KDAB contributions to Qt 5.4: qmllint

22.10.2014 Sérgio Martins 4 comments

One type of bug I see very often comes down to syntax errors in QML and JavaScript files. Most of the time these errors are simple typos; however, they creep in, go unnoticed by continuous integration and sometimes reach production — especially on delayed loaded components.

I like fixing bugs, but what I love even more is not fixing them, so I wrote a command line tool that can easily be integrated into our CI system so we can nip them in the bud at build time.

Enter qmllint: a command line QML syntax checker. Example usage:

$ qmllint ClickableImage.qml
ClickableImage.qml:17 : Expected token `numeric literal'

Verifying all files:

$ find -type f -name \*qml -exec qmllint \{\} +
./qml/SettingsPage.qml:35: Unexpected token `if'
./qml/AboutPage.qml:35: Expected a qualified name id

qmllint comes with Qt 5.4 and lives alongside other Qt tools.

Furthermore, I would also recommend using other linter tools in your project, such as json_verify or xmllint. Do not do manually what could be automated by the build system!

Caveats

qmllint cannot catch semantic errors. There are cases where QML files are syntactically correct but have other types of problems:

Bar.qml:1 TypeFoo is not a type
Bar.qml:2 module "QtQuickControls" is not installed
Bar.qml:6:9: QML Rectangle: Cannot anchor to an item that isn't a parent or
sibling.

There are no plans on my part to make this tool smarter, I like the “do one thing and do it well” philosophy, so it is finished. Checking for these kinds of mistakes is also very difficult, f.i. because we would need to depend on the QML context of a running application.

But don’t worry, my next blog post will address automating semantic validation, so stay tuned!