node.js - 循环异步写文件，如何管理流

Question

我正在尝试通过 Node Canvas 循环写入 100 多个 png 文件。仅生成 40 个文件，然后该过程完成。

我尝试通过 createPNGStream() 创建 png 流，并将结果通过管道传输到 fs.createWriteStream() 创建的写入流。

写入函数:

function writeFile(row, col) {
        // canvas code ...
        const stream = canvas.createPNGStream();
        let path = __dirname + '/imgs/heatmapRow' + row + "Col" + col + '.png';
        const out = fs.createWriteStream(path);
        stream.pipe(out);
        out.on('finish', () => console.log('The PNG file was created.'))
    }

调用函数:

function generateImages(){
    var numRows = 20;
    var numCols = 5;
    for(let row = 0; row < numRows; ++row) {
        for (let col = 0; col < numCols; ++col) {
            writeFile(row, col);
        }
    }
}

循环运行并完成，最后我一次得到一堆以下行:

The PNG file was created. The PNG file was created. The PNG file was created. The PNG file was created.

我认为在每个循环上都会创建一个 writeStream 并且是异步的。该进程正在终止，因为我只能打开这么多流。

如何异步写入所有文件以加快处理时间？ (我不喜欢同步写入文件)我需要将每个 writeFile 添加到队列中吗？如何确定我打开的流的数量限制以及如何管理它们？

Answer 1

您必须使用 Promise 来进行异步调用。我给你一个解决方案:

  function writeFile(row, col) {
    // canvas code ...
    const stream = canvas.createPNGStream();
    let path = __dirname + "/imgs/heatmapRow" + row + "Col" + col + ".png";

    return new Promise(resolve => {
      const out = fs.createWriteStream(path);
      stream.pipe(out);
      out.on("finish", () => {
        console.log("The PNG file was created.");
        resolve();
      });
    });
  }

  function generateImages() {
    var numRows = 20;
    var numCols = 5;
    var imagesToGenerate = [];
    for (let row = 0; row < numRows; ++row) {
      for (let col = 0; col < numCols; ++col) {
        imagesToGenerate.push(writeFile(row, col));
      }
    }

    Promise.all(imagesToGenerate).then(() => {
      console.log("All images generated");
    });
  }

看看Promise.all docs如果您不清楚它是如何工作的