import 'package:flutter/material.dart';
import 'package:go_router/go_router.dart';

/// Page transitions, kept in one place so every screen arrives the same way.
///
/// Two kinds only. Moving *deeper* (a shift, a document, a timecard) slides
/// forward, and the screen behind it slides back a little so the stack reads
/// as a stack. Moving *sideways* — between steps of sign-up, or replacing the
/// whole screen — cross-fades, because nothing is being pushed onto anything.
abstract final class Transitions {
  static const _forward = Duration(milliseconds: 320);
  static const _back = Duration(milliseconds: 260);

  /// Pushed detail screens.
  static CustomTransitionPage<T> push<T>(GoRouterState state, Widget child) {
    return CustomTransitionPage<T>(
      key: state.pageKey,
      child: child,
      transitionDuration: _forward,
      reverseTransitionDuration: _back,
      transitionsBuilder: (context, animation, secondaryAnimation, child) {
        final incoming = CurvedAnimation(
          parent: animation,
          curve: Curves.easeOutCubic,
          reverseCurve: Curves.easeInCubic,
        );
        // The outgoing screen eases back rather than sitting still, which is
        // what gives the push its depth.
        final outgoing = CurvedAnimation(
          parent: secondaryAnimation,
          curve: Curves.easeOutCubic,
          reverseCurve: Curves.easeInCubic,
        );

        // secondaryAnimation rests at 0 while this page is on top, so the
        // resting state has to be `begin`. Reversing these two leaves every
        // pushed screen sitting 12% off to the left.
        return SlideTransition(
          position: Tween(
            begin: Offset.zero,
            end: const Offset(-0.12, 0),
          ).animate(outgoing),
          child: FadeTransition(
            opacity: Tween(begin: 1.0, end: 0.0).animate(outgoing),
            child: SlideTransition(
              position: Tween(
                begin: const Offset(1, 0),
                end: Offset.zero,
              ).animate(incoming),
              child: child,
            ),
          ),
        );
      },
    );
  }

  /// Screens that replace what came before: splash, onboarding, sign-in steps.
  static CustomTransitionPage<T> fade<T>(GoRouterState state, Widget child) {
    return CustomTransitionPage<T>(
      key: state.pageKey,
      child: child,
      transitionDuration: _forward,
      reverseTransitionDuration: _back,
      transitionsBuilder: (context, animation, secondaryAnimation, child) {
        final curved = CurvedAnimation(
          parent: animation,
          curve: Curves.easeOutCubic,
        );
        return FadeTransition(
          opacity: curved,
          // A few pixels of rise stops the cross-fade feeling inert.
          child: SlideTransition(
            position: Tween(
              begin: const Offset(0, 0.015),
              end: Offset.zero,
            ).animate(curved),
            child: child,
          ),
        );
      },
    );
  }
}