title: Flutter simpler_animation migration
body: <p>I'm trying to update my app to flutter 2.0 and I'm stuck with the deprecated animations.
  I have a few errors in my <code>FadeIn.dart</code> file that manages a class I use in a few other places to fade in and out notifications and alerts to the user.</p>
  <p>Here is my Code in in that fad in File</p>
  <pre><code>import 'package:flutter/material.dart';
  import 'package:simple_animations/simple_animations.dart';
  
  class FadeIn extends StatelessWidget {
    final double delay;
    final Widget child;
  
    FadeIn(this.delay, this.child);
  
    @override
    Widget build(BuildContext context) {
      final tween = MultiTrackTween([
        Track(&quot;opacity&quot;)
            .add(Duration(milliseconds: 500), Tween(begin: 0.0, end: 1.0)),
        Track(&quot;translateX&quot;).add(
            Duration(milliseconds: 500), Tween(begin: 50.0, end: 0.0),
            curve: Curves.easeOut)
      ]);
  
      return CustomAnimation(
        delay: Duration(milliseconds: (300 * delay).round()),
        duration: tween.duration,
        tween: tween,
        child: child,
        builderWithChild: (context, child, animation) =&gt;
            Opacity(
              opacity: animation[&quot;opacity&quot;],
              child: Transform.translate(
                  offset: Offset(animation[&quot;translateX&quot;], 0), child: child),
            ),
      );
    }
  }
  </code></pre>
  <p>I'm looking at <a href="https://pub.dev/documentation/simple_animations/latest/" rel="nofollow noreferrer">https://pub.dev/documentation/simple_animations/latest/</a> &amp; <a href="https://pub.dev/packages/sa_v1_migration/versions/1.1.2" rel="nofollow noreferrer">https://pub.dev/packages/sa_v1_migration/versions/1.1.2</a> but I'm confused about the new syntax after I change to <code>MultiTween</code> when I go to the documentation I get stuck <a href="https://pub.dev/packages/sa_multi_tween" rel="nofollow noreferrer">https://pub.dev/packages/sa_multi_tween</a> with it saying nothing else needs to be changed. That is obviously not true and my Tracks are no longer recognized either.</p>
  <p>Flutter Error Messages.</p>
  <pre><code>error: The method 'MultiTrackTween' isn't defined for the type 'FadeIn'. (undefined_method at [orange_power] lib/component/FadeIn.dart:12)
  error: The method 'Track' isn't defined for the type 'FadeIn'. (undefined_method at [orange_power] lib/component/FadeIn.dart:13)
  error: The method 'Track' isn't defined for the type 'FadeIn'. (undefined_method at [orange_power] lib/component/FadeIn.dart:15)
  warning: The parameter 'builder' is required. (missing_required_param at [orange_power] lib/component/FadeIn.dart:20)
  error: The named parameter 'builderWithChild' isn't defined. (undefined_named_parameter at [orange_power] lib/component/FadeIn.dart:25)
  </code></pre>
  <p>I'm struggling to see what I need to change to meet the new syntax, any help would be most appreciated.</p>