22.Future链式使用

import 'dart:io';

main(List<String> args) {

  print('start'); 

  Future((){

    sleep(Duration(seconds: 3));

    return "第1次的结果";

  }).then((res){

    print(res);

    sleep(Duration(seconds: 3));

    return "第2次的结果";

  })..then((res){

    print(res);

    sleep(Duration(seconds: 3));

    return "第3次的结果";

  }).then((res){

    print(res);

    sleep(Duration(seconds: 3));

    return "第4次的结果";

  }).then((res){

    print(res);

  }).catchError((error) {

    print("$error");

  }).whenComplete((){

    print("全部执行结束...");

  });

  print("end");

}