handle about to be removed
+export function tb_on_add<T>(tb: Table<T>, f: fn(int) -> void) -> void {
+  let hk = new TbHook
+  hk.run = f
+  push(tb.on_add, hk)
+}
+export function tb_on_remove<T>(tb: Table<T>, f: fn(int) -> void) -> void {
+  let hk = new TbHook
+  hk.run = f
+  push(tb.on_remove, hk)
+}
diff --git a/packages/ludic.base/ecs_table.ludic b/packages/ludic.base/ecs_table.ludic
index 42cd5b32..83f18337 100644
--- a/packages/ludic.base/ecs_table.ludic
+++ b/packages/ludic.base/ecs_table.ludic
@@ -12,14 +12,18 @@ export property Table<T> {
   i: []words = null           # hot int columns
   added: words = null         # row -> the tick it was added at
   changed: words = null       # row -> the tick a setter last wrote it at
+  blk: words = null           # 64-row block -> the newest tick any row in it was added or written at
   tick: int = 1
   slot_gen: words = null      # slot -> generation
   slot_row: words = null      # slot -> row, -1 while free
   free: words = null
   grids: []Grid = null
   idx: []IntIndex = null
+  on_add: []TbHook = null     # observers: told a handle as its row is made, and before it goes
+  on_remove: []TbHook = null
 }
 
+
 # a table of nf float and ni int columns, every value 0 in a new row
 export function table_new<T>(nf: int, ni: int) -> Table<T> {
   let tb = new Table<T>
@@ -31,11 +35,14 @@ export function table_new<T>(nf: int, ni: int) -> Table<T> {
   for c in 0 .. ni { push(tb.i, words(0)) }
   tb.added = words(0)
   tb.changed = words(0)
+  tb.blk = words(0)
   tb.slot_gen = words(0)
   tb.slot_row = words(0)
   tb.free = words(0)
   tb.grids = new []Grid
   tb.idx = new []IntIndex
+  tb.on_add = new []TbHook
+  tb.on_remove = new []TbHook
   return tb
 }
 
@@ -75,5 +82,15 @@ export function tb_add<T>(tb: Table<T>, r: T) -> int {
   tb.n += 1
   for g in 0 .. len(tb.grids) { grid_grow(tb.grids[g]) }
   for k in 0 .. len(tb.idx) { ix_grow(tb.idx[k]) }
+  tb_stamp(tb, tb.n - 1)
+  for k in 0 .. len(tb.on_add) { tb.on_add[k].run(h) }
   return h
 }
+
+# the row's tick, and its block's: what change detection reads
+function tb_stamp<T>(tb: Table<T>, row: int) -> void {
+  tb.changed[row] = tb.tick
+  let b = row >> 6
+  while len(tb.blk) <= b { push(tb.blk, 0) }
+  tb.blk[b] = tb.tick
+}
diff --git a/packages/ludic.base/ecs_table_remove.ludic b/packages/ludic.base/ecs_table_remove.ludic
index 51b8a592..d7e0ccc7 100644
--- a/packages/ludic.base/ecs_table_remove.ludic
+++ b/packages/ludic.base/ecs_table_remove.ludic
@@ -1,6 +1,8 @@
 # ludic.base/ecs_table_remove.ludic - removing a row moves the last one into its place, so the
 # rows stay dense; every index is told, and the slot's generation moves on
 export function tb_remove<T>(tb: Table<T>, h: int) -> bool {
+  if tb_row(tb, h) < 0 { return false }
+  for k in 0 .. len(tb.on_remove) { tb.on_remove[k].run(h) }
   let r = tb_row(tb, h)
   if r < 0 { return false }
   for g in 0 .. len(tb.grids) { grid_unlink(tb.grids[g], r) }
@@ -23,6 +25,8 @@ function tb_move<T>(tb: Table<T>, from: int, to: int) -> void {
   for c in 0 .. len(tb.f) { tb.f[c][to] = tb.f[c][from] }
   for c in 0 .. len(tb.i) { tb.i[c][to] = tb.i[c][from] }
   tb.slot_row[tb.ent[to] & ECS_SLOT] = to
+  let b = to >> 6
+  if tb.blk[b] < tb.changed[to] { tb.blk[b] = tb.changed[to] }
   for g in 0 .. len(tb.grids) { grid_moved(tb.grids[g], from, to) }
   for k in 0 .. len(tb.idx) { ix_moved(tb.idx[k], from, to) }
 }
diff --git a/packages/ludic.base/ecs_table_set.ludic b/packages/ludic.base/ecs_table_set.ludic
index 33f98f61..8edb8bf8 100644
--- a/packages/ludic.base/ecs_table_set.ludic
+++ b/packages/ludic.base/ecs_table_set.ludic
@@ -5,7 +5,7 @@ export function tb_i<T>(tb: Table<T>, c: int, row: int) -> int { return tb.i[c][
 
 export function tb_set_f<T>(tb: Table<T>, c: int, row: int, v: float) -> void {
   tb.f[c][row] = v
-  tb.changed[row] = tb.tick
+  tb_stamp(tb, row)
   for g in 0 .. len(tb.grids) {
     let gr = tb.grids[g]
     if gr.cx == c or gr.cz == c { tb_grid_refile(tb, gr, row) }
@@ -16,7 +16,7 @@ export function tb_set_f<T>(tb: Table<T>, c: int, row: int, v: float) -> void {
 export function tb_set_xz<T>(tb: Table<T>, cx: int, cz: int, row: int, x: float, z: float) -> void {
   tb.f[cx][row] = x
   tb.f[cz][row] = z
-  tb.changed[row] = tb.tick
+  tb_stamp(tb, row)
   for g in 0 .. len(tb.grids) {
     let gr = tb.grids[g]
     if gr.cx == cx or gr.cz == cz or gr.cx == cz or gr.cz == cx { tb_grid_refile(tb, gr, row) }
@@ -25,7 +25,7 @@ export function tb_set_xz<T>(tb: Table<T>, cx: int, cz: int, row: int, x: float,
 
 export function tb_set_i<T>(tb: Table<T>, c: int, row: int, v: int) -> void {
   tb.i[c][row] = v
-  tb.changed[row] = tb.tick
+  tb_stamp(tb, row)
   for g in 0 .. len(tb.grids) { if tb.grids[g].gate == c { tb_grid_refile(tb, tb.grids[g], row) } }
   for k in 0 .. len(tb.idx) {
     let ix = tb.idx[k]
@@ -35,12 +35,12 @@ export function tb_set_i<T>(tb: Table<T>, c: int, row: int, v: int) -> void {
 
 # a row written without a setter (tb.f / tb.i directly, for speed) is refiled everywhere by this
 export function tb_refile<T>(tb: Table<T>, row: int) -> void {
-  tb.changed[row] = tb.tick
+  tb_stamp(tb, row)
   for g in 0 .. len(tb.grids) { tb_grid_refile(tb, tb.grids[g], row) }
   for k in 0 .. len(tb.idx) { tb_ix_refile(tb, tb.idx[k], row) }
 }
 
-export function tb_mark<T>(tb: Table<T>, row: int) -> void { tb.changed[row] = tb.tick }
+export function tb_mark<T>(tb: Table<T>, row: int) -> void { tb_stamp(tb, row) }
 
 function tb_gate_on<T>(tb: Table<T>, gate: int, row: int) -> bool { return gate < 0 or tb.i[gate][row] != 0 }
 
@@ -55,12 +55,22 @@ function tb_ix_refile<T>(tb: Table<T>, ix: IntIndex, row: int) -> void {
 # change detection: the rows a setter wrote after `since`, into `out` (cleared first); no allocation
 export function tb_changed_since<T>(tb: Table<T>, since: int, out: words) -> int {
   List.clear(out)
-  for r in 0 .. tb.n { if tb.changed[r] > since { push(out, r) } }
+  for b in 0 .. len(tb.blk) {
+    if tb.blk[b] <= since { continue }
+    var end = b * 64 + 64
+    if end > tb.n { end = tb.n }
+    for r in b * 64 .. end { if tb.changed[r] > since { push(out, r) } }
+  }
   return len(out)
 }
 
 export function tb_added_since<T>(tb: Table<T>, since: int, out: words) -> int {
   List.clear(out)
-  for r in 0 .. tb.n { if tb.added[r] > since { push(out, r) } }
+  for b in 0 .. len(tb.blk) {
+    if tb.blk[b] <= since { continue }
+    var end = b * 64 + 64
+    if end > tb.n { end = tb.n }
+    for r in b * 64 .. end { if tb.added[r] > since { push(out, r) } }
+  }
   return len(out)
 }
diff --git a/packages/ludic.base/index.ludic b/packages/ludic.base/index.ludic
index b3a92b9f..0d7aa63f 100644
--- a/packages/ludic.base/index.ludic
+++ b/packages/ludic.base/index.ludic
@@ -9,6 +9,7 @@ import "save.ludic"
 import "save_fields.ludic"
 import "system.ludic"
 import "ecs_table.ludic"
+import "ecs_hooks.ludic"
 import "ecs_table_remove.ludic"
 import "ecs_table_set.ludic"
 import "ecs_grid.ludic"
diff --git a/packages/ludic.base/tests/ecs_test.ludic b/packages/ludic.base/tests/ecs_test.ludic
index 91def3f2..328e0f1f 100644
--- a/packages/ludic.base/tests/ecs_test.ludic
+++ b/packages/ludic.base/tests/ecs_test.ludic
@@ -10,6 +10,17 @@ program EcsTest {
   const KIND: int = 0
   const ON: int = 1
 
+  state HookState {
+    added: int = 0
+    gone: int = 0
+    last: int = -1
+  }
+  function seen_add(hook_st: mut HookState, h: int) -> void {
+    hook_st.added += 1
+    hook_st.last = h
+  }
+  function seen_gone(hook_st: mut HookState, h: int) -> void { hook_st.gone += 1 }
+
   function toy(name: string) -> Toy {
     let t = new Toy
     t.name = name
@@ -108,6 +119,41 @@ program EcsTest {
     expect_eq(imap_get(m, 14, -1), 9)
   }
 
+  function hooks_case(hook_st: HookState) -> void {
+    let tb: Table<Toy> = table_new(2, 2)
+    tb_on_add(tb, fn seen_add)
+    tb_on_remove(tb, fn seen_gone)
+    let a = put(tb, "a", 0.0, 0.0, 1)
+    expect_eq(hook_st.last, a)
+    put(tb, "b", 0.0, 0.0, 1)
+    tb_remove(tb, a)
+    tb_remove(tb, a)
+    expect_eq(hook_st.added, 2)
+    expect_eq(hook_st.gone, 1)
+  }
+
+  # 300 rows over five blocks; a change in one block is found, the untouched blocks skipped
+  function blocks_case() -> void {
+    let tb: Table<Toy> = table_new(2, 2)
+    for i in 0 .. 300 { put(tb, "x", float(i), 0.0, 1) }
+    let seen = tb_tick(tb)
+    tb_advance(tb)
+    let out = words(0)
+    expect_eq(tb_changed_since(tb, seen, out), 0)
+    tb_set_f(tb, CX, 200, 9.0)
+    tb_set_f(tb, CX, 7, 9.0)
+    expect_eq(tb_changed_since(tb, seen, out), 2)
+    expect_eq(out[0], 7)
+    expect_eq(out[1], 200)
+    let before = tb_tick(tb)
+    tb_advance(tb)
+    tb_remove(tb, tb_handle(tb, 3))
+    expect_eq(tb_changed_since(tb, before, out), 0)   # a row moved into a gap was not written
+    expect_eq(tb_changed_since(tb, seen, out), 2)     # a removal is told by tb_on_remove
+  }
+
+  test "observers hear each add, and a remove once" (hook_st: HookState) { hooks_case(hook_st) }
+  test "change detection skips the blocks nobody wrote" () { blocks_case() }
   test "a removed handle misses, rows stay dense, a reused slot is a new handle" () { handles_case() }
   test "the grid answers nearest and within, gated, after moves" () { grid_case() }
   test "the kind index follows the setters, the gate and removals" () { index_case() }