Beru

I’ve download an entirely new development environment (lots of fun over a 3G cellular connection!) and I’m ready to learn QML. The most basic thing I have to work out is how to open a file for reading. Let’s find out how.

That’s odd—there’s nothing about opening files in the Ubuntu SDK. Let’s check the QML docs. Nothing there. Let’s ask Google. Here we go:

QML has no built-in file I/O.

Wat?

A poster points out that this isn’t entirely true. If you just want to read a file, you can use XMLHttpRequest. Let’s go ahead and try it:

var request = new XMLHttpRequest()
request.open('GET', 'test.txt', false)
console.log(request.send())

Since the request is just going to the filesystem, there’s no need to make an asynchronous request, so we can avoid messing with callbacks.

Error: Synchronous XMLHttpRequest calls are not supported

Or not.

To be fair, the QML docs do point out that synchronous requests are not supported. But other than that, it supports the W3C standard.

[Aside: Take a look at the breadcrumbs at the top of that doc page. I bet you think if you click on the “qtqml” link one level back, you’ll get taken to the QML docs for Qt5.0. Of course not, you silly goose—you get taken to the top level of the Qt5.1 docs! This makes navigating the site extra fun.]

Okay, we’ll do it with a callback. In anticipation of opening epub files, we’ll set the responseType so that we can load a binary file.

var request = new XMLHttpRequest()
request.open('GET', 'test.txt')
request.responseType = 'arraybuffer'
request.onload = function (event) {
    console.log(request.response)
}
request.send()

This doesn’t work either. The function is never called, so we switch back to the older onreadystatechange event. But response stays undefined. It’s looking like XMLHttpRequest2 isn’t actually supported, despite the docs referencing it. Back to version 1 we go, which means that we won’t be able to use an array buffer.

var request = new XMLHttpRequest()
request.open('GET', 'test.txt')
request.overrideMimeType('text/plain; charset=x-user-defined')
request.onreadystatechange = function(event) {
    if (request.readyState == XMLHttpRequest.DONE) {
        console.log(request.responseText)
    }
}
request.send()

TypeError: Object [object Object] has no method 'overrideMimeType'

Gah! Oh, and don’t bother to check status. That’s 0 regardless of success or failure. So I guess that list of exceptions to the standard in the docs wasn’t meant to be exhaustive. Luckily, some kind soul submitted a bug against this documentation. Two years ago. I’m sure it’ll be fixed Real Soon Now.

Summing up

If you want to open a text file in QML, you can, but you need to handle it in a callback set on an XMLHttpRequest. As best I can tell, this is the simplest way to do it:

var request = new XMLHttpRequest()
request.open('GET', 'test.txt')
request.onreadystatechange = function(event) {
    if (request.readyState == XMLHttpRequest.DONE) {
        process_file(request.responseText)
    }
}
request.send()

In comparison:

with file('test.txt', 'r') as f:
    process_file(f.read())

I’m reminded of INTERCAL.

For example, the easiest way to store the value of 65536 in a 32-bit INTERCAL variable is: > DO:1<-#0$#256 If that’s the easiest way, imagine the power at your fingertips if you’re trying to be deliberately obscure!

For completeness, let’s also give listings for binary files.

with file('test.txt', 'rb') as f:
    process_file(f.read())

// Hah!

I guess this is that convergence thing everyone keeps talking about.