skooma-js/render.md

# Rendering DOM nodes using `render.js`

```js
import {html} from "skooma/render.js"
```

A functional-friendly helper library for procedural DOM generation and templating, with support for reactive state objects.

## Summary

```js
html.button(
	"Click Me!",
	{
		class: "primary",
		click({target}) {
			console.log("User clicked", target)
		}
	},
	button => { console.log("Created", button) }
)
```

* elements as factory functions `content -> element`
* content as arguments or in arrays
* attributes in object arguments
* style, dataset, shadow root, etc. as magic attributes
* events as function attributes
* initialisers as `any -> void` functions

## Interface / Examples

### Basic DOM generation

Accessing the `html` proxy with any string key returns a new node generator function.
When called this function will generate a DOM node (HTML Tag).
The name of the function becomes the tag name of the node.

```js
html.div()
```

Content and attributes can be set via the function arguments:
Strings and DOM nodes are inserted as children, while other objects (except for some special cases) have their key-value pairs turned into attribute-value pairs on the 

```js
html.div("Big Text", {style: "font-size: 1.4em"})
```

Arrays are iterated and their values treated as arguments.
This works both for inserting children and setting attributes.

```js
const content = [" ps: hi", {class: "title"}]
html.h1({id: "main-heading"}, "Heading", content)
```

Function arguments are treated differently depending on their length:]
Functions with **no** named parameters are called, and their return value is then evaluated just like an argument to the generator.

All other functions are (immediately) called with the newly created node as their first and only argument.
These can be used to initialise the new node in a point-free style.

```js
const hello = () => html.bold("Hello, World!")
const init = node => console.log("Initialising", node)
html.div(hello, init)
```

Nested tags can be generated with nested function calls.
When properly formatted, this means simpler templates will have the same structure as if written in HTML (sans those pesky closing tags).

```js
html.div(
    html.p(
        html.b("Bold Text")
    )
)
```

### Attribute Processing

For convenience, arrays assigned as attributes will be joined with spaces:

```js
html.a({class: ["button", "important"]})
```

Assigning a function as an attribute will instead attach it as an event listener:

```js
html.button("Click me!", {click: event => {
    alert("You clicked the button.")
}})
```

The special `style` property can be set to an object and its key/value pairs will be inserted as CSS properties on the element's `style` object.

```js
const style = { color: "salmon" }
html.span("Salmon", { style })
```

The special property `shadowRoot` will attach a shadow-DOM to the element if none is present and append its content to the shadow root.
Arrays are iterated over and their elements appended individually.

```js
html.div({
   shadowRoot = ["Hello, ", html.b("World"), "!"]
})
```

The `dataset` property will add its key/value pairs to the new node's `dataset`,
as a more convenient alternative to setting individual `data-` attributes.

```js
const dataset = { foo: "bar" }
const div = html.div({dataset})
console.log(dataset.foo === div.dataset.foo)
console.log(div.getAttribute("data-foo") === "bar")
```

### Reactivity

Skooma supports reactivity through a simple protocol:

Observable objects identify themselves with the `observable` attribute,
which must return a truthy value.

Observables are expected to expose a `value` attribute that is both readable and writeable,
and to emit a "change" event whenever its vale has changed.

Observables can be passed to skooma's node functions as both
attribute values (values in an object) or
child elements (direct arguments or in an array).

#### Reactive Children

Passing an observable as a child element will attempt to insert its current
value into the new node as if it was passed in directly, but will also hook into
the observable to replace the value when the state changes.

```js
const state = new Observable.value(0)

const button = html.button(state, {
	click(event) { state.value++ }
})
```

**Note** that to keep the replacement logic simple, it is not currently possible
to insert use document fragments, as these could insert several top-level
children into a component that would then all have to be replaced. When an
observable contains or changes to a document fragment, skooma will raise an
error.

When replacing a reactive child, the following events are dispatched:

- `skooma:beforereplace` (cancelable) on the element being replaced while it is
  still in the DOM.
- `skooma:afterreplace` on the old element after it was removed from the DOM and
  replaced with a new one.
- `skooma:replaced` (bubbles) on the new event after it has replaced the old one
  and inserted into the DOM.

Before replacing an element, a `"replace"` event is emitted from the old
element. This event bubbles and is cancelable, and can thus be used both to
completely prevent the replacement according to custom logic, to alter or
initialise the new element before it is inserted, or even to modify the old
object instead of replacing it.

#### Reactive Attributes

Passing an observable as an object value will, likewise, treat its value as the
attribute value, and update it whenever the state's value changes.

```js
const state = new Observable.value(0)

const input_1 = html.input({ type: "number", value: state })
const input_2 = html.input({ type: "number", value: state })
```

When a reactive attribute is updated to a new value, a `skooma:attribute` event
is dispatched on the element. Canceling the event will prevent the attribute
from being changed.

The element has the properties `attribute`, `to` and `from`, representing the
attribute name new and old values.

## Custom Renderers

Skooma uses a class hierarchy to handle the complexity of generating DOM nodes.
The `Renderer` class provides the more generic utility functions to build custom
renderers for targets other than the DOM.

This allows building custom renderers that can work on the back-end by
outputting elements in a virtual dom or even plain HTML.