nodejs 中关于阻塞的疑惑

[Sirormy]

上传使用了 formidable, 有些处理用到了 listener, 那么问题来了, 监听是异步的, 如何中断返回结果呢. 看下代码

var received = 0;
form.on('progress', function(bytesReceived, bytesExpected) {
    received += bytesReceived;
    if (received > 1024*1024) {
    	res.json({err: "图片过大"})
    	return;
    }
});

...

form.parse(req, function(err, fields, files) {
    var file = files.icon;
    res.json({url: file.path.replace('public', '')})
});

这个写法是错误的, 会报 Can't set headers after they are sent. 应该如何改进呢

[broadliyn]

检查 http body 的大小不是应该先去检查 http 请求头的 content-length 大小么？

[Arrowing]

不太明白你为什么这样写
因为执行了 2 次 res.json
可以加个标识变量

```
var isSended = false;
var received = 0;
form.on('progress', function(bytesReceived, bytesExpected) {
received += bytesReceived;
if (!isSended && received > 1024*1024) {
res.json({err: "图片过大"})
return;
}
});
form.parse(req, function(err, fields, files) {
var file = files.icon;
!isSended && res.json({url: file.path.replace('public', '')})
});
```

[Arrowing]

忘记加上更改了 0.0
res.json 后面加上 isSended = true;

[Sirormy]

@broadliyn 对 Http 请求没搞明白, 先这么写，再慢慢改，这个不是主要的问题

[Sirormy]

@Arrowing 你那个写法不行，没法中断程序的进行，后边是执行写文件的操作，我那样做是先判断文件大小，要阻止后边的代码

[magicdawn]

不用自己立标志变量...

res.headerSent https://nodejs.org/dist/latest-v6.x/docs/api/http.html#http_response_headerssent