An Empty Component

As mentioned in the introduction, the simplest Asteracea component is E()()[].

In context, and written more like what you'd see in the wild:

fn main() {
asteracea::component! {


(All Asteracea component examples are followed by their output as rendered by lignin-html, but in this case it's an empty string.)

This component expands to the following Rust code, with use imports extracted by hand to improve readability:


As you can see, the component! macro created a struct type, with one constructor called new and one method called render, as well as a few helper types and functions that enable named arguments, and a Drop implementation. The output of component!, as far as you're supposed to touch it, always has this shape. No exceptions.

Identifiers containing __Asteracea__ are considered internal and may change at any point in time. Please don't use them directly, even if technically accessible!

You may find small bits of similar useless syntax like those empty {} blocks in new. Some of these pieces of code nudge Rust into giving you a better error message or block off certain edge cases (usually inner attributes) that either would be confusing to read or haven't been properly evaluated yet, while others, like the empty unsafe {} in drop are slots where code is placed when generating more complex components, and which should be effectively removed by the compiler if empty. (If you notice such an empty construct that impacts runtime performance or Wasm assembly size, please file a bug report.)

The breakdown

There are five distinct pieces of syntax that are translated into the output here: pub, Empty, (), () and [].

pub (visibility)

This is a plain Rust visibility and inserted just before the struct keyword in the macro output above, controlling where the component can be used directly. Leave it out to for current-module-only visibility.

new and render are always declared pub; They inherit their visibility from the component structure.

Empty (component name)

This identifier is inserted verbatim into the output as shown.

There aren't any requirements regarding which identifier to use, but I encourage you to avoid generic suffixes like "…Component".

Consider e.g. "…ListItem", "…Button" or, if nothing more specific applies, "…Panel" as more descriptive alternatives, or leave the suffix off entirefly if there's no confusion regarding which types are components and which are not.

() (constructor argument list)

This is the first pair of parenthese in the input and also appears before the other in the output. As you can see, it is inserted verbatim after new here.

You can use any normal argument declaration here, with the exception of self parameters.

The constructor argument list also supports a shorthand to declare and assign fields on the component instance, but more on that [later].

() (render argument list)

The second pair of parentheses is used to declare additional render arguments.

This one is never pasted verbatim into the resulting component, despite supporting only plain Rust argument declarations (with the exception of self parameters and, usually, bump).

Instead, its items are inserted at the end of render's argument list above, after the implicit arguments &self and bump: &'bump Bump. You can access instance fields through self in the component body (more on that later) and bump is a Bump from bumpalo, a bump allocation arena that makes the VDOM more efficient.

Do not place anything into bump that needs to be dropped! Bump allocators are speedy, but this speed is bought by not running any logic before the memory is reused. Some workarounds for common use cases exist, but for the most part Asteracea handles this for you. See bumpalo's documentation for more information.

[] (body / empty Multi Node)

The location of [] in this example component is called the body of the component.

[] itself is an empty Multi Node, which expands to Node::Multi(&*bump.alloc_with(|| [])).

The contents of this node are placed in the bump allocation arena which, in this case, is effectively no operation. Location and length of this list are stored in the containing [Node], which here is returned directly from render.

It's legal to reuse [Node] instances in multiple places in the VDOM tree. You can also cache long-lived [Node]s and then refer to them across multiple render cycles, to avoid re-rendering part of the VDOM.

Multi Nodes are a VDOM concept that doesn't translate into DOM: Their contents are replicated without nesting in the surrounding DOM structure. You can use them, for example, to return multiple elements at the top level of a component.

Another use is to represent a variable number of elements, including none. The diffing algorithm in lignin-dom advances by a single VDOM sibling when processing a multi node. This means that you can avoid shifting any following sibling nodes, which can avoid expensively recreating their DOM representation or confusing the user by moving their selection to an unexpected place.