Position can emit state changes

gaphas/position.py

@@ -1,10 +1,10 @@
 from __future__ import annotations
 
-from typing import SupportsFloat, Tuple, Union
+from typing import Callable, Set, SupportsFloat, Tuple, Union
 
 from gaphas.matrix import Matrix
 from gaphas.solver import NORMAL, BaseConstraint, Variable
-from gaphas.types import SupportsFloatPos, TypedProperty
+from gaphas.types import Pos, SupportsFloatPos, TypedProperty
 
 
 class Position:
@@ -22,6 +22,32 @@ class Position:
     def __init__(self, x, y, strength=NORMAL):
         self._x = Variable(x, strength)
         self._y = Variable(y, strength)
+        self._handlers: Set[Callable[[Position, Pos], None]] = set()
+        self._setting_pos = 0
+
+    def add_handler(self, handler: Callable[[Position, Pos], None]) -> None:
+        if not self._handlers:
+            self._x.add_handler(self._propagate_x)
+            self._y.add_handler(self._propagate_y)
+        self._handlers.add(handler)
+
+    def remove_handler(self, handler: Callable[[Position, Pos], None]) -> None:
+        self._handlers.discard(handler)
+        if not self._handlers:
+            self._x.remove_handler(self._propagate_x)
+            self._y.remove_handler(self._propagate_y)
+
+    def notify(self, oldpos: Pos) -> None:
+        for handler in self._handlers:
+            handler(self, oldpos)
+
+    def _propagate_x(self, variable, oldval):
+        if not self._setting_pos:
+            self.notify((oldval, self._y.value))
+
+    def _propagate_y(self, variable, oldval):
+        if not self._setting_pos:
+            self.notify((self._x.value, oldval))
 
     @property
     def strength(self) -> int:
@@ -42,7 +68,13 @@ def _set_y(self, v: SupportsFloat) -> None:
 
     def _set_pos(self, pos: Union[Position, SupportsFloatPos]) -> None:
         """Set handle position (Item coordinates)."""
-        self._x.value, self._y.value = pos
+        oldpos = (self._x.value, self._y.value)
+        self._setting_pos += 1
+        try:
+            self._x.value, self._y.value = pos
+        finally:
+            self._setting_pos -= 1
+        self.notify(oldpos)
 
     pos: TypedProperty[Tuple[Variable, Variable], Union[Position, SupportsFloatPos]]
     pos = property(lambda s: (s._x, s._y), _set_pos, doc="The position.")

tests/conftest.py

@@ -85,3 +85,14 @@ def revert_undo(undo_fixture):
     yield
     state.observers.remove(state.revert_handler)
     state.subscribers.remove(undo_fixture[1])
+
+
+@pytest.fixture
+def handler():
+    events = []
+
+    def handler(*args):
+        events.append(args)
+
+    handler.events = events  # type: ignore[attr-defined]
+    return handler

tests/test_position.py

@@ -23,6 +23,39 @@ def test_position_can_compare_with_tuple():
     assert pos < (4, 4)
 
 
+def test_position_notifies_on_x_change(handler):
+    pos = Position(3, 3)
+    pos.add_handler(handler)
+
+    pos.x = 4
+
+    assert handler.events
+    assert handler.events[0][0] is pos
+    assert handler.events[0][1] == (3.0, 3.0)
+
+
+def test_position_notifies_on_y_change(handler):
+    pos = Position(3, 3)
+    pos.add_handler(handler)
+
+    pos.y = 4
+
+    assert handler.events
+    assert handler.events[0][0] is pos
+    assert handler.events[0][1] == (3.0, 3.0)
+
+
+def test_position_notifies_on_pos_change(handler):
+    pos = Position(3, 3)
+    pos.add_handler(handler)
+
+    pos.pos = (4, 4)
+
+    assert len(handler.events) == 1
+    assert handler.events[0][0] is pos
+    assert handler.events[0][1] == (3.0, 3.0)
+
+
 def test_matrix_projection_exposes_variables():
     proj = MatrixProjection(Position(0, 0), Matrix())
 

tests/test_solver_constraint.py

@@ -4,17 +4,6 @@
 from gaphas.solver.constraint import Constraint, ContainsConstraints
 
 
-@pytest.fixture
-def handler():
-    events = []
-
-    def handler(e):
-        events.append(e)
-
-    handler.events = events  # type: ignore[attr-defined]
-    return handler
-
-
 def test_base_constraint_implements_constraint_protocol():
     c = BaseConstraint(Variable())
 
@@ -35,7 +24,7 @@ def test_constraint_propagates_variable_changed(handler):
 
     v.value = 3
 
-    assert handler.events == [c]
+    assert handler.events == [(c,)]
 
 
 def test_multi_constraint(handler):
@@ -46,7 +35,7 @@ def test_multi_constraint(handler):
 
     v.value = 3
 
-    assert handler.events == [c]
+    assert handler.events == [(c,)]
 
 
 def test_default_constraint_can_not_solve():