How automatic updates work¶

Anvil Reactive watches which values your bindings, computed values, and effects read. When one of those values changes, Reactive updates only the code that used it.

You do not need to maintain a separate list of dependencies.

Reactive watches the values you read¶

Suppose a Form stores a quantity and a note:

from anvil_reactive.main import bind, reactive_dict

self.state = reactive_dict({"quantity": 1, "note": ""})

bind(
    self.quantity_label,
    "text",
    lambda: f"Quantity: {self.state['quantity']}",
)

The binding reads quantity, so changing that key updates the Label. Changing note does not update this binding because the binding did not read it.

The values being watched can change¶

A binding can choose which value to read:

bind(
    self.value_label,
    "text",
    lambda: (
        self.state["primary"] if self.state["use_primary"] else self.state["fallback"]
    ),
)

If use_primary changes, the binding runs again and watches the newly selected value. A value that is no longer used stops triggering that binding.

Several changes are grouped together¶

If an event handler changes several reactive values, Reactive groups the resulting work. Your bindings and effects update when client Python next yields control to the browser, usually after the handler returns. A server call can also yield control before the handler finishes.

Setting a string or number to the value it already contains does not cause an update. If you need changes inside a dictionary or list to trigger updates, use a reactive collection.

Automatic updates run in client code¶

Reactive watches values in Anvil client code. Server code receives normal serialized values and does not become reactive. If an effect waits for a server call, there is an additional rule about which values are watched; see Server calls inside effects.