完成可写流

Question

I am using gulp to process, minify and bundle my css files (and js too). But I don't want to write file immediately. I want to check if md5 sum of the processed content is not equal to md5 sum of the existing version of bundled file and write only in this case.

Rather then write file immediately I try to get is as string from the process.

But the construction doesn't work.

const Stream = require('stream');
const writableStream = new Stream.Writable({ objectMode: true });

var file = "";

writableStream._write = (chunk, encoding, next) => {
    file += chunk.contents.toString();
    next();
}

let bundlePipe = gulp.src([...vendorLinks.map(e => path.resolve(htmlPath + e)), ...resourceLinks.map(e => path.resolve(htmlPath + e))], { sourcemaps: true })
    .pipe(postcss([cssnext, cssnano]))
    .pipe(concat("style.bundle.min.css"))
    .pipe(writableStream);
    //.pipe(gulp.dest(path.resolve(paths["html/static/css"]))); 


bundlePipe.on("end", () => {
    // It never goes there!
    console.log("get file contents", file); 
}

Seems writableStream which I create doesn't know that all data was already passed to it and never issues end signal.

What can I do so that writableStream close after it get all data from pipe?

Answer 1

Writable streams don't emit end , only readable streams emit this kind of event.

The equivalent on a writable stream is finish . You can find the details on this event in the documentation of Node.js.

The reason why readable and writable streams have different events to signal that they have ended / finished is that there are also hybrid streams (such as the DuplexStream ) which consist of a readable stream and a writable stream at the same time, and hence you need to be able to differ between one and the other type of event.