Behaviour for Harlock applications.

An app defines three callbacks:

• init/1 — returns the initial model from an arbitrary init argument.
• update/2 — given an event and the current model, returns the next model, optionally paired with a Cmd. Return :quit to exit the app.
• view/1 — given the current model, returns an element tree.

The simplest app:

defmodule Counter do
  use Harlock.App

  def init(_), do: %{n: 0}

  def update({:key, {:char, ?+}, []}, m), do: %{m | n: m.n + 1}
  def update({:key, {:char, ?q}, []}, _), do: :quit
  def update(_event, m), do: m

  def view(m) do
    vbox(constraints: [length: 1, fill: 1], children: [
      text("Count: #{m.n}"),
      text("(+ to inc, q to quit)")
    ])
  end
end

Event vocabulary

update/2 receives messages from several sources. The runtime guarantees the following tuple shapes; apps pattern-match on these directly.

Tab / Shift-Tab are consumed by the runtime for focus traversal whenever the current tree contains at least one focusable element and do not reach update/2 in that case. (When the tree has no focusables to cycle, the keys fall through as raw {:key, :tab, …} events.) Apps that previously dispatched on {:key, :tab, []} for sub-navigation — for example, cycling a custom highlight — should bind that gesture to a non-Tab key the moment a focusable widget enters the tree, or the binding will be silently shadowed.

Raw input:

• {:key, key, [modifier()]} — terminal key events. Examples: {:key, :down, []}, {:key, {:char, ?q}, []}, {:key, :tab, [:shift]}. key is an atom for named keys (:up, :enter, :escape, …), {:char, codepoint} for printables, {:f, n} for function keys.

Focus-aware widget routing (R2, v0.4). When a focusable widget (viewport, tabs, text_input) carries a :focusable id and is focused, the runtime translates relevant keys into widget-shaped messages before calling update/2. The raw {:key, …} is swallowed — apps see the routed message or the raw key, never both. Opt out per-element with handle_keys: false. The four messages:

• {:harlock_scroll, focus_id, new_offset} — focused viewport handled a scroll key (:up/:down/:page_up/:page_down/ :home/:end). The app's clause typically just writes the offset back to the model:

  def update({:harlock_scroll, :log, n}, m),
    do: %{m | log_offset: n}

• {:harlock_select, focus_id, new_id} — focused tabs widget moved selection (:left/:right/:home/:end):

  def update({:harlock_select, :nav, id}, m), do: %{m | tab: id}

• {:harlock_edit, focus_id, {new_value, new_cursor}} — focused text_input accepted a printable character, arrow, backspace, or delete:

  def update({:harlock_edit, :search, {v, c}}, m),
    do: %{m | search: v, search_cursor: c}

• {:harlock_submit, focus_id} — focused text_input saw Enter:

  def update({:harlock_submit, :search}, m), do: run_search(m)

No-op key events (e.g. :up on a viewport already at offset 0, or a modifier-only press in a text input) fall through to update/2 as raw {:key, …} events so apps can still react if they want.

Cmd results and Sub-produced messages have whatever shape the app defined via Cmd.map/2 and friends — those are not part of the runtime's vocabulary.