---
title: 'Properties'
description: 'Parent to child communication'
---

Properties enable child and parent components to communicate with each other.
Every component has an associated properties type which describes what is passed down from the parent.
In theory, this can be any type that implements the `Properties` trait, but in practice, there is no
reason for it to be anything but a struct where each field represents a property.

## Derive macro

Instead of implementing the `Properties` trait yourself, you should use `#[derive(Properties)]` to
automatically generate the implementation instead.
Types for which you derive `Properties` must also implement `PartialEq`.

### Field attributes

When deriving `Properties`, all fields are required by default.
The following attributes allow you to give your props initial values which will be used unless they are set to another value.

:::tip
Attributes aren't visible in Rustdoc generated documentation.
The doc strings of your properties should mention whether a prop is optional and if it has a special default value.
:::

#### `#[prop_or_default]`

Initialize the prop value with the default value of the field's type using the `Default` trait.

#### `#[prop_or(value)]`

Use `value` to initialize the prop value. `value` can be any expression that returns the field's type.
For example, to default a boolean prop to `true`, use the attribute `#[prop_or(true)]`.

#### `#[prop_or_else(function)]`

Call `function` to initialize the prop value. `function` should have the signature `FnMut() -> T` where `T` is the field type.

## `PartialEq`

`Properties` require `PartialEq` to be implemented. This is so that they can be compared by Yew to call the `changed` method
only when they change.

## Memory/speed overhead of using Properties

Internally properties are reference counted. This means that only a pointer is passed down the component tree for props.
It saves us from the cost of having to clone the entire props, which might be expensive.

:::tip
Make use of `AttrValue` which is our custom type for attribute values instead of defining them as String or another similar type.
:::

## Example

```rust
use yew::Properties;
/// Importing the AttrValue from virtual_dom
use yew::virtual_dom::AttrValue;

#[derive(Clone, PartialEq)]
pub enum LinkColor {
    Blue,
    Red,
    Green,
    Black,
    Purple,
}

fn create_default_link_color() -> LinkColor {
    LinkColor::Blue
}

#[derive(Properties, PartialEq)]
pub struct LinkProps {
    /// The link must have a target.
    href: AttrValue,
    /// Also notice that we are using AttrValue instead of String
    text: AttrValue,
    /// Color of the link. Defaults to `Blue`.
    #[prop_or_else(create_default_link_color)]
    color: LinkColor,
    /// The view function will not specify a size if this is None.
    #[prop_or_default]
    size: Option<u32>,
    /// When the view function does not specify active, it defaults to true.
    #[prop_or(true)]
    active: bool,
}
```

## Props macro

The `yew::props!` macro allows you to build properties the same way the `html!` macro does it.

The macro uses the same syntax as a struct expression except that you cannot use attributes or a base expression (`Foo { ..base }`).
The type path can either point to the props directly (`path::to::Props`) or the associated properties of a component (`MyComp::Properties`).

```rust
use yew::{props, Properties, virtual_dom::AttrValue};

#[derive(Clone, PartialEq)]
pub enum LinkColor {
    Blue,
    Red,
    Green,
    Black,
    Purple,
}

fn create_default_link_color() -> LinkColor {
    LinkColor::Blue
}

#[derive(Properties, PartialEq)]
pub struct LinkProps {
    /// The link must have a target.
    href: AttrValue,
    /// Also notice that we're using AttrValue instead of String
    text: AttrValue,
    /// Color of the link. Defaults to `Blue`.
    #[prop_or_else(create_default_link_color)]
    color: LinkColor,
    /// The view function will not specify a size if this is None.
    #[prop_or_default]
    size: Option<u32>,
    /// When the view function doesn't specify active, it defaults to true.
    #[prop_or(true)]
    active: bool,
}

impl LinkProps {
    /// Notice that this function receives href and text as String
    /// We can use `AttrValue::from` to convert it to a `AttrValue`
    pub fn new_link_with_size(href: String, text: String, size: u32) -> Self {
        // highlight-start
        props! {LinkProps {
            href: AttrValue::from(href),
            text: AttrValue::from(text),
            size,
        }}
        // highlight-end
    }
}
```
