class _FlexibleSpaceBarState extends State<FlexibleSpaceBar> {
  bool _getEffectiveCenterTitle(ThemeData theme) {
    if (widget.centerTitle != null)
      return widget.centerTitle;
    assert(theme.platform != null);
    switch (theme.platform) {
      case TargetPlatform.android:
      case TargetPlatform.fuchsia:
        return false;
      case TargetPlatform.iOS:
        return true;
    }
    return null;
  }

  Alignment _getTitleAlignment(bool effectiveCenterTitle) {
    if (effectiveCenterTitle)
      return Alignment.bottomCenter;
    final TextDirection textDirection = Directionality.of(context);
    assert(textDirection != null);
    switch (textDirection) {
      case TextDirection.rtl:
        return Alignment.bottomRight;
      case TextDirection.ltr:
        return Alignment.bottomLeft;
    }
    return null;
  }

  double _getCollapsePadding(double t, _FlexibleSpaceBarSettings settings) {
    switch (widget.collapseMode) {
      case CollapseMode.pin:
        return -(settings.maxExtent - settings.currentExtent);
      case CollapseMode.none:
        return 0.0;
      case CollapseMode.parallax:
        final double deltaExtent = settings.maxExtent - settings.minExtent;
        return -Tween<double>(begin: 0.0, end: deltaExtent / 4.0).transform(t);
    }
    return null;
  }

  @override
  Widget build(BuildContext context) {
    final _FlexibleSpaceBarSettings settings = context.inheritFromWidgetOfExactType(_FlexibleSpaceBarSettings);
    assert(settings != null, 'A FlexibleSpaceBar must be wrapped in the widget returned by FlexibleSpaceBar.createSettings().');

    final List<Widget> children = <Widget>[];

    final double deltaExtent = settings.maxExtent - settings.minExtent;

    // 0.0 -> Expanded
    // 1.0 -> Collapsed to toolbar
    final double t = (1.0 - (settings.currentExtent - settings.minExtent) / deltaExtent).clamp(0.0, 1.0);

    // background image
    if (widget.background != null) {
      final double fadeStart = math.max(0.0, 1.0 - kToolbarHeight / deltaExtent);
      const double fadeEnd = 1.0;
      assert(fadeStart <= fadeEnd);
      final double opacity = 1.0 - Interval(fadeStart, fadeEnd).transform(t);
      if (opacity > 0.0) {
        children.add(Positioned(
          top: _getCollapsePadding(t, settings),
          left: 0.0,
          right: 0.0,
          height: settings.maxExtent,
          child: Opacity(
            opacity: opacity,
            child: widget.background
          )
        ));
      }
    }

    if (widget.title != null) {
      Widget title;
      switch (defaultTargetPlatform) {
        case TargetPlatform.iOS:
          title = widget.title;
          break;
        case TargetPlatform.fuchsia:
        case TargetPlatform.android:
          title = Semantics(
            namesRoute: true,
            child: widget.title,
          );
      }

      final ThemeData theme = Theme.of(context);
      final double opacity = settings.toolbarOpacity;
      if (opacity > 0.0) {
        TextStyle titleStyle = theme.primaryTextTheme.title;
        titleStyle = titleStyle.copyWith(
          color: titleStyle.color.withOpacity(opacity)
        );
        final bool effectiveCenterTitle = _getEffectiveCenterTitle(theme);
        final double scaleValue = Tween<double>(begin: 1.5, end: 1.0).transform(t);
        final Matrix4 scaleTransform = Matrix4.identity()
          ..scale(scaleValue, scaleValue, 1.0);
        final Alignment titleAlignment = _getTitleAlignment(effectiveCenterTitle);
        children.add(Container(
          padding: EdgeInsetsDirectional.only(
            start: effectiveCenterTitle ? 0.0 : 72.0,
            bottom: 16.0                                       // here is the bottom padding,
          ),
          child: Transform(
            alignment: titleAlignment,
            transform: scaleTransform,
            child: Align(
              alignment: titleAlignment,
              child: DefaultTextStyle(
                style: titleStyle,
                child: title,
              )
            )
          )
        ));
      }
    }

    return ClipRect(child: Stack(children: children));
  }
}