/**
 * Root sagas can be exported from every file, and are all bundled together in one master saga.
 * This saga listens for requests to sync, as well as failure events.
 */
export default function* rootSaga() {
  yield fork(takeLatest, cacheManagerActionTypes.SYNC_ALL, startSync);
  yield fork(takeLatest, cacheManagerActionTypes.SYNC_FAILED, showSyncFailed);
}

/**
 * Sync everything (download/upload from the server), but halt if a SYNC_FAILED event occurs.
 *
 * `race` starts multiple effects at once and stops everything else as soon as one completes.
 * The object keys here ('sync' and 'cancelSync') are not being used.
 */
function* startSync() {
  yield race({
    sync: call(syncAll),
    cancelSync: take(cacheManagerActionTypes.SYNC_FAILED),
  });
}

/**
 * Sync everything, showing status messages to the user as it progresses.
 *
 * `select` gets a value from the global state;
 * `call` calls a function;
 * `put` dispatches an event;
 * `take` waits until an event occurs;
 * `fork` calls another saga asynchronously.
 */
function* syncAll() {
  const { syncInProgress } = yield select((state) => ({
    syncInProgress: state.sync.syncInProgress,
  }));
  
  if (syncInProgress) return;
                                          
  try {
    yield call(Snackbar.show, 'Downloading...');

    yield put(CacheManagerActions.syncStarted());

    yield fork(syncWorkingState);

    // Wait until all downloads are done.
    yield take(cacheManagerActionTypes.SYNC_FINISHED);

    yield fork(syncUrlCache);

    yield call(Snackbar.show, 'Uploading...');

    yield put(CacheManagerActions.syncStarted());

    yield fork(syncCheckins);
    yield fork(syncCompletions);
    yield fork(syncAttachments);

    // Wait until all uploads are done.
    yield take(cacheManagerActionTypes.SYNC_FINISHED);

    yield call(Snackbar.show, 'Sync successful!');
  } catch (error) {
    yield put(CacheManagerActions.syncFailed());
  }
}

/**
 * Show an error message to the user, and clean up.
 */
function* showSyncFailed() {
  yield call(Snackbar.show, 'Sync failed!');
  yield put(CacheManagerActions.syncFinished());
}