Hydrate, splay, dehydrate

Three operations

Operation	From	To
hydrate(kit, b, store, value, parentId?)	A value	An instance tree
splay(kit, b, store, id, options?)	An instance tree	An output of the type T
dehydrate(b, store, id)	An instance tree	The value

The options of splay are all optional:

Option	Effect
mutate, addChild, replace	The callbacks of the edits. Without them, the render is read-only
cache	The memo of the outputs (SplayCache)
view	The view policy: the level of detail of each instance. See level of detail

All three are class methods. The engine has no knowledge of a specific class. It resolves the method of each instance through the extends chain.

The kit

A kit configures one output type:

• classFor: the dispatch from a value to a class name. defaultClassFor uses the JavaScript type. exprClassFor also sends expressions to ExprLit, ExprRef and ExprApp, thus an expression renders as a tree of ops.
• ops: the op registry of the render methods, with the view atoms of the output, for example textView and stack.
• fallbackRender: the render of a class without a render method, and of a failed render.
• frame: an optional wrapper of each collapsible instance, for example a disclosure control.

Render methods are expressions

The render method of Text is this expression:

app("textView", app("get", ref("self", "cells"), lit("value")), ref("self", "setCell"))

Splay evaluates it against the render context of the instance (self). The context has the cells, the children, renderChild, dehydrate, setCell, addChild and replace. It also has the level of detail: detail, level, collapsible and toggle. The view atom textView is the only opaque part. Thus the type graph can show the method as data, and a person can edit it.

When a render method gives none, splay uses fallbackRender with the issues of the evaluation. The viewer shows them, thus an error is visible and not a blank.

New values for a subtree

Two functions give an instance a new value:

• rehydrate(kit, b, store, id, value) keeps the instance and its class. It writes the value cell, and the hydrate method of the class makes new children. A class without a hydrate method keeps its children.
• replaceValue(kit, b, store, id, value) builds a new subtree with the class of the new value, at the same place among the children of the parent. An instance of the same class, or in a typed cell, keeps its ID.

hydrateAs(kit, b, store, className, value, parentId?) hydrates a value with a named class at the root, for example the ClassDef of a class card in the viewer.

The memo

splay takes an optional memo (SplayCache). A hit skips the full subtree. The host owns the invalidation: invalidateSplay(b, cache, changed) deletes the entries of the changed instances and of all their ancestors. The viewer reads changed from the epoch stats. After a change of a class, it deletes the entries of the instances of the class and of its subclasses. An expand or a collapse deletes the entry of its instance.

The standard classes

Class	Cells	Render
Top	(none)	No render. It gives the defaults of splash, flow, deref, targets, dehydrate and summary
Text, Num, Bool	value	textView, numView, boolView
KeyValuePair	width, height	kvp: the key child, then the value child
VStack, HStack	width, height	stack
Grid	cols, width, height	grid
HtmlElement	tag, width, height	stack
ExprLit, ExprRef, ExprApp	value (the expression)	exprLitView, exprRefView, exprAppView