如何在回圈中等待方法的執行結束？-有解無憂

如您所見，我有一個 js，它采用 .csv 并為每一行呼叫一個異步函式（4 個不同的迭代函式）。

問題是在進行 i 1 迭代之前，我需要等待第 i 次迭代中的函式結束。

const csv = require('csv-parser');
const fs = require('fs');

var i=1;

fs.createReadStream('table.csv')
  .pipe(csv())
  .on('data', (row) => {
      switch(i%4){
          case 1: org1createPatient(row.patientId, row.FirstName, row.LastName, row.Age, row.Sex, row.ChestPainType, row.RestingBP, row.Cholesterol, row.FastingBS, row.RestingECG, row.MaxHR, row.ExerciseAngina, row.Oldpeak, row.ST_Slope, row.HeartDisease); break;
          case 2: org2createPatient(row.patientId, row.FirstName, row.LastName, row.Age, row.Sex, row.ChestPainType, row.RestingBP, row.Cholesterol, row.FastingBS, row.RestingECG, row.MaxHR, row.ExerciseAngina, row.Oldpeak, row.ST_Slope, row.HeartDisease); break;
          case 3: org3createPatient(row.patientId, row.FirstName, row.LastName, row.Age, row.Sex, row.ChestPainType, row.RestingBP, row.Cholesterol, row.FastingBS, row.RestingECG, row.MaxHR, row.ExerciseAngina, row.Oldpeak, row.ST_Slope, row.HeartDisease); break;
          case 0: org4createPatient(row.patientId, row.FirstName, row.LastName, row.Age, row.Sex, row.ChestPainType, row.RestingBP, row.Cholesterol, row.FastingBS, row.RestingECG, row.MaxHR, row.ExerciseAngina, row.Oldpeak, row.ST_Slope, row.HeartDisease); break;
      }
    i  ;
  })
  .on('end', () => {
    console.log('CSV file successfully processed');
  });





  async function org1createPatient(patientId, FirstName, LastName, Age, Sex, ChestPainType, RestingBP, Cholesterol, FastingBS, RestingECG, MaxHR, ExerciseAngina, Oldpeak, ST_Slope, HeartDisease) {
    ...
  }

  async function org2createPatient( patientId, FirstName, LastName, Age, Sex, ChestPainType, RestingBP, Cholesterol, FastingBS, RestingECG, MaxHR, ExerciseAngina, Oldpeak, ST_Slope, HeartDisease) {
    ...
  }

  async function org3createPatient( patientId, FirstName, LastName, Age, Sex, ChestPainType, RestingBP, Cholesterol, FastingBS, RestingECG, MaxHR, ExerciseAngina, Oldpeak, ST_Slope, HeartDisease) {
   ...
  }

  async function org4createPatient( patientId, FirstName, LastName, Age, Sex, ChestPainType, RestingBP, Cholesterol, FastingBS, RestingECG, MaxHR, ExerciseAngina, Oldpeak, ST_Slope, HeartDisease) {
   ...
  }

我怎樣才能得到我想要的？希望我的問題足夠清楚！

uj5u.com熱心網友回復：

在readStream您使用的是這里是異步的，也就是說.on(event, callback)將觸發每一個新的資料塊被讀取時，獨立于任何的callback觸發。換句話說，callback這里函式的執行不影響這個程序，它會并行運行，每次event接收。

這意味著，如果callback要執行一段異步代碼，您很可能會遇到這樣的情況：在event接收到下一次讀取時，此函式的多個實體可能仍在運行。

注意：這適用于任何事件，包括'end'事件。

如果使用async/awaiton callbackif 只會使該函式的內部邏輯同步。它仍然不會影響讀取資料的速率。

為了做到這一點，你會希望同時使用async/await上callback（使其內部同步），并callback手動暫停和恢復讀操作并行發生的。

const csv = require('csv-parser');
const fs = require('fs');

let i = 1;

const stream = fs.createReadStream('table.csv').pipe(csv());

stream.on('data', async (row) => {
   // pause overall stream until this row is processed
   stream.pause();

   // process row
   switch (i%4){
      case 1: await org1createPatient(row.patientId, row.FirstName, row.LastName, row.Age, row.Sex, row.ChestPainType, row.RestingBP, row.Cholesterol, row.FastingBS, row.RestingECG, row.MaxHR, row.ExerciseAngina, row.Oldpeak, row.ST_Slope, row.HeartDisease); break;
      case 2: await org2createPatient(row.patientId, row.FirstName, row.LastName, row.Age, row.Sex, row.ChestPainType, row.RestingBP, row.Cholesterol, row.FastingBS, row.RestingECG, row.MaxHR, row.ExerciseAngina, row.Oldpeak, row.ST_Slope, row.HeartDisease); break;
      case 3: await org3createPatient(row.patientId, row.FirstName, row.LastName, row.Age, row.Sex, row.ChestPainType, row.RestingBP, row.Cholesterol, row.FastingBS, row.RestingECG, row.MaxHR, row.ExerciseAngina, row.Oldpeak, row.ST_Slope, row.HeartDisease); break;
      case 0: await org4createPatient(row.patientId, row.FirstName, row.LastName, row.Age, row.Sex, row.ChestPainType, row.RestingBP, row.Cholesterol, row.FastingBS, row.RestingECG, row.MaxHR, row.ExerciseAngina, row.Oldpeak, row.ST_Slope, row.HeartDisease); break;
   }
   i  ;

   // resume overall stream
   stream.resume();
});

stream.on('end', () => {
  // now guaranteed that no instances of `callback` is still running in parallel when this event is fired
  console.log('CSV file successfully processed');
});

uj5u.com熱心網友回復：

下面的解決方案是使用iter-ops庫，在這種情況下它非常有效，因為pipe(csv())回傳一個AsyncIterable，因此應該對其進行相應處理。

由于您不關心這些處理函式回傳什么，我們可以限制每一行的處理：

const {pipe, throttle, onEnd} = require('iter-ops');
const csv = require('csv-parser');
const fs = require('fs');

const asyncIterable = fs.createReadStream('table.csv').pipe(csv());

const i = pipe(
    asyncIterable,
    throttle(async (row, index) => {
        switch (index % 4) {
            case 1: await org1createPatient(row.patientId, ...); break;
            case 2: await org2createPatient(row.patientId, ...); break;
            case 3: await org3createPatient(row.patientId, ...); break;
            case 0: await org4createPatient(row.patientId, ...); break;
            default: break;
        }
    }),
    onEnd(s => {
        console.log(`Finished in ${s.duration}ms`);
    })
);

async function processCSV() {
    // this will trigger the iteration:
    for await(const a of i) {
        // iterating through entire CSV
    }
}

PS 我是iter-ops的作者。

轉載請註明出處，本文鏈接：https://www.uj5u.com/qiye/387118.html

標籤：javascript 节点.js 循环异步等待

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