Helpers for the viewport element.

The viewport renders a child taller than its allocated region and blits a window starting at offset. The application owns the offset (same TEA discipline as text_input); this module gives you a pure function to translate scroll keys into a new offset.

Example

def init(_), do: %{items: build_items(500), offset: 0, viewport_h: 20}

def update({:key, key, _mods}, m) do
  offset =
    Harlock.Viewport.apply_key(
      m.offset,
      length(m.items),
      m.viewport_h,
      key
    )

  %{m | offset: offset}
end

def view(m) do
  viewport(
    child: vbox(children: Enum.map(m.items, &text(content: &1))),
    offset: m.offset,
    content_height: length(m.items)
  )
end

:up/:down move one row, :page_up/:page_down move by viewport_h - 1 (leaves one row of context like most pagers), :home/:end jump to the extremes. Any other key returns the offset unchanged.

Auto-routing (v0.4)

When a viewport element carries a :focusable id, the runtime routes the six scroll keys to apply_key/4 automatically and delivers the result to update/2 as {:harlock_scroll, focus_id, new_offset} — the app only has to write where the offset lives on the model:

viewport(focusable: :log, offset: m.offset, content_height: ...)

def update({:harlock_scroll, :log, new_offset}, m),
  do: %{m | offset: new_offset}

Calling apply_key/4 from update/2 directly is still supported (and required for viewports without a :focusable id, or with handle_keys: false).