Reactive Apps

      
constructor(initialState) {
  // Deep-clone the initial state, cache proxy identity,
  // and expose the proxied tree.
  this.raw = deepClone(initialState);
  this.proxyCache = new WeakMap();
  this.state = this.createProxy(this.raw, []);
}

createProxy(target, path) {
  if (!isObject(target)) {
    return target;
  }
  
  if (this.proxyCache.has(target)) {
    return this.proxyCache.get(target);
  }
  
  // Proxy adds behavior, while Reflect keeps object
  // semantics predictable.
  // Proxy is the watcher: intercept reads and writes
  // so the store can react.
  const proxy = new Proxy(target, {
    get: (raw, key, receiver) => {
      if (key === "__raw") return raw;
      if (key === "__path") return path;
      
      // Reflect performs the real native read after
      // the trap inspects the access.
      const value = Reflect.get(raw, key, receiver);
      
      if (isObject(value)) {
        return this.createProxy(value, [...path, key]);
      }
      
      return value;
    },
    set: (raw, key, value, receiver) => {
      const nextPath = [...path, key];
      const oldValue = raw[key];
      const prepared = clonePlainValue(value);
      
      // Reflect performs the real native write after
      // the store prepares the value.
      const result = Reflect.set(
        raw, 
        key, 
        prepared, 
        receiver
      );
      
      if (oldValue !== prepared) {
        this.emitChange(nextPath, oldValue, prepared);
      }
      
      return result;
    },
    deleteProperty: (raw, key) => {
      const oldValue = raw[key];
      const result = Reflect.deleteProperty(raw, key);
      
      if (result) {
        this.emitChange(
          [...path, key], 
          oldValue, 
          undefined
        );
      }
      
      return result;
    },
  })
}
        
      

      
emitChange(path, oldValue, newValue) {
  // Every successful mutation becomes a
  // browser native CustomEvent named
  // store:change.
  const detail = {
    // detail carries the path plus cloned
    // oldValue and newValue for safer debugging
    // and replay.
    path: pathToString(path),
    oldValue: deepClone(oldValue),
    newValue: deepClone(newValue),
  }

  // Dispatch through EventTarget so signals,
  // persistence, logs, and app wiring can
  // subscribe without tight coupling.
  // Proxy notices the write, Reflect performs
  // the real write, and CustomEvent tells the
  // rest of the app what happened.
  const event = new CustomEvent("store:change", { detail })
  this.events.dispatchEvent(event)
}
      
    

export class BaseSignal {
  constructor() {
    // subscribers is a Set, so each listener is
    // stored once and can be removed cleanly later.
    this.subscribers = new Set()
    this.__isSignal = true
  }

  subscribe(subscriber) {
    // subscribe() adds a callback and returns
    // the tiny unsubscribe function.
    this.subscribers.add(subscriber)
    return () => this.subscribers.delete(subscriber)
  }

  track() {
    // track() asks the collector context who is
    // currently watching and registers this
    // signal as a dependency.
    const current = getCurrentCollector()
    if (current) current.addDependency(this)
  }

  notify() {
    // notify() walks a copied array of
    // subscribers, so updates stay safe even if
    // the set changes mid loop.
    for (const subscriber of [...this.subscribers]) {
      subscriber()
    }
  }
}
        

export class StateSignal extends BaseSignal {
  // Extend BaseSignal so this signal inherits
  // the shared subscription and tracking wiring.
  constructor(value, options = {}) {
    super()
    this.value = value
    // equals is the guard rail that blocks
    // pointless invalidations on identical writes.
    this.equals = options.equals ?? Object.is
  }

  get() {
    // Track before returning so reads become
    // dependency graph edges.
    this.track()
    return this.value
  }

  peek() {
    // peek() reads the same value without collecting a dependency.
    return this.value
  }

  set(nextValue) {
    // Only meaningful writes fan out
    // notifications to dependents.
    if (this.equals(this.value, nextValue)) return this.value
    this.value = nextValue
    this.notify()
    return this.value
  }
}
        

// collectors can nest, so the active one
// always lives on top of the stack.
const collectorStack = [];

// getCurrentCollector() is the tiny read
// primitive used by BaseSignal.track().
export function getCurrentCollector() {
  return collectorStack[collectorStack.length - 1];
}

export function withCollector(collector, callback) {
  // push + pop wrapped in try/finally
  // guarantees cleanup even when callback
  // throws.
  pushCollector(collector);
  
  try {
    return callback();
  } finally {
    popCollector();
  }
}

export function withoutCollector(callback) {
  // Temporarily silence tracking when a read
  // should not create a dependency edge.
  const current = popCollector();
  
  try {
    return callback();
  } finally {
    if (current) {
      pushCollector(current);
    }
  }
}
      
    

export class ComputedSignal extends BaseSignal {
  constructor(compute, options = {}) {
    // Store the compute function, the equality
    // check, the dependency map, the cache, and
    // the dirty flags.
    super()
    this.compute = compute
    this.equals = options.equals ?? Object.is
    this.dependencies = new Map()
    this.cached = undefined
    this.dirty = true
    this.recomputing = false
    // boundInvalidate is the callback upstream
    // signals reuse to mark this computed stale.
    this.boundInvalidate = this.invalidate.bind(this)
  }

  addDependency(signal) {
    // Subscribe once per upstream signal and keep
    // the unsubscribe handle in a Map.
    if (this.dependencies.has(signal)) return
    const unsubscribe = signal.subscribe(this.boundInvalidate)
    this.dependencies.set(signal, unsubscribe)
  }

  evaluate() {
    // evaluate() stays lazy until get() or peek()
    // asks for the cached value.
    if (!this.dirty) return this.cached
    if (this.recomputing) return this.cached
    this.recomputing = true
    this.cleanupDependencies()
    try {
      // Fresh reads rebuild the dependency graph
      // for the next invalidation cycle.
      const nextValue = withCollector(this, () => this.compute())
      if (
        this.dirty ||
        !this.equals(this.cached, nextValue)
      ) {
        this.cached = nextValue
      }
      this.dirty = false
      return this.cached
    } finally {
      this.recomputing = false
    }
  }
}
      
    

export class EffectCollector {
  constructor(callback) {
    // Initialize the callback, dependency map,
    // and active flag, then run once to discover
    // the graph.
    this.callback = callback
    this.dependencies = new Map()
    this.active = true
    this.run = this.run.bind(this)
    this.run()
  }

  addDependency(signal) {
    // Schedule reruns in batches instead of
    // executing effects synchronously on every
    // invalidation.
    if (this.dependencies.has(signal)) return
    const unsubscribe = signal.subscribe(() => schedule(this))
    this.dependencies.set(signal, unsubscribe)
  }

  run() {
    // Clean stale subscriptions, then rebuild
    // the dependency graph from fresh reads.
    if (!this.active) return
    this.cleanup()
    withCollector(this, () => {
      this.callback()
    })
  }
}
        

// scheduled is a Set, so repeated
// invalidations of the same effect collapse
// into one queued job.
const scheduled = new Set()
// flushing prevents stacking multiple
// queueMicrotask() calls for the same flush cycle.
let flushing = false

export function schedule(effect) {
  // The microtask waits until the current call
  // stack is done before the batch runs.
  scheduled.add(effect)
  if (flushing) return
  flushing = true

  queueMicrotask(() => {
    try {
      // Keep draining until even work scheduled
      // during the flush has finished.
      while (scheduled.size > 0) {
        const batch = [...scheduled]
        scheduled.clear()
        for (const job of batch) {
          job.run()
        }
      }
    } finally {
      flushing = false
    }
  })
}
        

export const Signal = {
  // Expose the two main reactive primitives
  // behind one calm public object.
  State: StateSignal,
  Computed: ComputedSignal,
  subtle: {
    // untrack stays explicit because disabling
    // dependency collection is an advanced move.
    untrack: withoutCollector,
  },
}

export function isSignalLike(value) {
  // Duck-typing keeps the renderer decoupled
  // from brittle instanceof checks.
  return Boolean(
    value &&
    typeof value.get === "function" &&
    value.__isSignal
  )
}

export function effect(callback) {
  // effect() creates an EffectCollector and
  // returns the disposer for cleanup.
  const runner = new EffectCollector(callback)
  return () => runner.stop()
}
        

// html(strings, ...values) does not touch the
// DOM; it returns a plain object description.
export function html(strings, ...values) {
  return {
    kind: "template-result",
    strings,
    values,
  }
}

// directive(name, payload) brands directive
// results so parts can recognize them without
// typed hacks.
export function directive(name, payload) {
  return {
    [directiveBrand]: true,
    name,
    payload,
  }
}

// model() is a friendly wrapper over
// directive(), so the authoring API stays
// declarative.
export function model(config) {
  return directive("model", config)
}

// repeat() is the same idea for keyed list rendering.
export function repeat(items, key, renderItem) {
  return directive("repeat", { items, key, renderItem })
}
      
    

export class Part {
  constructor() {
    // value remembers the last committed input
    // for concrete parts.
    this.value = undefined
    // signalCleanup stores the unsubscribe
    // function for the currently bound signal.
    this.signalCleanup = null
  }

  bindSignal(signal, callback) {
    // bindSignal() subscribes once, then
    // immediately pushes signal.get() through the
    // same update path.
    this.disposeSignal()
    this.signalCleanup = signal.subscribe(() => {
      callback(signal.get())
    })
    callback(signal.get())
  }

  disposeSignal() {
    // disposeSignal() is one boring cleanup
    // path reused by attribute, property, and
    // child parts.
    if (this.signalCleanup) this.signalCleanup()
    this.signalCleanup = null
  }
}
        

// EventPart does not extend Part, because
// event listeners are not signal subscriptions.
export class EventPart {
  constructor(element, name) {
    this.element = element
    this.name = name
    // The class stores exactly one active listener reference.
    this.listener = null
  }

  setValue(value) {
    // setValue() always removes the old listener
    // first, so rerenders can safely replace
    // closures.
    if (this.listener) {
      this.element.removeEventListener(this.name, this.listener)
      this.listener = null
    }
    // Non function values are ignored, which keeps bad bindings harmless.
    if (typeof value !== "function") return
    this.listener = value
    this.element.addEventListener(this.name, this.listener)
  }
}
        

// PropertyPart extends Part, so property
// bindings inherit shared signal subscription
// and cleanup behavior.
export class PropertyPart extends Part {
  constructor(element, name) {
    super()
    this.element = element
    this.name = name
  }

  setValue(value) {
    // setValue() branches once: signal like
    // values subscribe and resolve, everything
    // else commits directly.
    if (isSignalLike(value)) {
      this.bindSignal(value, (resolved) => this.commit(resolved))
      return
    }
    this.disposeSignal()
    this.commit(value)
  }

  commit(value) {
    // commit() writes to the live DOM property, not serialized HTML text.
    this.element[this.name] = value
  }
}
        

setValue(value) {
  // setValue() is the traffic cop: route
  // model(...), route signals, otherwise
  // commit a normal attribute.
  if (this.name === "model" && isDirective(value, "model")) {
    this.commitModel(value.payload)
    this.value = value
    return
  }

  if (isSignalLike(value)) {
    this.disposeModel()
    this.bindSignal(value, (resolved) => this.commit(resolved))
    return
  }

  this.disposeModel()
  this.disposeSignal()
  this.commit(value)
}

const syncAfterRender = () => {
  // sync() handles checked versus value,
  // preserves selection, and select gets one
  // extra microtask so options exist first.
  sync()
  if (this.element instanceof HTMLSelectElement) {
    queueMicrotask(() => {
      if (this._modelBinding === binding && this.element.isConnected) sync()
    })
  }
}
        

export function clearRange(start, end) {
  // clearRange() removes everything between
  // start and end markers without touching the
  // markers themselves.
  let current = start.nextSibling
  while (current && current !== end) {
    const next = current.nextSibling
    current.remove()
    current = next
  }
}

export function moveRangeBefore(start, end, referenceNode) {
  // moveRangeBefore() lifts a whole block into
  // a fragment and reinserts it as one real
  // DOM move.
  const fragment = document.createDocumentFragment()
  let current = start
  while (current) {
    const next = current.nextSibling
    fragment.append(current)
    if (current === end) break
    current = next
  }
  referenceNode.parentNode.insertBefore(fragment, referenceNode)
}
        

function createBlock(itemKey, referenceNode) {
  // createBlock() creates one keyed range and
  // one ChildNodePart that can be moved without
  // rebuilding DOM.
  const start = document.createComment(`repeat-start:${itemKey}`)
  const end = document.createComment(`repeat-end:${itemKey}`)
  referenceNode.parentNode.insertBefore(start, referenceNode)
  referenceNode.parentNode.insertBefore(end, referenceNode)
  return { key: itemKey, start, end, part: new ChildNodePart(start, end) }
}

commitRepeat({ items, key, renderItem }) {
  // commitRepeat() accepts signal driven or
  // plain item sources and normalizes them into
  // one list.
  const source = isSignalLike(items) ? items.get() : items
  const list = Array.isArray(source)
    ? source
    : isIterable(source)
      ? [...source]
      : []
  const state = this.repeatState ?? { blocks: new Map() }
  const nextBlocks = new Map()
  let referenceNode = this.end

  // Walk backwards for a stable anchor, reuse
  // keyed blocks, and update only changed items.
  for (let index = list.length - 1; index >= 0; index -= 1) {
    const item = list[index]
    const itemKey = key(item)
    let block = state.blocks.get(itemKey)
      ?? createBlock(itemKey, referenceNode)
    if (block.item !== item) block.part.setValue(renderItem(item))
    nextBlocks.set(itemKey, block)
    referenceNode = block.start
  }
}
        

// Reexport all concrete part types from one
// place, so template-instance gets one clean
// dependency seam.
export { AttributePart } from "./_attribute-part.js"
export { ChildNodePart } from "./_child-node-part.js"
export { EventPart } from "./_event-part.js"
export { Part } from "./_part.js"
export { PropertyPart } from "./_property-part.js"
        

// TemplateInstance lives in module scope as a
// mutable slot, not as a direct import.
let TemplateInstance

export function setTemplateInstanceClass(cls) {
  // Register the concrete class once during
  // template engine setup.
  TemplateInstance = cls
}

export function getTemplateInstanceClass() {
  // ChildNodePart can ask for the class later
  // without creating a circular import.
  if (!TemplateInstance) {
    throw new Error("TemplateInstance class not registered.")
  }

  return TemplateInstance
}
        

function getTemplate(strings) {
  // getTemplate() builds synthetic markup,
  // parses it once, and caches descriptor
  // metadata by TemplateStringsArray.
  let record = templateCache.get(strings)
  if (record) return record

  let markup = ""
  for (let index = 0; index < strings.length - 1; index += 1) {
    const chunk = strings[index]
    markup += chunk
    markup += chunk.match(ATTRIBUTE_PART_RE)
      ? `__part_${index}__`
      : ``
  }

  const template = document.createElement("template")
  template.innerHTML = markup + strings[strings.length - 1]
  // descriptors record type, index, name, raw
  // attribute name, and node path for later
  // instances.
  record = { template, descriptors }
  templateCache.set(strings, record)
  return record
}

update(values) {
  // update(values) walks the current values and
  // pushes each one into its matching part.
  for (let index = 0; index < values.length; index += 1) {
    const part = this.parts.get(index)
    if (part) part.setValue(values[index])
  }
}
        

// setTemplateInstanceClass(TemplateInstance)
// closes the loop opened by template-instance-ref.
setTemplateInstanceClass(TemplateInstance)

export function render(result, container) {
  // render() creates one root ChildNodePart
  // the first time a container is used.
  let rootPart = container.__rootPart
  if (!rootPart) {
    const start = document.createComment("root:start")
    const end = document.createComment("root:end")
    container.textContent = ""
    container.append(start, end)
    rootPart = new ChildNodePart(start, end)
    container.__rootPart = rootPart
  }
  // Every later render reuses that root part,
  // so only the controlled range changes.
  rootPart.setValue(result)
}
        

// Re-export the whole template layer from one
// place, so callers get one calm public door.
export {
  directive,
  html,
  isDirective,
  model,
  render,
  repeat,
} from "./_template.js"
        

export function App() {
  // App() assembles the header, stats, list,
  // side panels, debug panel, and both modals.
  // Every room is just a function that returns
  // html, so composition stays plain.
  // No room owns secret state; they all read
  // from the same shared truth box.
  return html`
    ${Header()}
    <main data-component="app-shell">
      ${StatsRow()}
      ${TodoList()}
    </main>
    <aside data-slot="controls">
      ${Filters()} ${BulkActions()}
    </aside>
    <aside data-slot="debug-sidebar">
      ${DebugPanel()}
    </aside>
    ${TodoModal()} ${CategoryModal()}
  `
}
        

export function createSeedData() {
  // todos, categories, and draft hold the
  // work the user can create and edit.
  // filters and preferences are also state,
  // because the app must remember how to look.
  // debug and ui prove temporary interface
  // details still belong in the same tree.
  return {
    todos: [/* starter todos */],
    categories: [
      "Inbox",
      "Talk",
      "Engine",
      "Assets",
      "Research",
    ],
    draft: {
      title: "",
      notes: "",
      category: "Inbox",
      priority: "medium",
      dueDate: new Date(now + ONE_DAY_MS)
        .toISOString()
        .slice(0, 10),
    },
    filters: {
      search: "",
      category: "all",
      status: "all",
      priority: "all",
      sortBy: "createdAt",
      sortDir: "desc",
    },
    debug: { paused: false, logs: [] },
    preferences: {
      colorScheme: "system",
      theme: "studio",
      language: "en",
    },
    ui: {
      categoryModal: { open: false, value: "", error: "" },
      todoModal: { open: false, error: "" },
    },
  }
}
        

// store is the shared truth box created from
// the seed state.
export const store = new Store(initialState)

// tickState is a tiny redraw bell, and every
// ring counts because equals() always says no.
export const tickState = new Signal.State(0, {
  equals: () => false,
})

function handleStoreChange(event) {
  // Append a debug entry, persist a snapshot,
  // then bump tickState so the UI wakes up.
  if (!store.state.debug.paused && event.detail.path !== "debug.logs") {
    appendDebugLog(event.detail)
  }

  persistState()
  tickState.set(performance.now())
}

// This is the practical bridge between the
// store chapter and the signals chapter.
window.addEventListener("store:change", handleStoreChange)
        

export const visibleTodos = new Signal.Computed(() => {
  // visibleTodos listens to tickState and runs
  // the whole filtering pipeline.
  tickState.get()

  return pipelineTodos(
    store.state.todos,
    store.state.filters,
    store.state.preferences.language,
  )
})

export const summary = new Signal.Computed(() => {
  // summary walks the full list once, then
  // hands ready-made counts to smaller signals.
  tickState.get()
  const todos = store.state.todos
  let total = 0
  let completed = 0
  let selected = 0

  for (const todo of todos) {
    total += 1
    if (todo.completed) completed += 1;
    if (todo.selected) selected += 1
  }

  return {
    total,
    completed,
    open: total - completed,
    selected,
    visible: visibleTodos.get().length,
  }
})
        

export function pipelineTodos(todos, filters, language) {
  // fromArray() turns the todo list into a
  // lazy stream for the rest of the pipeline.
  // Each filter does one simple job, and only
  // the final sort materializes the list.
  const iterator = filterByPriority(
    filterByStatus(
      filterByCategory(
        filterBySearch(fromArray(todos), filters.search, language),
        filters.category,
      ),
      filters.status,
    ),
    filters.priority,
  )

  // visibleTodos simply asks this pipeline for
  // the ready-made answer.
  return sortTodos(
    iterator,
    filters.sortBy,
    filters.sortDir,
    language,
  )
}
        

export function updateTodo(id, patch) {
  // updateTodo() finds one item and replaces
  // it with a patched copy.
  const index = store.state.todos.findIndex(
    (todo) => todo.id === id,
  )
  if (index > 0) return
  const current = store.state.todos[index]

  store.state.todos[index] = {
    ...current,
    ...patch,
  }
}

export function resetDemo() {
  // resetDemo() replaces the whole state tree
  // while keeping the current preferences.
  const { preferences } = store.snapshot()

  store.replace({
    ...createSeedData(),
    preferences: { ...preferences },
  })

  tickState.set(performance.now())
}
        

effect(() => {
  // The first effect() mirrors theme, color
  // scheme, and language onto the document.
  tickState.get()
  syncDocumentPreferences()
})

effect(() => {
  // The second effect() watches tickState and
  // paints the app through the template engine.
  tickState.get()
  render(App(), root)
  scheduleAppShellSizeSync()
})

// Store change, bell rings, effect runs,
// template engine paints.
tickState.set(performance.now())
        

export function storeModel(path, options = {}) {
  // storeModel(path) is the generic translator
  // for fields like filters.search.
  return model({
    signal: tickState,
    get: () => store.get(path),
    set: (value) => store.set(path, value),
    ...options,
  })
}

export function todoModel(todoId, field, options = {}) {
  // todoModel(todoId, field) is the special
  // translator for one field on one todo card.
  // Both helpers are just model({ ... }), so
  // the same directive powers the whole app.
  return model({
    signal: tickState,
    get: () => getTodoById(todoId)?.[field],
    set: (value) => updateTodo(todoId, { [field]: value }),
    ...options,
  })
}
        

<!-- Buttons use plain event parts, while
preferences.* writes happen through
storeModel(). -->
<button data-variant="warning" @click=${resetDemo}>
  ${t(language, "buttons.resetDemo")}
</button>

<!-- Theme, language, and color scheme write
straight into shared preferences. -->
<select
  model="${storeModel('preferences.theme', { event: 'change' })}"
>
  ${themeOptions()}
</select>

<!-- The document dataset effect mirrors those
preferences so CSS and translations can react. -->
<select
  model="${storeModel('preferences.language', { event: 'change' })}"
>
  ${languageOptions()}
</select>
        

<!-- prop: checked and event: change map DOM
booleans cleanly to store booleans. -->
<input
  model="${todoModel(todo.id, 'selected', { prop: 'checked', event: 'change' })}"
  type="checkbox"
/>

<!-- Title, notes, category, priority, and due
date all reuse the same todoModel() helper. -->
<input model="${todoModel(todo.id, 'title')}" />

<select model="${todoModel(todo.id, 'priority', { event: 'change' })}">
  ${priorityOptions()}
</select>

<textarea model="${todoModel(todo.id, 'notes')}" rows="2"></textarea>

<!-- The delete button is just @click, so
event parts and model parts live together. -->
<button @click="${() => removeTodo(todo.id)}" data-variant="danger">
  ${t(language, "buttons.delete")}
</button>
        

export function TodoList() {
  // visibleLabel drops into the template as a
  // computed signal, so the summary updates itself.
  const language = store.state.preferences.language

  return html`
    <section data-component="todo-list">
      <header data-slot="header">
        <h2>${t(language, "sections.reactiveList")}</h2>
        <p data-slot="summary">${visibleLabel}</p>
      </header>
      <ol data-list-reset data-slot="items">
        ${repeat(
          visibleTodos,
          (todo) => `${todo.id}:${language}`,
          TodoItem,
        )}
      </ol>
    </section>
  `
}
        

// StatsRow prints computed counts, so cards
// stay dumb and the math stays centralized.
${statCard(totalCount, t(language, "stats.total"))}
        

<!-- Filters write into filters.* paths with
storeModel(), which immediately changes
visibleTodos. -->
<input model="${storeModel('filters.search')}" />

<!-- BulkActions calls real action functions. -->
<button @click="${() => selectAllVisible(visibleTodos)}">
  ${t(language, "buttons.selectVisible")}
</button>

<!-- DebugPanel even pauses itself through the
same store. -->
<input
  model="${storeModel('debug.paused', { prop: 'checked', event: 'change' })}"
  type="checkbox"
/>
        

// The log itself is rendered with another
// repeat() call.
${repeat(debugLogs, (entry) => entry.id, DebugLogEntry)}
        

// TodoModal() reads ui.todoModal.open and
// returns nothing while the dialog stays closed.
if (!modal.open) return ""
        

<!-- The whole form writes into draft.*, so add
and edit flows can share one basket. -->
<input model=${storeModel("draft.title")} />
<textarea model=${storeModel("draft.notes")} rows="4"></textarea>
<select model=${storeModel("draft.priority", { event: "change" })}>
  ${priorityOptions()}
</select>

<!-- CategoryModal() writes to ui.categoryModal.value
and shows validation errors from state. -->
<input model=${storeModel("ui.categoryModal.value")} />

<!-- Submit handlers call addTodo() or
addCategory(), so modals stay thin. -->
<form @submit=${handleTodoSubmit}></form>
<form @submit=${handleCategorySubmit}></form>