---
title: 'Agents'
description: "Yew's Actor System"
---

import useBaseUrl from '@docusaurus/useBaseUrl'
import ThemedImage from '@theme/ThemedImage'

Agents are similar to Angular's [Services](https://angular.io/guide/architecture-services)
\(but without dependency injection\), and provide Yew with an
[Actor Model](https://en.wikipedia.org/wiki/Actor_model). Agents can be used to route messages
between components independently of where they sit in the component hierarchy, or they can be used
to create shared state between different components. Agents can also be used to offload
computationally expensive tasks from the main thread which renders the UI. There is also planned
support for using agents to allow Yew applications to communicate across tabs \(in the future\).

In order for agents to run concurrently, Yew uses
[web-workers](https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Using_web_workers).

## Lifecycle

<ThemedImage
    alt="agent lifecycle diagram"
    sources={{
        light: useBaseUrl('/img/agent-lifecycle-light.svg'),
        dark: useBaseUrl('/img/agent-lifecycle-dark.svg'),
    }}
/>

## Types of Agents

### Reaches

-   Context - There will exist at most one instance of a Context Agent at any given time. Bridges will
    spawn or connect to an already spawned agent on the UI thread. This can be used to coordinate
    state between components or other agents. When no bridges are connected to this agent, the agent
    will disappear.

-   Job - Spawn a new agent on the UI thread for every new bridge. This is good for moving shared but
    independent behavior that communicates with the browser out of components. \(TODO verify\) When
    the task is done, the agent will disappear.

-   Public - Same as Context, but runs on its own web worker.

-   Private - Same as Job, but runs on its own web worker.

-   Global \(WIP\)

## Communication between Agents and Components

### Bridges

A bridge allows bi-directional communication between an agent and a component. Bridges also allow agents to communicate with one another.

A `use_bridge` hook is also provided to create bridges in a function component.

### Dispatchers

A dispatcher allows uni-directional communication between a component and an agent. A dispatcher allows a component to send messages to an agent.

## Overhead

Agents that use web workers \(i.e. Private and Public\) will incur a serialization overhead on the
messages they send and receive. They use [bincode](https://github.com/servo/bincode) to communicate
with other threads, so the cost is substantially higher than just calling a function. Unless the
cost of computation will outweigh the cost of message passing, you should use agents running on the
UI thread \(i.e. Job or Context\).

## Further reading

-   The [pub_sub](https://github.com/yewstack/yew/tree/yew-v0.19.3/examples/pub_sub) example shows how
    components can use agents to communicate with each other.
