[Development] QProperty and library coding guide

Dominik Holland dominik.holland at qt.io
Sat Jul 18 10:21:15 CEST 2020

Am 17.07.20 um 21:35 schrieb Ulf Hermann:

>> Mhh, it there are no change signals anymore, i guess i also can't use
>> QObject::connect() anymore and rather use a binding instead ? This is
>> fine, but if two objects are living in different threads how does it
>> work then ? With a Queued connection how it works with signals/slots is
>> pretty clear, but in which thread is the binding evaluated ?
> Binding properties from different threads is not supported. Bindings
> should be lightweight. Usually they'd be used for closely related bits
> of data. Those rarely live in different threads. Building thread
> synchronization into bindings would make them more expensive for
> little gain.
> You can, however, still use queued signals in this scenario. QProperty
> has a subscribe() method, and if the change of a property is actually
> something you want to communicate across thread boundaries, you can
> attach a function that sends a signal to a QProperty via subscribe().
> There is also QNotifiedProperty. I would advise against all of this,
> though. Sending the signal forces the property and everything it
> depends on to be eagerly evaluated. There likely are other ways to
> organize your thread synchronization.
ok, i guess we need a example for this then. Sure properties are not
something you use across threads very often, but sometimes you still do
and the old way works perfectly fine as you just connect the signals. If
there is a way on how it can be done, even if this is not as convenient
as connecting the signals, but has other benefits like the lazy
evaluation, if this has a good documentation, then we are good.

We just need to make sure to be better than the QML documentation in the
first days, which was really horrible...
>> With the old properties it's also possible to define a custom behavior
>> for the setter and getter. E.g. for the getter to not emit the changed
>> signal, but just call a worker thread to to this and wait for the
>> feedback to do the actual change and emit the change signal. I guess all
>> this is not possibly anymore with the new QProperty ? If we define that
>> behavior like this can not be done with QProperty but the old way should
>> be used instead everything is fine, but doesn't make the API
>> inconsistent ?
> You get value guards with QNotifiedProperty, and soon also with
> QProperty: https://codereview.qt-project.org/c/qt/qtbase/+/307894
> With those, you can introspect and/or modify a new value before it is
> set. You can do the additional work in a value guard, and you can even
> reject values that way. Using a value guard does not even force eager
> evaluation.
great, sounds exactly what i would need ;-)
>> As I understood it, we don't want to break SC too much so we will keep
>> all setters/getters in our API.
>> Let's say we create a new type derived from QQuickItem and use a
>> QProperty there, let's name this property "foo".
>> Doesn't it mean we now have a class, which has functions like setWidth
>> and setHeight, but not a function for setFoo ?
> Q_PRIVATE_QPROPERTY generates a setter, setFoo in this case, that just
> calls the setValue() function of the propery. If you add the property
> directly in the public object, you can do the same manually.



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