等待给函数的回调的问题

Question

我有一个向服务器发送消息以获取答案的函数，如果答案为真，我希望我的应用程序向用户发送错误。问题是我无法等待我编写的 Fetch 函数中的回调。 这是向服务器发送问题的函数。

async donglePaired(){
    if (Platform.OS !=='ios'){
        var pairedDevices = await BluetoothScanner.getPairedDevices();
        console.log('Sending........');
        let data={
            data:pairedDevices,
        };
        new Api().fetch("bluetoothCheck",{devices:JSON.stringify(data),userid:this.state.probe.UID},(result) => {
            if (!result.err) return false;
            console.log("Dongle already paired");
            return true;
            //logNetworkState
        });

    }

}

这是我写的 Api.fetch 函数

fetch(action,data,cb){
    let url=Config.URL+"?version="+Config.VERSION+"&action="+action;
    let post="";
    let formData=new FormData();
    for(let k in data) formData.append(k,data[k]);
    for(let k in data) post+="&"+k+"="+encodeURIComponent(data[k]).replace(/%20/g,'+');
    console.log(url+post);
    console.log(url);
    if (data.batch) console.log(data.batch);
    let sending=true;
    fetch(url,{
        method: 'post',
        body: formData
    })
    .then(function(response){
        if (true) return response.json();
        let txt=response.text();
        console.log(txt);
        return JSON.parse(txt);
    })
    .then(
        (result)=>{
            if (!sending) return;
            sending=false;
            console.log(JSON.stringify(result));
            if (cb) cb(result);
        },
        (error)=>{
            if (!sending) return;
            sending=false;
            console.log("fetch error");
            console.log(error);
            if (cb) cb();
        }
    );
    setTimeout(()=>{
        console.log("http timeout")
        if (!sending) return console.log("nothing to abort");
        if (cb) cb();
    },Config.HTTP_TIMEOUT*1000)

}

}

这是我等待第一个函数 donglePaired 的主要代码，如果 donglePaired 返回 true，我会向用户发送错误。

let donglePaired = await this.props.app.donglePaired();
    if (donglePaired) return this.props.app.setError("ERR_DONGLE");

问题是程序不等待加密狗配对，尽管等待

Answer 1

你的代码不合适

let donglePaired = await this.props.app.donglePaired();
if (donglePaired) return this.props.app.setError("ERR_DONGLE");

除非是 Promise，否则异步函数无法正常返回值 请看下面我的简单演示！

async function test() {
  const result = await asyncRequest()
  return result
}

function asyncRequest() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve('success')
    }, 2000)
  })
}

test().then((data) => {
  console.log(data)
})

Answer 2

sn-ps 应该让你知道如何等待回调

发送到 API

async function remove_configuration(filename) {

const data = { filename };
const options = {
    method: 'POST',
    headers: {
        'Content-Type': 'application/json'
    },
    body: JSON.stringify(data)
};
    await fetch('/delete', options);

}

只是检索数据

async function display() {
    
        let response = await fetch('/get-available-configurations')
        let data = await response.json(); // JSON or Text what do you prefer
         // do something with data
}

Answer 3

一种可能性是删除async 并将其更改为：

   donglePaired() {
        return new Promise( function(resolve, reject) { 
            if (Platform.OS !=='ios'){
              var pairedDevices = await BluetoothScanner.getPairedDevices();
              console.log('Sending........');
              let data={
                  data:pairedDevices,
              };
              new Api().fetch("bluetoothCheck",{devices:JSON.stringify(data),userid:this.state.probe.UID},(result) => {
                  if (!result.err) reject(false);
                  console.log("Dongle already paired");
                  resolve(true);
                  //logNetworkState
              });
            }
            reject(false);
        });  
     }

还有：

this.props.app.donglePaired().then( (response) => { 
  // do something here, this will only run if the response is true
});

Answer 4

您可以使用超时函数返回 Promise.race()。

fetch(action, data, cb) {
  let url = Config.URL + "?version=" + Config.VERSION + "&action=" + action;
  let post = "";
  let formData = new FormData();
  for (let k in data) formData.append(k, data[k]);
  for (let k in data)
    post += "&" + k + "=" + encodeURIComponent(data[k]).replace(/%20/g, "+");
  console.log(url + post);
  console.log(url);
  if (data.batch) console.log(data.batch);
  let sending = true;
  return Promise.race([
    fetch(url, {
      method: "post",
      body: formData
    })
      .then(res => res.json())
      .then(result => {
        if (!sending) return;
        sending = false;
        return result;
      }),
    sleep(Config.HTTP_TIMEOUT * 1000)
  ]);
}

const sleep = ms => new Promise((_, rej) => setTimeout(rej("TIMEOUT"), ms));

它要么返回值，要么用TIMEOUT 拒绝，或者用 fetch 的错误拒绝

然后donglePaired 看起来像这样。我已经用try / catch 包裹了它

async donglePaired() {
  if (Platform.OS !== "ios") {
    var pairedDevices = await BluetoothScanner.getPairedDevices();
    console.log("Sending........");
    let data = {
      data: pairedDevices
    };
    try {
      let result = await new Api().fetch("bluetoothCheck", {
        devices: JSON.stringify(data),
        userid: this.state.probe.UID
      });

      if (!result.err) return false;
      console.log("Dongle already paired");
      return true;
      //logNetworkState
    } catch (err) {
      console.log(err);
    }
  }
}