import type { CellHandle } from "@commonfabric/runtime-client";

/**
 * What a component displays of `handle`'s cell: its value, or nothing while
 * the worker refuses the handle's read, or before the handle has read
 * anything. Unlike `get()`, it does not throw for a refusal, so a render in
 * progress displays nothing of the cell in its place, and a component that
 * displays the cell's contents displays a placeholder where `handle.refusal`
 * is set.
 *
 * For display only. It reads a refusal, and a handle that has read nothing,
 * as nothing, and nothing is what a cell that holds nothing reads as too, so
 * a value computed from it, to write or to decide whether to write, would
 * replace a value the host was never shown. Code that writes reads with
 * `get()`, which throws for a refusal, or checks `handle.lastRead()` first.
 */
export function valueForDisplay<T>(
  handle: CellHandle<T>,
): Readonly<T> | undefined {
  const read = handle.lastRead();
  return "value" in read ? read.value : undefined;
}