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EVENT is a reactive reference to $event — the value produced by the interaction that triggered an action handler. What $event contains depends on which component fired the action:
Component$event value
Input / TextareaCurrent text (string)
SliderCurrent position (number)
Checkbox / SwitchChecked state (boolean)
SelectSelected value (string)
RadioGroupSelected value (string)
Buttonundefined

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Usage

EVENT is most useful when you need to capture an interaction value and store it under a different key, or pass it to a server action: 
from prefab_ui.components import Slider
from prefab_ui.actions import SetState
from prefab_ui.rx import EVENT

Slider(
    name="volume",
    on_change=SetState("last_adjusted", EVENT),
)
For form controls, the component’s own state key (from name) updates automatically. A separate SetState with EVENT is only needed when you want to write the value somewhere else.

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In Templates

In raw template strings, use $event directly: 
from prefab_ui.components import Input
from prefab_ui.actions import ShowToast

Input(
    name="search",
    on_change=ShowToast("Searching for: {{ $event }}"),
)

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Import

from prefab_ui.rx import EVENT