yew/website/docs/concepts/components/introduction.mdx

---
title: "Introduction"
description: "Components and their lifecycle hooks"
---

## What are Components?

Components are the building blocks of Yew. They manage their own state and can render themselves to the DOM. Components are created by implementing the `Component` trait for a type. The `Component`
trait has a number of methods which need to be implemented; Yew will call these at different stages
in the lifecycle of a component.

## Lifecycle

:::important contribute
`Contribute to our docs:` [Add a diagram of the component lifecycle](https://github.com/yewstack/yew/issues/1915)
:::

## Lifecycle Methods

### Create

When a component is created, it receives properties from its parent component and is stored within 
the `Context<Self>` thats passed down to the `create` method. The properties can be used to 
initialize the component's state and the "link" can be used to register callbacks or send messages to the component.

```rust
use yew::{Component, Context, html, Html, Properties};

#[derive(PartialEq, Properties)]
pub struct Props;

pub struct MyComponent;

impl Component for MyComponent {
    type Message = ();
    type Properties = Props;

    // highlight-start
    fn create(ctx: &Context<Self>) -> Self {
        MyComponent
    }
    // highlight-end

    fn view(&self, _ctx: &Context<Self>) -> Html {
        html! {
            // impl
        }
    }
}
```

### View

The `view` method allows you to describe how a component should be rendered to the DOM. Writing
HTML-like code using Rust functions can become quite messy, so Yew provides a macro called `html!`
for declaring HTML and SVG nodes (as well as attaching attributes and event listeners to them) and a
convenient way to render child components. The macro is somewhat similar to React's JSX (the
differences in programming language aside).
One difference is that Yew provides a shorthand syntax for properties, similar to Svelte, where instead of writing `onclick={onclick}`, you can just write `{onclick}`.

```rust
use yew::{Component, Context, html, Html, Properties};

enum Msg {
    Click,
}

#[derive(PartialEq, Properties)]
struct Props {
    button_text: String,
}

struct MyComponent;

impl Component for MyComponent {
    type Message = Msg;
    type Properties = Props;

    fn create(_ctx: &Context<Self>) -> Self {
        Self
    }

    // highlight-start
    fn view(&self, ctx: &Context<Self>) -> Html {
        let onclick = ctx.link().callback(|_| Msg::Click);
        html! {
            <button {onclick}>{ &ctx.props().button_text }</button>
        }
    }
    // highlight-end
}
```

For usage details, check out [the `html!` guide](../html/introduction).

### Rendered

The `rendered` component lifecycle method is called once `view` has been called and Yew has rendered
the results to the DOM, but before the browser refreshes the page. This method is useful when you
want to perform actions that can only be completed after the component has rendered elements. There
is also a parameter called `first_render` which can be used to determine whether this function is
being called on the first render, or instead a subsequent one.

```rust
use web_sys::HtmlInputElement;
use yew::{
    Component, Context, html, Html, NodeRef,
};

pub struct MyComponent {
    node_ref: NodeRef,
}

impl Component for MyComponent {
    type Message = ();
    type Properties = ();

    fn create(_ctx: &Context<Self>) -> Self {
        Self {
            node_ref: NodeRef::default(),
        }
    }

    fn view(&self, ctx: &Context<Self>) -> Html {
        html! {
            <input ref={self.node_ref.clone()} type="text" />
        }
    }

    // highlight-start
    fn rendered(&mut self, _ctx: &Context<Self>, first_render: bool) {
        if first_render {
            if let Some(input) = self.node_ref.cast::<HtmlInputElement>() {
                input.focus();
            }
        }
    }
    // highlight-end
}
```

:::tip note
Note that this lifecycle method does not require an implementation and will do nothing by default.
:::

### Update

Communication with components happens primarily through messages which are handled by the
`update` lifecycle method. This allows the component to update itself
based on what the message was, and determine if it needs to re-render itself. Messages can be sent
by event listeners, child components, Agents, Services, or Futures.

Here's an example of what an implementation of `update` could look like:

```rust
use yew::{Component, Context, html, Html};

// highlight-start
pub enum Msg {
    SetInputEnabled(bool)
}
// highlight-end

struct MyComponent {
    input_enabled: bool,
}

impl Component for MyComponent {
    // highlight-next-line
    type Message = Msg;
    type Properties = ();

    fn create(_ctx: &Context<Self>) -> Self {
        Self {
            input_enabled: false,
        }
    }

    // highlight-start
    fn update(&mut self, _ctx: &Context<Self>, msg: Self::Message) -> bool {
        match msg {
            Msg::SetInputEnabled(enabled) => {
                if self.input_enabled != enabled {
                    self.input_enabled = enabled;
                    true // Re-render
                } else {
                    false
                }
            }
        }
    }
    // highlight-end

    fn view(&self, _ctx: &Context<Self>) -> Html {
        html! {
            // impl
        }
    } 

}
```

### Changed

Components may be re-rendered by their parents. When this happens, they could receive new properties
and need to re-render. This design facilitates parent to child component communication by just
changing the values of a property. There is a default implementation which re-renders the component
when props are changed.

### Destroy

After Components are unmounted from the DOM, Yew calls the `destroy` lifecycle method; this is
necessary if you need to undertake operations to clean up after earlier actions of a component
before it is destroyed. This method is optional and does nothing by default.

### Infinite loops

Infinite loops are possible with Yew's lifecycle methods, but are only caused when trying to update
the same component after every render when that update also requests the component to be rendered.

A simple example can be seen below:

```rust
use yew::{Context, Component, Html};

struct Comp;

impl Component for Comp {
    type Message = ();
    type Properties = ();

    fn create(_ctx: &Context<Self>) -> Self {
        Self
    }

    fn update(&mut self, _ctx: &Context<Self>, _msg: Self::Message) -> bool {
        // We are going to always request to re-render on any msg
        true
    }

    fn view(&self, _ctx: &Context<Self>) -> Html {
        // For this example it doesn't matter what is rendered
        Html::default()
    }

    fn rendered(&mut self, ctx: &Context<Self>, _first_render: bool) {
        // Request that the component is updated with this new msg
        ctx.link().send_message(());
    }
}
```

Let's run through what happens here:
1. Component is created using the `create` function.
2. The `view` method is called so Yew knows what to render to the browser DOM.
3. The `rendered` method is called, which schedules an update message using the `Context` link.
4. Yew finishes the post-render phase.
5. Yew checks for scheduled events and sees the update message queue is not empty so works through
the messages.
6. The `update` method is called which returns `true` to indicate something has changed and the
component needs to re-render.
7. Jump back to 2.

You can still schedule updates in the `rendered` method and it's often useful to do so, but
consider how your component will terminate this loop when you do.


## Associated Types

The `Component` trait has two associated types: `Message` and `Properties`.

```rust ,ignore
impl Component for MyComponent {
    type Message = Msg;
    type Properties = Props;

    // ...
}
```

The `Message` type is used to send messages to a component after an event has taken place; for
example you might want to undertake some action when a user clicks a button or scrolls down the
page. Because components tend to have to respond to more than one event, the `Message` type will
normally be an enum, where each variant is an event to be handled.

When organizing your codebase, it is sensible to include the definition of the `Message` type in the
same module in which your component is defined. You may find it helpful to adopt a consistent naming
convention for message types. One option (though not the only one) is to name the types
`ComponentNameMsg`, e.g. if your component was called `Homepage` then you might call the type
`HomepageMsg`.

```rust
enum Msg {
    Click,
    FormInput(String)
}
```

`Properties` represents the information passed to a component from its parent. This type must implement the `Properties` trait \(usually by deriving it\) and can specify whether certain properties are required or optional. This type is used when creating and updating a component. It is common practice to create a struct called `Props` in your component's module and use that as the component's `Properties` type. It is common to shorten "properties" to "props". Since props are handed down from parent components, the root component of your application typically has a `Properties` type of `()`. If you wish to specify properties for your root component, use the `App::mount_with_props` method.

## Context

All component lifecycle methods take a context object. This object provides a reference to component's scope, which
allows sending messages to a component and the props passed to the component.