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@override
  Widget build(BuildContext context) {
    List<Widget> stackChildren = [];

    switch (currentScreenState) {
      case CURRENT_SCREEN_STATE.INIT_STATE:
        stackChildren.addAll(_getBgWidgets());
        stackChildren.addAll(_getDefaultWidgets());
        stackChildren.addAll(_getInitScreenWidgets());
        stackChildren.add(_getBrandTitle());

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//Animation Controller for setting bounce animation for "Swipe up" text widget
    _swipeUpBounceAnimationController =
        AnimationController(duration: Duration(milliseconds: 800), vsync: this)
          ..repeat(reverse: true);

    //Animation for actual bounce effect
    _swipeUpBounceAnimation = Tween<double>(begin: 0, end: -20).animate(
        CurvedAnimation(
            parent: _swipeUpBounceAnimationController,
            curve: Curves.easeOutBack))

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//A simple container with a SingleChildScrollView. The trick is to set the child of SingleChildScrollView height
//exceed the height of parent scroll widget so it can be scrolled. The BouncingScrollPhysics helps the scroll retain its
//original position if it doesn't cross the threshold to play reveal animation.
//This widget is added by _getInitScreenWidgets() method 
Positioned(
        right: 0,
        left: 0,
        bottom: 0,
        child: Container(
          height: widget.height * .5,

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//Update scene state to "reveal" and start corresponding animation
//This method is called when drag excced our defined threshold
  void _playRevealAnimation() {
    setState(() {
      currentScreenState = CURRENT_SCREEN_STATE.REVEALING_ANIMATING_STATE;
      _revealAnimationController.forward();
      _amoebaAnimationController.forward();
    });
  }

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//Animation controller for showing animation after reveal
    _postRevealAnimationController =
        AnimationController(duration: Duration(milliseconds: 600), vsync: this);

 //Scale animation for showing center logo after reveal is completed
    _centerIconScale = Tween<double>(begin: 0, end: .5).animate(CurvedAnimation(
      parent: _postRevealAnimationController,
      curve: Curves.fastOutSlowIn,
    ));

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//The paint method of "CurvePageSwitcher" to draw tab selection arc
void _drawSwipeAbleArc(Canvas canvas, Size size) {
    Path path = Path();

    path.moveTo(-2, size.height - archBottomOffset);
    path.cubicTo(
        -2,
        size.height - archBottomOffset,
        size.width / 2,
        size.height - arcHeight - archBottomOffset,

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///The background for selected tab. On the basis of tab selected, the foreground container is translated away,
///revealing the underlying background container. If the screen state is just set to reveal, then in the
///initial state no foreground container is added which is signified by _tabSelectionAnimation set to null.
///_tabSelectionAnimation is only set when either of the tab is pressed.
  List<Widget> _getBgWidgets() {
    List<Widget> widgets = [];
    Color foreGroundColor;
    Color backgroundColor;
    if (isLeftTabSelected) {
      foreGroundColor = Colors.deepPurple;

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abstract class BaseUseCase<P, R> {

  Stream<R> buildUseCase(P param);

  StreamSubscription<R> execute(P param,void Function(R) result,void Function(String) error)
  {
    return buildUseCase(param).listen(result,onError: (Object exception, StackTrace stackTrace){
      error(exception.toString());
    });
  }

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  //Usecase Implementation
  class PostBillingInfoUseCase
    extends UseCase<BillingInfoRequest, BillingInfoResponse> {
  final BillingInfoRepository _billingInfoRepository;

  PostBillingInfoUseCase(this._billingInfoRepository);

  @override
  Stream<BillingInfoResponse> buildUseCase(BillingInfoRequest param) {
    return _billingInfoRepository.postBillingInfo(param).asStream();

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//Usecase Implementation
class ValidateBillingAndShippingAddressUseCase extends UseCase<
    BillingAndShippingAddressValidationRequest,
    BillingAndShippingAddressValidationResponse> {
  final AddressRespository _addressRespository;

  ValidateBillingAndShippingAddressUseCase(this._addressRespository);

  @override
  Stream<BillingAndShippingAddressValidationResponse> buildUseCase(