import 'dart:async';

import 'package:flutter/material.dart';

/// Keeps the data of a widget up to date by reloading it every
/// [refreshInterval].
///
/// Mix this into the [State] of every widget that shows data of the backend and
/// implement [RefreshData]. The mixin only owns the timer, fetching the data and
/// putting it on screen stays in the widget itself.
///
/// The backend has no push channel, so the only way to notice new agents,
/// tasks or reports is polling. The interval is therefore a compromise between
/// fresh data and load on the api.
///
/// A refresh happens quietly: implementations are expected to keep the data
/// that is already on screen while the request runs, so that the user neither
/// sees a spinner nor loses the scroll position every half minute. Failures of
/// a refresh are swallowed on purpose - the data on screen simply stays as it
/// is and the next tick tries again.
mixin AutoRefreshState<TWidget extends StatefulWidget> on State<TWidget> {

  /// Interval in which the api data of the whole app is refreshed.
  static const Duration refreshInterval = Duration(seconds: 30);

  Timer? _refreshTimer;

  /// True while a refresh triggered by the timer is running. A slow api must
  /// not pile up requests.
  bool mIsAutoRefreshing = false;

  @override
  void initState() {
    super.initState();
    _refreshTimer = Timer.periodic(refreshInterval, (_) => _onTick());
  }

  @override
  void dispose() {
    _refreshTimer?.cancel();
    _refreshTimer = null;
    super.dispose();
  }

  /// Fetches the data of this widget again. Called every [refreshInterval] as
  /// long as the widget is mounted.
  ///
  /// Implementations may let api errors escape, [_onTick] catches them.
  Future<void> RefreshData();

  Future<void> _onTick() async {

    if (mIsAutoRefreshing || !mounted) {
      return;
    }

    mIsAutoRefreshing = true;

    try {
      await RefreshData();
    } catch (error) {
      // An automatic refresh must never replace data that is already on screen
      // with an error message, so a failed tick is ignored.
    } finally {
      mIsAutoRefreshing = false;
    }
  }
}