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State

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Overview

State allows us to create interactive apps that can respond to user input. It defines the variables that can change over time, and the functions that can modify them.

You can define state by creating a class that inherits from rx.State:

import reflex as rx


class State(rx.State):
    """Define your app state here."""

A state class is made up of two parts: vars and event handlers.

Vars are variables in your app that can change over time.

Event handlers are functions that modify these vars in response to events.

These are the main concepts to understand how state works in Reflex:

Base Var

  • Any variable in your app that can change over time.
  • Defined as a field in a State class
  • Can only be modified by event handlers.

Computed Var

  • Vars that change automatically based on other vars.
  • Defined as functions using the @rx.var decorator.
  • Cannot be set by event handlers, are always recomputed when the state changes.

Event Trigger

  • A user interaction that triggers an event, such as a button click.
  • Defined as special component props, such as on_click.
  • Can be used to trigger event handlers.

Event Handlers

  • Functions that update the state in response to events.
  • Defined as methods in the State class.
  • Can be called by event triggers, or by other event handlers.

Here is a example of how to use state within a Reflex app. Click the text to change its color.

Welcome to Reflex!

class ExampleState(rx.State):
    # A base var for the list of colors to cycle through.
    colors: list[str] = [
        "black",
        "red",
        "green",
        "blue",
        "purple",
    ]

    # A base var for the index of the current color.
    index: int = 0

    def next_color(self):
        """An event handler to go to the next color."""
        # Event handlers can modify the base vars.
        # Here we reference the base vars `colors` and `index`.
        self.index = (self.index + 1) % len(self.colors)

    @rx.var
    def color(self) -> str:
        """A computed var that returns the current color."""
        # Computed vars update automatically when the state changes.
        return self.colors[self.index]


def index():
    return rx.heading(
        "Welcome to Reflex!",
        # Event handlers can be bound to event triggers.
        on_click=ExampleState.next_color,
        # State vars can be bound to component props.
        color=ExampleState.color,
        _hover={"cursor": "pointer"},
    )

The base vars are colors and index. They are the only vars in the app that may be directly modified within event handlers.

There is a single computed var, color, that is a function of the base vars. It will be computed automatically whenever the base vars change.

The heading component links its on_click event to the ExampleState.next_color event handler, which increments the color index.

Each user who opens your app has a unique ID and their own copy of the state. This means that each user can interact with the app and modify the state independently of other users.

Because Reflex internally creates a new instance of the state for each user, your code should never directly initialize a state class.

Try opening an app in multiple tabs to see how the state changes independently.

All user state is stored on the server, and all event handlers are executed on the server. Reflex uses websockets to send events to the server, and to send state updates back to the client.

Similar to backend vars, any method defined in a State class that begins with an underscore _ is considered a helper method. Such methods are not usable as event triggers, but may be called from other event handler methods within the state.

Functionality that should only be available on the backend, such as an authenticated action, should use helper methods to ensure it is not accidentally or maliciously triggered by the client.

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