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");
}