How to read a file with JavaScript to WebAssembly?

How can I pass a File to be read within the WebAssembly memory context?

Reading a file in the browser with JavaScript is easy:

<input class="file-selector" type="file" id="files" name="files[]" />

I was able to bootstrap WebAssembly code written in Rust with the crate stdweb, add an event listener to the DOM element and fire up a FileReader:

let reader = FileReader::new();
let file_input_element: InputElement = document().query_selector(".file-selector").unwrap().unwrap().try_into().unwrap();
file_input_element.add_event_listener(enclose!( (reader, file_input_element) move |event: InputEvent| {
    // mystery part
}));

In JavaScript, I would get the file from the element and pass it to the reader, however, the API of stdweb needs the following signature:

pub fn read_as_array_buffer<T: IBlob>(&self, blob: &T) -> Result<(), TODO>

I have no idea how to implement IBlob and I am sure that I am missing something obvious either with the stdweb API or in my understanding of WebAssembly/Rust. I was hoping that there is something less verbose than converting stuff to UTF-8.

Solution

It works when the FileReader itself is passed from JavaScript to WebAssembly. It also seems like a clean approach because the data has to be read by the JavaScript API anyway - no need to call JS from WASM.

index.html

<!doctype html>
<html lang="en">
<head>
    <meta charset="utf-8">
    <title>Read to wasm</title>
</head>
<body>
<input type="file" id="file-input"/>
<script src="reader.js"></script>
<script>
    var fileReader = new FileReader();
    fileReader.onloadend = e => Rust.reader
            .then(reader=> {
                window.alert(reader.print_result(fileReader));
            });

    var fileInputElement = document.getElementById("file-input");
    fileInputElement.addEventListener("change", e => fileReader.readAsText(fileInputElement.files[0]));
</script>
</body>
</html>

main.rs

#![feature(proc_macro)]

#[macro_use]
extern crate stdweb;

use stdweb::js_export;
use stdweb::web::FileReader;
use stdweb::web::FileReaderResult;

#[js_export]
fn print_result(file_reader: FileReader) -> String {
    match file_reader.result() {
        Some(value) => match value {
            FileReaderResult::String(value) => value,
            _ => String::from("not a text"),
        }
        None => String::from("empty")
    }
}

fn main() {
    stdweb::initialize();

    stdweb::event_loop();
}