feat: computed reactive variables

Creates a reactive variable that is set to the return value of the function.

The value will be updated when any of the reactive variables used in the function
change.

Example:
```solara
import solara
import solara.lab

a = solara.reactive(1)
b = solara.reactive(2)

@solara.lab.computed
def total():
    return a.value + b.value

def reset():
    a.value = 1
    b.value = 2

@solara.component
def Page():
    print(a, b, total)
    solara.IntSlider("a", value=a)
    solara.IntSlider("b", value=b)
    solara.Text(f"a + b = {total.value}")
    solara.Button("reset", on_click=reset)
```

z.value will be lazily executed the first time, and will be updated
when one of the dependencies changes.

solara/toestand.py

@@ -381,22 +381,36 @@ def __set__(self, obj, value):
 
 
 class Computed(Reactive[S]):
+    _storage: KernelStore[S]
+
     def __init__(self, f: Callable[[], S], key=None):
         self.f = f
 
         def on_change(*ignore):
             with self._auto_subscriber.value:
                 self.set(f())
 
-        self._auto_subscriber = Singleton(lambda: AutoSubscribeContextManager(on_change))
+        import functools
+
+        self._auto_subscriber = Singleton(functools.wraps(AutoSubscribeContextManager)(lambda: AutoSubscribeContextManager(on_change)))
 
+        @functools.wraps(f)
         def factory():
             v = self._auto_subscriber.value
             with v:
                 return f()
 
         super().__init__(KernelStoreFactory(factory, key=key))
 
+        # reset on kernel restart (e.g. hot reload)
+        def reset():
+            def cleanup():
+                self._storage.clear()
+
+            return cleanup
+
+        solara.server.kernel_context.on_kernel_start(reset)
+
     def __repr__(self):
         value = super().__repr__()
         return "<Computed" + value[len("<Reactive") : -1]

tests/unit/toestand_computed_reload.py

@@ -0,0 +1,15 @@
+import solara
+import solara.lab
+
+value_reactive = solara.reactive(1.0)
+
+
+@solara.lab.computed
+def computed_reactive():
+    return value_reactive.value + 1.0
+
+
+@solara.component
+def Page():
+    solara.FloatSlider("test", value=value_reactive)
+    solara.InputFloat("test", value=computed_reactive.value)

tests/unit/toestand_test.py

@@ -1,6 +1,7 @@
 import dataclasses
 import threading
 import unittest.mock
+from pathlib import Path
 from typing import Callable, Dict, List, Optional, Set, TypeVar
 
 import ipyvuetify as v
@@ -15,6 +16,8 @@
 
 from .common import click
 
+HERE = Path(__file__).parent
+
 
 @dataclasses.dataclass(frozen=True)
 class Bears:
@@ -1127,3 +1130,41 @@ def conditional_add():
     assert z.value == 42
     assert z._auto_subscriber.value.reactive_used == {x}
     assert calls == 4
+
+
+def test_computed_reload(no_kernel_context):
+    import solara.server.reload
+    from solara.server.app import AppScript
+
+    name = str(HERE / "toestand_computed_reload.py")
+    app = AppScript(name)
+    try:
+        assert len(app.routes) == 1
+        route = app.routes[0]
+        c = app.run()
+        kernel_shared = kernel.Kernel()
+        kernel_context = solara.server.kernel_context.VirtualKernelContext(id="1", kernel=kernel_shared, session_id="session-1")
+        with kernel_context:
+            root = solara.RoutingProvider(children=[c], routes=app.routes, pathname="/")
+            box, rc = solara.render(root, handle_error=False)
+            kernel_context.app_object = rc
+            text = rc.find(v.TextField)
+            assert text.widget.v_model == "2.0"
+            route.module.value_reactive.value = 2  # type: ignore
+            module = route.module
+            assert text.widget.v_model == "3.0"
+            solara.server.reload.reloader._on_change("--unused--")
+            kernel_context.restart()
+        c = app.run()
+        with kernel_context:
+            route = app.routes[0]
+            root = solara.RoutingProvider(children=[c], routes=app.routes, pathname="/")
+            box, rc = solara.render(root, handle_error=False)
+            kernel_context.app_object = rc
+            text = rc.find(v.TextField)
+            assert text.widget.v_model == "3.0"
+            assert route.module is not module
+            route.module.value_reactive.value = 3  # type: ignore
+            assert text.widget.v_model == "4.0"
+    finally:
+        app.close()