Trigger download from the backend

Trigger the download of a file after the process that generates it is finished.

module App
using GenieFramework

@app begin
    @out linkhref = "/data.txt"
    @in dfile = false
    @onbutton dfile begin
        # do some processing and write to a file
        write("./public/", string(randn(10)))
        # change the link url to trigger the file download script. The URL will still point to
        # the same file, but the link href will have a different anchor after the #
        linkhref = "/$(rand(1)))"

# Define a script that will watch the href attribute in the link, and click it when it changes.
# This @mounted block will be run when the page is loaded in the browser. The script is activated
# with a 2 second delay to allow the browser to render the link first
@mounted """
    setTimeout(() => {
        var linkToWatch = document.getElementById('dynamicLink');
        if (!linkToWatch) {
            console.error('Link element not found');

        var observer = new MutationObserver(function(mutations) {
            mutations.forEach(function(mutation) {
                if (mutation.type === 'attributes' && mutation.attributeName === 'href') {
                    console.log('Href changed to:', linkToWatch.href);

        observer.observe(linkToWatch, { attributes: true });
    }, 2000); // Delay in milliseconds

ui() = [
    # "dynamicLink" is the id of the link to be watched by the script
    # var":href" is set so that the href attribute takes its value from a reactive variable
    a(id="dynamicLink", var":href"="linkhref", "Dynamic Link"),
    btn("Download", @click(:dfile), loading=:dfile)

@page("/", ui)