Dynamic Rendering

Sometimes you want to render different things depending on the state/props. With Dioxus, just describe what you want to see using Rust control flow – the framework will take care of making the necessary changes on the fly if the state or props change!

Conditional Rendering

To render different elements based on a condition, you could use an if-else statement:

if *is_logged_in {
    cx.render(rsx! {
        "Welcome!"
        button {
            onclick: move |_| on_log_out.call(()),
            "Log Out",
        }
    })
} else {
    cx.render(rsx! {
        button {
            onclick: move |_| on_log_in.call(()),
            "Log In",
        }
    })
}

You could also use match statements, or any Rust function to conditionally render different things.

Improving the if-else Example

You may have noticed some repeated code in the if-else example above. Repeating code like this is both bad for maintainability and performance. Dioxus will skip diffing static elements like the button, but when switching between multiple rsx calls it cannot perform this optimization. For this example either approach is fine, but for components with large parts that are reused between conditionals, it can be more of an issue.

We can improve this example by splitting up the dynamic parts and inserting them where they are needed.

cx.render(rsx! {
    // We only render the welcome message if we are logged in
    // You can use if statements in the middle of a render block to conditionally render elements
    if *is_logged_in {
        // Notice the body of this if statement is rsx code, not an expression
        "Welcome!"
    }
    button {
        // depending on the value of `is_logged_in`, we will call a different event handler
        onclick: move |_| if *is_logged_in {
            on_log_in.call(())
        }
        else{
            on_log_out.call(())
        },
        if *is_logged_in {
            // if we are logged in, the button should say "Log Out"
            "Log Out"
        } else {
            // if we are not logged in, the button should say "Log In"
            "Log In"
        }
    }
})

Inspecting Element props

Since Element is a Option<VNode>, components accepting Element as a prop can inspect its contents, and render different things based on that. Example:

fn Clickable<'a>(cx: Scope<'a, ClickableProps<'a>>) -> Element {
    match cx.props.children {
        Some(VNode { dynamic_nodes, .. }) => {
            todo!("render some stuff")
        }
        _ => {
            todo!("render some other stuff")
        }
    }
}

You can't mutate the Element, but if you need a modified version of it, you can construct a new one based on its attributes/children/etc.

Rendering Nothing

To render nothing, you can return None from a component. This is useful if you want to conditionally hide something:

if *is_logged_in {
    return None;
}

cx.render(rsx! {
    a {
        "You must be logged in to comment"
    }
})

This works because the Element type is just an alias for Option<VNode>

Again, you may use a different method to conditionally return None. For example the boolean's then() function could be used.

Rendering Lists

Often, you'll want to render a collection of components. For example, you might want to render a list of all comments on a post.

For this, Dioxus accepts iterators that produce Elements. So we need to:

• Get an iterator over all of our items (e.g., if you have a Vec of comments, iterate over it with iter())
• .map the iterator to convert each item into a LazyNode using rsx!(...)
  • Add a unique key attribute to each iterator item
• Include this iterator in the final RSX (or use it inline)

Example: suppose you have a list of comments you want to render. Then, you can render them like this:

let comment_field = use_state(cx, String::new);
let mut next_id = use_state(cx, || 0);
let comments = use_ref(cx, Vec::<Comment>::new);

let comments_lock = comments.read();
let comments_rendered = comments_lock.iter().map(|comment| {
    rsx!(CommentComponent {
        key: "{comment.id}",
        comment: comment.clone(),
    })
});

cx.render(rsx!(
    form {
        onsubmit: move |_| {
            comments.write().push(Comment {
                content: comment_field.get().clone(),
                id: *next_id.get(),
            });
            next_id += 1;

            comment_field.set(String::new());
        },
        input {
            value: "{comment_field}",
            oninput: move |event| comment_field.set(event.value.clone()),
        }
        input {
            r#type: "submit",
        }
    },
    comments_rendered,
))

Inline for loops

Because of how common it is to render a list of items, Dioxus provides a shorthand for this. Instead of using .iter, .map, and rsx, you can use a for loop with a body of rsx code:

let comment_field = use_state(cx, String::new);
let mut next_id = use_state(cx, || 0);
let comments = use_ref(cx, Vec::<Comment>::new);

cx.render(rsx!(
    form {
        onsubmit: move |_| {
            comments.write().push(Comment {
                content: comment_field.get().clone(),
                id: *next_id.get(),
            });
            next_id += 1;

            comment_field.set(String::new());
        },
        input {
            value: "{comment_field}",
            oninput: move |event| comment_field.set(event.value.clone()),
        }
        input {
            r#type: "submit",
        }
    },
    for comment in &*comments.read() {
        // Notice the body of this for loop is rsx code, not an expression
        CommentComponent {
            key: "{comment.id}",
            comment: comment.clone(),
        }
    }
))

The key Attribute

Every time you re-render your list, Dioxus needs to keep track of which items go where to determine what updates need to be made to the UI.

For example, suppose the CommentComponent had some state – e.g. a field where the user typed in a reply. If the order of comments suddenly changes, Dioxus needs to correctly associate that state with the same comment – otherwise, the user will end up replying to a different comment!

To help Dioxus keep track of list items, we need to associate each item with a unique key. In the example above, we dynamically generated the unique key. In real applications, it's more likely that the key will come from e.g. a database ID. It doesn't matter where you get the key from, as long as it meets the requirements:

• Keys must be unique in a list
• The same item should always get associated with the same key
• Keys should be relatively small (i.e. converting the entire Comment structure to a String would be a pretty bad key) so they can be compared efficiently

You might be tempted to use an item's index in the list as its key. That’s what Dioxus will use if you don’t specify a key at all. This is only acceptable if you can guarantee that the list is constant – i.e., no re-ordering, additions, or deletions.

Note that if you pass the key to a component you've made, it won't receive the key as a prop. It’s only used as a hint by Dioxus itself. If your component needs an ID, you have to pass it as a separate prop.