New in Qt 5.10: Diagnostics when breaking QML bindings

Property bindings are one of the most interesting features of the QML language. In QML, when we set a value on a property, the right hand side expression isn't evaluated just once to produce a value, like in a ordinary imperative language.

In particular, if the expression involves other properties, then the property we're setting becomes bound to the properties in the expression: whenever they change their values, then the expression is automatically evaluated again, and the target property value updated.

For instance:

In the snippet above, the color property of the Rectangle is the target of a binding: it is bound to the containsMouse property of the internal MouseArea. When the mouse moves inside or outside of the mouse area, its containsMouse property will change, causing the expression for color to be re-evaluated.

In layman's terms: the rectangle will be red if the mouse cursor is hovering over it, and blue otherwise:

Property bindings allow to build our UIs in a very declarative way: we don't need to write boilerplate "slots" to update our UI elements when some other property changes value. The expression that we can write on the right hand side can be as complex as we want, even involving function calls; the engine will do all the work for us, and will make the binding work as expected.

Breaking property bindings

Sounds too good to be true? Well, property bindings have a limitation: if we use an imperative assignment to set a value on a property, its binding will be lost. For instance, suppose we want the rectangle to turn green when we click over it. This is a possible implementation:

In the onClicked signal handler we set the color property to green using an ordinary assignment from imperative JavaScript code. With this code, if we click on the rectangle, and then move the mouse over it and outside it, we'll see that the rectangle doesn't change color any more -- it stays green. This happens because the imperative assignment destroyed the binding, and set the color property to one specific value.

Are broken bindings a problem?

In general, the above example is 100% correct QML code, following the documented QML semantics. Therefore, one cannot claim that such code should be rejected by the QML engine or unconditionally raise warnings.

However, in my experience at KDAB, I have never encountered one single occasion when breaking a binding was intended by the developer. Broken bindings were always caused by accident (during refactorings, not realizing that the original property was bound to something, etc.), or by excessive usage of JavaScript to manage state, instead of using more declarative approaches.

On some occasions, a silent overwriting of a binding with a value did actually introduce a runtime bug, which would then stay latent, maybe for a long time. Debugging such issues could be very challenging, as it is not normally clear what the problem is when reading the QML source code -- the bug would manifest itself only when the JavaScript code containing an imperative assignment is run.

Debugging broken bindings

In order for a developer to be able to more easily track down these cases, KDAB contributed a new feature to the QML engine that will appear in Qt 5.10: the QML engine can now print debugging information whenever a binding is broken.

In order to enable the debug output you just need to enable output for the qt.qml.binding.removal logging category, for instance by exporting the QT_LOGGING_RULES environment variable:

With this variable set, the above example:

now prints this debug message:

As you can see, the message tells us which property was bound, where the binding had been established, and where the imperative statement is that breaks the binding. Pretty much everything we need to debug any issues with broken bindings!