Error handling

A selling point of Rust for web development is the reliability of always knowing where errors can occur and being forced to handle them

However, we haven't talked about error handling at all in this guide! In this chapter, we'll cover some strategies in handling errors to ensure your app never crashes.

The simplest – returning None

Astute observers might have noticed that Element is actually a type alias for Option<VNode>. You don't need to know what a VNode is, but it's important to recognize that we could actually return nothing at all:

fn App() -> Element {
    None
}

This lets us add in some syntactic sugar for operations we think shouldn't fail, but we're still not confident enough to "unwrap" on.

The nature of Option<VNode> might change in the future as the try trait gets upgraded.

fn App() -> Element {
    // immediately return "None"
    let name = use_hook(|| Some("hi"))?;

    todo!()
}

Early return on result

Because Rust can't accept both Options and Results with the existing try infrastructure, you'll need to manually handle Results. This can be done by converting them into Options or by explicitly handling them. If you choose to convert your Result into an Option and bubble it with a ?, keep in mind that if you do hit an error you will lose error information and nothing will be rendered for that component.

fn App() -> Element {
    // Convert Result to Option
    let name: i32 = use_hook(|| "1.234").parse().ok()?;

    // Early return
    let count = use_hook(|| "1.234");
    let val: i32 = match count.parse() {
        Ok(val) => val,
        Err(err) => return rsx! {"Parsing failed"},
    };

    todo!()
}

Notice that while hooks in Dioxus do not like being called in conditionals or loops, they are okay with early returns. Returning an error state early is a completely valid way of handling errors.

Match results

The next "best" way of handling errors in Dioxus is to match on the error locally. This is the most robust way of handling errors, but it doesn't scale to architectures beyond a single component.

To do this, we simply have an error state built into our component:

let mut error = use_signal(|| None);

Whenever we perform an action that generates an error, we'll set that error state. We can then match on the error in a number of ways (early return, return Element, etc).

fn Commandline() -> Element {
    let mut error = use_signal(|| None);

    match error() {
        Some(error) => rsx! { h1 { "An error occurred" } },
        None => rsx! { input { oninput: move |_| error.set(Some("bad thing happened!")) } },
    }
}

Passing error states through components

If you're dealing with a handful of components with minimal nesting, you can just pass the error handle into child components.

fn Commandline() -> Element {
    let error = use_signal(|| None);

    if let Some(error) = error() {
        return rsx! {"An error occurred"};
    }

    rsx! {
        Child { error }
        Child { error }
        Child { error }
        Child { error }
    }
}

#[component]
fn Child(error: Signal<Option<&'static str>>) -> Element {
    rsx! { input { oninput: move |_| error.set(Some("bad thing happened!")) } }
}

Much like before, our child components can manually set the error during their own actions. The advantage to this pattern is that we can easily isolate error states to a few components at a time, making our app more predictable and robust.

Throwing errors

Dioxus provides a much easier way to handle errors: throwing them. Throwing errors combines the best parts of an error state and early return: you can easily throw and error with ?, but you keep information about the error so that you can handle it in a parent component.

You can call throw on any Result type that implements Debug to turn it into an error state and then use ? to return early if you do hit an error. You can capture the error state with an ErrorBoundary component that will render the a different component if an error is thrown in any of its children.

fn Parent() -> Element {
    rsx! {
        ErrorBoundary {
            handle_error: |error| {
                rsx! {
                    "Oops, we encountered an error. Please report {error} to the developer of this application"
                }
            },
            ThrowsError {}
        }
    }
}

fn ThrowsError() -> Element {
    let name: i32 = use_hook(|| "1.234").parse().throw()?;

    todo!()
}

You can even nest ErrorBoundary components to capture errors at different levels of your app.

fn App() -> Element {
    rsx! {
        ErrorBoundary {
            handle_error: |error| {
                rsx! {
                    "Hmm, something went wrong. Please report {error} to the developer of this application"
                }
            },
            Parent {}
        }
    }
}

fn Parent() -> Element {
    rsx! {
        ErrorBoundary {
            handle_error: |error| {
                rsx! {
                    "The child component encountered an error: {error}"
                }
            },
            ThrowsError {}
        }
    }
}

fn ThrowsError() -> Element {
    let name: i32 = use_hook(|| "1.234").parse().throw()?;

    todo!()
}

This pattern is particularly helpful whenever your code generates a non-recoverable error. You can gracefully capture these "global" error states without panicking or handling state for each error yourself.