diff --git a/gaphas/connections.py b/gaphas/connections.py
index d26e1cf..de23976 100644
--- a/gaphas/connections.py
+++ b/gaphas/connections.py
@@ -1,6 +1,8 @@
 """This module contains a connections manager."""
 
-from typing import Callable, Iterator, NamedTuple, Optional
+from __future__ import annotations
+
+from typing import Callable, Iterator, NamedTuple, Optional, Set
 
 from gaphas import table
 from gaphas.connector import Handle, Port
@@ -40,8 +42,27 @@ class Connections:
     def __init__(self, solver: Optional[Solver] = None) -> None:
         self._solver = solver or Solver()
         self._connections: table.Table[Connection] = table.Table(
-            Connection, tuple(range(4))
+            Connection, tuple(range(5))
         )
+        self._handlers: Set[Callable[[Connection], None]] = set()
+
+        self._solver.add_handler(self._on_constraint_solved)
+
+    def add_handler(self, handler):
+        """Add a callback handler.
+
+        Handlers are triggered when a constraint has been solved.
+        """
+        self._handlers.add(handler)
+
+    def remove_handler(self, handler):
+        """Remove a previously assigned handler."""
+        self._handlers.discard(handler)
+
+    def _on_constraint_solved(self, constraint):
+        for cinfo in self._connections.query(constraint=constraint):
+            for handler in self._handlers:
+                handler(cinfo)
 
     @property
     def solver(self) -> Solver:
@@ -107,7 +128,6 @@ class Connections:
         If handle is not None, only the connection for that handle is
         disconnected.
         """
-        # disconnect on canvas level
         for cinfo in list(self._connections.query(item=item, handle=handle)):
             self._disconnect_item(*cinfo)
 
diff --git a/gaphas/solver/solver.py b/gaphas/solver/solver.py
index 937c7f9..0256677 100644
--- a/gaphas/solver/solver.py
+++ b/gaphas/solver/solver.py
@@ -32,7 +32,7 @@ every constraint is being asked to solve itself
 (`constraint.Constraint.solve_for()` method) changing appropriate
 variables to make the constraint valid again.
 """
-from typing import Collection, List, Set
+from typing import Callable, Collection, List, Set
 
 from gaphas.solver.constraint import Constraint
 from gaphas.state import observed, reversible_pair
@@ -49,6 +49,19 @@ class Solver:
         self._constraints: Set[Constraint] = set()
         self._marked_cons: List[Constraint] = []
         self._solving = False
+        self._handlers: Set[Callable[[Constraint], None]] = set()
+
+    def add_handler(self, handler):
+        """Add a callback handler, triggered when a constraint is resolved."""
+        self._handlers.add(handler)
+
+    def remove_handler(self, handler):
+        """Remove a previously assigned handler."""
+        self._handlers.discard(handler)
+
+    def _notify(self, constraint):
+        for handler in self._handlers:
+            handler(constraint)
 
     @property
     def constraints(self) -> Collection[Constraint]:
@@ -154,6 +167,7 @@ class Solver:
         10.0
         """
         marked_cons = self._marked_cons
+        notify = self._notify
         try:
             self._solving = True
 
@@ -164,6 +178,7 @@ class Solver:
             while n < len(marked_cons):
                 c = marked_cons[n]
                 c.solve()
+                notify(c)
                 n += 1
 
             self._marked_cons = []
diff --git a/tests/test_connections.py b/tests/test_connections.py
index 617cb33..103af3f 100644
--- a/tests/test_connections.py
+++ b/tests/test_connections.py
@@ -66,3 +66,38 @@ def test_remove_item_constraint_when_item_is_disconnected(connections):
     connections.disconnect_item(i)
 
     assert list(connections.get_connections(item=i)) == []
+
+
+def test_notify_on_constraint_solved(connections):
+    events = []
+
+    def on_notify(cinfo):
+        events.append(cinfo)
+
+    i = item.Line(connections)
+    c = EqualsConstraint(i.handles()[0].pos.x, i.handles()[0].pos.x)
+    connections.add_constraint(i, c)
+
+    connections.add_handler(on_notify)
+    connections.solve()
+
+    assert events
+    assert events[0].constraint is c
+
+
+def test_connection_remove_handler(connections):
+    events = []
+
+    def on_notify(cinfo):
+        events.append(cinfo)
+
+    i = item.Line(connections)
+    c = EqualsConstraint(i.handles()[0].pos.x, i.handles()[0].pos.x)
+    connections.add_constraint(i, c)
+
+    connections.add_handler(on_notify)
+    connections.remove_handler(on_notify)
+    connections.remove_handler(on_notify)
+    connections.solve()
+
+    assert not events