Custom Renderer

Dioxus is an incredibly portable framework for UI development. The lessons, knowledge, hooks, and components you acquire over time can always be used for future projects. However, sometimes those projects cannot leverage a supported renderer or you need to implement your own better renderer.

Great news: the design of the renderer is entirely up to you! We provide suggestions and inspiration with the 1st party renderers, but only really require processing Mutations and sending UserEvents.

The specifics:

Implementing the renderer is fairly straightforward. The renderer needs to:

1. Handle the stream of edits generated by updates to the virtual DOM
2. Register listeners and pass events into the virtual DOM's event system

Essentially, your renderer needs to process edits and generate events to update the VirtualDOM. From there, you'll have everything needed to render the VirtualDOM to the screen.

Internally, Dioxus handles the tree relationship, diffing, memory management, and the event system, leaving as little as possible required for renderers to implement themselves.

For reference, check out the javascript interpreter or tui renderer as a starting point for your custom renderer.

Templates

Dioxus is built around the concept of Templates. Templates describe a UI tree known at compile time with dynamic parts filled at runtime. This is useful internally to make skip diffing static nodes, but it is also useful for the renderer to reuse parts of the UI tree. This can be useful for things like a list of items. Each item could contain some static parts and some dynamic parts. The renderer can use the template to create a static part of the UI once, clone it for each element in the list, and then fill in the dynamic parts.

Mutations

The Mutation type is a serialized enum that represents an operation that should be applied to update the UI. The variants roughly follow this set:

enum Mutation {
	AppendChildren,
	AssignId,
	CreatePlaceholder,
	CreateTextNode,
	HydrateText,
	LoadTemplate,
	ReplaceWith,
	ReplacePlaceholder,
	InsertAfter,
	InsertBefore,
	SetAttribute,
	SetText,
	NewEventListener,
	RemoveEventListener,
	Remove,
	PushRoot,
}

The Dioxus diffing mechanism operates as a stack machine where the LoadTemplate, CreatePlaceholder, and CreateTextNode mutations pushes a new "real" DOM node onto the stack and AppendChildren, InsertAfter, InsertBefore, ReplacePlaceholder, and ReplaceWith all remove nodes from the stack.

Node storage

Dioxus saves and loads elements with IDs. Inside the VirtualDOM, this is just tracked as as a u64.

Whenever a CreateElement edit is generated during diffing, Dioxus increments its node counter and assigns that new element its current NodeCount. The RealDom is responsible for remembering this ID and pushing the correct node when id is used in a mutation. Dioxus reclaims the IDs of elements when removed. To stay in sync with Dioxus you can use a sparse Vec (Vec

An Example

For the sake of understanding, let's consider this example – a very simple UI declaration:

rsx! {
	h1 { "count: {x}" }
}

Building Templates

The above rsx will create a template that contains one static h1 tag and a placeholder for a dynamic text node. The template contains the static parts of the UI, and ids for the dynamic parts along with the paths to access them.

The template will look something like this:

Template {
	// Some id that is unique for the entire project
	name: "main.rs:1:1:0",
	// The root nodes of the template
	roots: &[
		TemplateNode::Element {
			tag: "h1",
			namespace: None,
			attrs: &[],
			children: &[
				TemplateNode::DynamicText {
					id: 0
				},
			],
		}
	],
	// the path to each of the dynamic nodes
	node_paths: &[
		// the path to dynamic node with a id of 0
		&[
			// on the first root node
			0,
			// the first child of the root node
			0,
		]
	],
	// the path to each of the dynamic attributes
	attr_paths: &'a [&'a [u8]],
}

For more detailed docs about the structure of templates see the Template api docs

This template will be sent to the renderer in the list of templates supplied with the mutations the first time it is used. Any time the renderer encounters a LoadTemplate mutation after this, it should clone the template and store it in the given id.

For dynamic nodes and dynamic text nodes, a placeholder node should be created and inserted into the UI so that the node can be modified later.

In HTML renderers, this template could look like this:

<h1>""</h1>

Applying Mutations

After the renderer has created all of the new templates, it can begin to process the mutations.

When the renderer starts, it should contain the Root node on the stack and store the Root node with an id of 0. The Root node is the top-level node of the UI. In HTML, this is the <div id="main"> element.

instructions: []
stack: [
	RootNode,
]
nodes: [
	RootNode,
]

The first mutation is a LoadTemplate mutation. This tells the renderer to load a root from the template with the given id. The renderer will then push the root node of the template onto the stack and store it with an id for later. In this case, the root node is an h1 element.

instructions: [
	LoadTemplate {
		// the id of the template
		name: "main.rs:1:1:0",
		// the index of the root node in the template
		index: 0,
		// the id to store
		id: ElementId(1),
	}
]
stack: [
	RootNode,
	<h1>""</h1>,
]
nodes: [
	RootNode,
	<h1>""</h1>,
]

Next, Dioxus will create the dynamic text node. The diff algorithm decides that this node needs to be created, so Dioxus will generate the Mutation HydrateText. When the renderer receives this instruction, it will navigate to the placeholder text node in the template and replace it with the new text.

instructions: [
	LoadTemplate {
		name: "main.rs:1:1:0",
		index: 0,
		id: ElementId(1),
	},
	HydrateText {
		// the id to store the text node
		id: ElementId(2),
		// the text to set
		text: "count: 0",
	}
]
stack: [
	RootNode,
	<h1>"count: 0"</h1>,
]
nodes: [
	RootNode,
	<h1>"count: 0"</h1>,
	"count: 0",
]

Remember, the h1 node is not attached to anything (it is unmounted) so Dioxus needs to generate an Edit that connects the h1 node to the Root. It depends on the situation, but in this case, we use AppendChildren. This pops the text node off the stack, leaving the Root element as the next element on the stack.

instructions: [
	LoadTemplate {
		name: "main.rs:1:1:0",
		index: 0,
		id: ElementId(1),
	},
	HydrateText {
		id: ElementId(2),
		text: "count: 0",
	},
	AppendChildren {
		// the id of the parent node
		id: ElementId(0),
		// the number of nodes to pop off the stack and append
		m: 1
	}
]
stack: [
	RootNode,
]
nodes: [
	RootNode,
	<h1>"count: 0"</h1>,
	"count: 0",
]

Over time, our stack looked like this:

[Root]
[Root, <h1>""</h1>]
[Root, <h1>"count: 0"</h1>]
[Root]

Conveniently, this approach completely separates the Virtual DOM and the Real DOM. Additionally, these edits are serializable, meaning we can even manage UIs across a network connection. This little stack machine and serialized edits make Dioxus independent of platform specifics.

Dioxus is also really fast. Because Dioxus splits the diff and patch phase, it's able to make all the edits to the RealDOM in a very short amount of time (less than a single frame) making rendering very snappy. It also allows Dioxus to cancel large diffing operations if higher priority work comes in while it's diffing.

This little demo serves to show exactly how a Renderer would need to process a mutation stream to build UIs.

Event loop

Like most GUIs, Dioxus relies on an event loop to progress the VirtualDOM. The VirtualDOM itself can produce events as well, so it's important for your custom renderer can handle those too.

The code for the WebSys implementation is straightforward, so we'll add it here to demonstrate how simple an event loop is:

pub async fn run(&mut self) -> dioxus_core::error::Result<()> {
	// Push the body element onto the WebsysDom's stack machine
	let mut websys_dom = crate::new::WebsysDom::new(prepare_websys_dom());
	websys_dom.stack.push(root_node);

	// Rebuild or hydrate the virtualdom
	let mutations = self.internal_dom.rebuild();
	websys_dom.apply_mutations(mutations);

	// Wait for updates from the real dom and progress the virtual dom
	loop {
		let user_input_future = websys_dom.wait_for_event();
		let internal_event_future = self.internal_dom.wait_for_work();

		match select(user_input_future, internal_event_future).await {
			Either::Left((_, _)) => {
				let mutations = self.internal_dom.work_with_deadline(|| false);
				websys_dom.apply_mutations(mutations);
			},
			Either::Right((event, _)) => websys_dom.handle_event(event),
		}

		// render
	}
}

It's important to decode what the real events are for your event system into Dioxus' synthetic event system (synthetic meaning abstracted). This simply means matching your event type and creating a Dioxus UserEvent type. Right now, the virtual event system is modeled almost entirely around the HTML spec, but we are interested in slimming it down.

fn virtual_event_from_websys_event(event: &web_sys::Event) -> VirtualEvent {
	match event.type_().as_str() {
		"keydown" => {
			let event: web_sys::KeyboardEvent = event.clone().dyn_into().unwrap();
			UserEvent::KeyboardEvent(UserEvent {
				scope_id: None,
				priority: EventPriority::Medium,
				name: "keydown",
				// This should be whatever element is focused
				element: Some(ElementId(0)),
				data: Arc::new(KeyboardData{
					char_code: event.char_code(),
					key: event.key(),
					key_code: event.key_code(),
					alt_key: event.alt_key(),
					ctrl_key: event.ctrl_key(),
					meta_key: event.meta_key(),
					shift_key: event.shift_key(),
					location: event.location(),
					repeat: event.repeat(),
					which: event.which(),
				})
			})
		}
		_ => todo!()
	}
}

Custom raw elements

If you need to go as far as relying on custom elements/attributes for your renderer – you totally can. This still enables you to use Dioxus' reactive nature, component system, shared state, and other features, but will ultimately generate different nodes. All attributes and listeners for the HTML and SVG namespace are shuttled through helper structs that essentially compile away. You can drop in your elements any time you want, with little hassle. However, you must be sure your renderer can handle the new namespace.

For more examples and information on how to create custom namespaces, see the dioxus_html crate.

Native Core

If you are creating a renderer in rust, the native-core crate provides some utilities to implement a renderer. It provides an abstraction over Mutations and Templates and contains helpers that can handle the layout and text editing for you.

The RealDom

The RealDom is a higher-level abstraction over updating the Dom. It uses an entity component system to manage the state of nodes. This system allows you to modify insert and modify arbitrary components on nodes. On top of this, the RealDom provides a way to manage a tree of nodes, and the State trait provides a way to automatically add and update these components when the tree is modified. It also provides a way to apply Mutations to the RealDom.

Example

Let's build a toy renderer with borders, size, and text color.

rsx!{
	div{
		color: "red",
		p{
			border: "1px solid black",
			"hello world"
		}
	}
}

In this tree, the color depends on the parent's color. The layout depends on the children's layout, the current text, and the text size. The border depends on only the current node.

In the following diagram arrows represent dataflow:

To help in building a Dom, native-core provides the State trait and a RealDom struct. The State trait provides a way to describe how states in a node depend on other states in its relatives. By describing how to update a single node from its relations, native-core will derive a way to update the states of all nodes for you. Once you have a state you can provide it as a generic to RealDom. RealDom provides all of the methods to interact and update your new dom.

Native Core cannot create all of the required methods for the State trait, but it can derive some of them. To implement the State trait, you must implement the following methods and let the #[partial_derive_state] macro handle the rest:

Lets take a look at how to implement the State trait for a simple renderer.

Now that we have our state, we can put it to use in our RealDom. We can update the RealDom with apply_mutations to update the structure of the dom (adding, removing, and changing properties of nodes) and then update_state to update the States for each of the nodes that changed.

Layout

For most platforms, the layout of the Elements will stay the same. The layout_attributes module provides a way to apply HTML attributes a Taffy layout style.

Text Editing

To make it easier to implement text editing in rust renderers, native-core also contains a renderer-agnostic cursor system. The cursor can handle text editing, selection, and movement with common keyboard shortcuts integrated.

Conclusion

That should be it! You should have nearly all the knowledge required on how to implement your renderer. We're super interested in seeing Dioxus apps brought to custom desktop renderers, mobile renderers, video game UI, and even augmented reality! If you're interested in contributing to any of these projects, don't be afraid to reach out or join the community.