---
title: "Scope"
description: "Component's Scope"
---

## Component's `Scope<_>` API

The component "`Scope`" is the mechanism through which components are able to create callbacks and update themselves 
using messages. We obtain a reference to this by calling `link()` on the context object passed to the component.

### `send_message`

Sends a message to the component.
Messages are handled by the `update` method which determines whether the component should re-render.

### `send_message_batch`

Sends multiple messages to the component at the same time.
This is similar to `send_message` but if any of the messages cause the `update` method to return `true`,
the component will re-render after all messages in the batch have been processed.

If the given vector is empty, this function doesn't do anything.

### `callback`

Create a callback that will send a message to the component when it is executed.
Under the hood, it will call `send_message` with the message returned by the provided closure.

There is a different method called `callback_once` which accepts a `FnOnce` instead of a `Fn`.
You should use this with care though, as the resulting callback will panic if executed more than once.

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

enum Msg {
    Text(String),
}

struct Comp;

impl Component for Comp {

    type Message = Msg;
    type Properties = ();

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

    fn view(&self, ctx: &Context<Self>) -> Html {
        // Create a callback that accepts some text and sends it 
        // to the component as the `Msg::Text` message variant.
        // highlight-next-line
        let cb = ctx.link().callback(|text: String| Msg::Text(text));
            
        // The previous line is needlessly verbose to make it clearer.
        // It can be simplified it to this:
        // highlight-next-line
        let cb = ctx.link().callback(Msg::Text);
            
        // Will send `Msg::Text("Hello World!")` to the component.
        // highlight-next-line
        cb.emit("Hello World!".to_owned());

        html! {
            // html here
        }
    }
}
```

### `batch_callback`

Create a callback that will send a batch of messages to the component when it is executed.
The difference to `callback` is that the closure passed to this method doesn't have to return a message.
Instead, the closure can return either `Vec<Msg>` or `Option<Msg>` where `Msg` is the component's message type.

`Vec<Msg>` is treated as a batch of messages and uses `send_message_batch` under the hood.

`Option<Msg>` calls `send_message` if it is `Some`. If the value is `None`, nothing happens.
This can be used in cases where, depending on the situation, an update isn't required.

This is achieved using the `SendAsMessage` trait which is only implemented for these types.
You can implement `SendAsMessage` for your own types which allows you to use them in `batch_callback`.

Like `callback`, this method also has a `FnOnce` counterpart, `batch_callback_once`.
The same restrictions apply as for `callback_once`.