This is a text-only version of the following page on https://raymii.org:
---
Title       : 	Qt/QML Property bindings break after a JavaScript assignment
Author      : 	Remy van Elst
Date        : 	19-05-2022 05:30
URL         : 	https://raymii.org/s/articles/Qt_QML_Property_bindings_break_after_javascript_assignment.html
Format      : 	Markdown/HTML
---



Property bindings are one of the most powerful concepts in Qt/QML. Property bindings let you specify relationships between different object properties. When a properties dependencies change in value, the property is automatically updated according to the specified relationship. The QML engine monitors the properties dependencies (that is, the variables in the binding expression). When a change is detected, the QML engine re-evaluates the binding expression and applies the new result to the property.
One little known caveat with property bindings is that they break after a static JavaScript assignment (`property = value`). This post shows you the different behaviors and how to use `Qt.binding()` to assign property bindings via JavaScript.


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Do note that this behavior is intended, [documented][2] and can be useful in
situations where you want to purposely want to break a property binding. 

This post is intended to inform you of the different behaviors, as it can be
confusing if you're unaware of what is happening. A colleague of mine was
working on a bug, he was not aware of this behavior and it cost him two hours
to figure out that it was the underlying issue. Could happen to me as well
when you're knee-deep into a debugging session.

### Demonstration

![demo][1]

The image above demonstrates the issue.

The top `Button` (`id: boundButton`) `text` property is bound to the
`TextField` (`id: bindableTextField`) `text` property, so when you edit the
text, the text on the button updates automatically.

The second button breaks the binding by doing a JavaScript assignment in it's
`onClicked` function:

    onClicked: boundButton.text = "Binding broken"

If you've clicked the button and then change the text in the `TextField`,
the button's text no longer reflects the text you've typed. 

The last button restores the property binding using the `Qt.binding()` function:

    onClicked: boundButton.text = Qt.binding(function () {
                return bindableTextField.text
            })
        }

A bit convoluted syntax wise, but it gets the job done. 

### Qt.binding()

The [documentation][3] states the following:

> Returns a JavaScript object representing a property binding, with a function
  that evaluates the binding. There are two main use-cases for the function:
  firstly, to apply a property binding imperatively from JavaScript code and
  secondly, to apply a property binding when initializing property values of
  dynamically constructed objects (via `Component.createObject()` or
  `Loader.setSource()`).


Confusingly, that page has examples for the second use case, not the first. 
Simpler examples are on [the other doc page][2], transcribed below:

The Rectangle below initially ensures that its height is always twice its
`width`. However, when the space key is pressed, the current value of
`width*3` will be **assigned to height as a static value**. After that, the
height will remain fixed at this value, even if the width changes. The
assignment of the static value removes the binding.

    Rectangle {
        width: 100
        height: width * 2

        focus: true
        Keys.onSpacePressed: {
            height = width * 3
        }
    }

If the intention is to give the rectangle a fixed height and stop automatic
updates, then this is not a problem. However, if the intention is to
establish a new relationship between width and height, then the new binding
expression must be wrapped in the `Qt.binding()` function instead:

    Rectangle {
        width: 100
        height: width * 2

        focus: true
        Keys.onSpacePressed: {
            height = Qt.binding(function() { return width * 3 })
        }
    }

Now, after the space key is pressed, the rectangle's height will continue
auto-updating to always be three times its width.

---

The other use case is, in my experience, not used often, or at least not in
the projects I've worked on. Dynamically creating QML objects is fun at
first, but you loose type safety and it's all strings, so changing one name
will work fine at first, until you get runtime errors somewhere else. Maybe
I'm spoiled by the compile-time safety, but such a small typo or rename has
caught me more than I'd like to admit. Whereas, in C++ it'll just won't
compile anymore.



[1]: /s/inc/img/qt-binding.gif
[2]: https://web.archive.org/web/20220516083722/https://doc.qt.io/qt-5/qtqml-syntax-propertybinding.html
[3]: https://web.archive.org/web/20220516084540/https://doc.qt.io/qt-5/qml-qtqml-qt.html#binding-method






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