export const BEFORE_PAGE_REFRESH = "app:before-page-refresh";
export const PAGE_DOWNLOAD_ACTIVE_ATTRIBUTE = "data-page-download-active";
export type PageRefreshRequest = {
  save: (action: () => Promise<void>) => void;
  block: (message: string) => void;
};

const downloadListeners = new Set<() => void>();
let activePageDownloads = 0;

export function isPageDownloadActive() {
  return activePageDownloads > 0;
}

export function subscribeToPageDownloadActivity(listener: () => void) {
  downloadListeners.add(listener);
  return () => { downloadListeners.delete(listener); };
}

function publishDownloadActivity() {
  if (typeof document !== "undefined") {
    document.documentElement?.toggleAttribute(PAGE_DOWNLOAD_ACTIVE_ATTRIBUTE, isPageDownloadActive());
  }
  for (const listener of downloadListeners) listener();
}

export async function withActivePageDownload<T>(download: () => Promise<T>): Promise<T> {
  if (++activePageDownloads === 1) publishDownloadActivity();
  try {
    return await download();
  } finally {
    if (--activePageDownloads === 0) publishDownloadActivity();
  }
}

function assertPageDownloadFinished() {
  if (isPageDownloadActive()) throw new Error("文件正在下载，请等下载完成后再刷新。");
}

export async function preparePageRefresh() {
  assertPageDownloadFinished();
  const actions: Array<() => Promise<void>> = [];
  let blocked = "";
  window.dispatchEvent(new CustomEvent<PageRefreshRequest>(BEFORE_PAGE_REFRESH, {
    detail: { save: action => actions.push(action), block: message => { blocked ||= message; } },
  }));
  if (blocked) throw new Error(blocked);
  for (const action of actions) await action();
  assertPageDownloadFinished();
}