Qtilities: Custom property types for the property browsers?

Question

I'm writing a program that requires the user to be very flexible in manipulating data on a given object. I figured I would use a property browser of some kind; Qtilities' ObjectDynamicPropertyBrowser caught my eye.

However, I need to be able to add my own data types. The documentation is not clear on how to do so.

How can I allow my own data types to be represented in Qtilities' property browser widgets?

Also, more about my needs:

• The data types are not part of Qt, nor are they even Q_OBJECTs.
• Qt-specific modifications to the relevant classes are not an option.
• Declaring the relevant classes via Q_DECLARE_METATYPE is okay.
• In particular, I need to represent vector and matrix types (and possibly more later).

Answer 1

The browser you refer to depends on the QObject property system. So, unless your classes are QObjects, it won't work - but don't despair, Qt 5.5 to the rescue (read on). The browser seems to use a QTreeView and provides an adapter model that exposes the QObject property system. So, it leverages Qt's type and delegate system.

In Qt 5.5, there is a general purpose property system, known as gadgets, that can be used on any class, as long as there is a QMetaObject describing that class. By adding the Q_GADGET macro to a class deriving from the subject class, and describing the properties using Q_PROPERTY macro, you can leverage moc and the gadget system to access your unmodified types' properties.

The only reason you'd do that is to require minimal changes to the ObjectPropertyBrowser system. You don't want the ObjectDynamicPropertyBrowser, since it works on dynamic properties, and your objects don't have any. They have static properties, given through Q_PROPERTY macros and code generated by moc.

So, you'll proceed as you would for implementing your own type support for QVariant and views in general. You also need Qt 5.5, since you need gadget support for it to work. A solution for Qt 5.4 and below requires a different approach and might be less cumbersome to implement in another way.

See this answer for a reference on using gadget property system for object serialization, it's fundamentally what the property browser would do, sans the serialization proper of course.

There are three steps. First, you need to address simple custom types that don't have a structure, but represent a single value (such as a date, or time, or geographic position, etc.), or a collection of simple values (such as a matrix).

1. Ensure that QVariant can carry the simple types. Add the Q_DECLARE_METATYPE macro right after the type's definition in an interface (header file).

2. Implement delegates for the types. For types with table structure, such as a matrix, you can leverage QTableView and provide an adaptor model that exposes the type's contents as a table model.

Secondly, you get to your complex types that have internal structure:

1. Create a wrapper class that derives from the complex type, declares all properties using Q_PROPERTY, and has the Q_GADGET macro (not Q_OBJECT since they are not QObjects). Such class should not have any members of its own. Its only methods should be optional property accessors. The Q_GADGET macro adds the static (class) member staticMetaObject.

2. The underlying type can be static_cast to the wrapper class if needed, but that's normally not necessary.

3. At this point, any class that you wrote a wrapper for is accessible to the QMetaProperty system directly, without casting! You'd use the wrapper's staticMetaObject for its static metaobject, but the QMetaProperty readOnGadget and writeOnGadget will take the pointers to the base class directly.

Thirdly, since ObjectPropertyBrowser most likely doesn't implement the support for gadgets in Qt 5.5, as that's quite new, you'll have to modify it to provide such support. The changes will be minimal and have to do with using QMetaProperty::readOnGadget and QMetaProperty::writeOnGadget instead of QMetaProperty::read and QMetaProperty::write. See the serialization answer for comparison between the two.