等待而不将功能声明为异步 [英] await without declaring function as async

问题描述

实际上，在Dart中，为了在函数体中使用await，需要将整个函数声明为async:

Actually in Dart, in order to use await in function body, one need to declare whole function as async:

import "dart:async";

void main()  async {
  var x = await funcTwo();
  print(x);  
}

funcTwo() async {
  return 42;
}

如果不将main()标记为async

Error: Unexpected token 'await'. 

但是，医生说"await表达式计算e，然后挂起当前正在运行的函数，直到结果准备好，即直到Future完成为止"( Dart语言异步支持)

But, doc says "The await expressions evaluates e, and then suspends the currently running function until the result is ready–that is, until the Future has completed" (Dart Language Asynchrony Support)

那么，也许我错过了一些东西，但是没有必要强制功能异步吗?使异步声明成为必需的理由是什么?

So, maybe I miss something, but there is no need to force function to be asynchronous? What is a rationale for making async declaration obligatory ?

推荐答案

在async函数中，await被重写为使用.then(...)而不是await的代码.

In async functions await is rewritten to code where .then(...) is used instead of await.

async修饰符将这样的功能标记为必须重写且支持await的功能.

The async modifier marks such a function as one that has to be rewritten and with that await is supported.

没有async，您将不得不写

void main() {
  return funcTwo().then((x) {
    print(x);  
  });
}

这是一个非常简单的示例，但是当使用更多异步功能(例如try/catch，await for(...)，...

This is a very simple example but the rewriting can be rather complex when more of the async features are uses, like try/catch, await for(...), ...

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