lottie-flutter/example/lib/examples/custom_load.dart

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:flutter/widgets.dart';
import 'package:lottie/lottie.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: MyWidget(),
      ),
    );
  }
}

//--- example
class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  Future<LottieComposition> _composition;

  @override
  void initState() {
    super.initState();

    _composition = _loadComposition();
  }

  Future<LottieComposition> _loadComposition() async {
    var assetData = await rootBundle.load('assets/LottieLogo1.json');
    return await LottieComposition.fromByteData(assetData);
  }

  @override
  Widget build(BuildContext context) {
    return FutureBuilder<LottieComposition>(
      future: _composition,
      builder: (context, snapshot) {
        var composition = snapshot.data;
        if (composition != null) {
          return Lottie(composition: composition);
        } else {
          return Center(child: CircularProgressIndicator());
        }
      },
    );
  }
}
//---