[Development] How to skip write property in qml's property binding?

[JiDe Zhang]

Thank you, I need

Text {
     Binding on text {  value: fooObject.title; when: fooObject }
}

But It's troublesome to write like this, I think the Binding is superfluous.

Just like how, when detecting that the value is `undefined`, it tries to reset the property (if it has a corresponding reset function) or reports "Unable to assign [undefined] to QString," could we add a special value that, when detected, does nothing? This would be less costly than using Binding to achieve this effect, and we are already doing something similar for the special value `undefined`.
________________________________
From: Fabian Kosmale <fabian.kosmale at qt.io>
Sent: Friday, July 19, 2024 14:37
To: JiDe Zhang <zccrs at live.com>; Qt邮件列表 <development at qt-project.org>
Subject: AW: How to skip write property in qml's property binding?

Hi,

as always, it depends. If  you really want to set a binding based on a condition, but otherwise leave the value as it is, you want to use

Text {
     Binding on text {  value: fooObject.title; when: fooObject }
}
Compare https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdoc.qt.io%2Fqt-6%2Fqml-qtqml-binding.html%23conditional-bindings&data=05%7C02%7C%7Ceb6b834d4f2d4f8c493808dca7bd4e7c%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C638569678723442236%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C0%7C%7C%7C&sdata=6FLxZhLwF96F13r%2Bdm%2BwPXi4G5w4chdANcCyky8n1yg%3D&reserved=0<https://doc.qt.io/qt-6/qml-qtqml-binding.html#conditional-bindings>. Set restoreMode depending on your needs.

If you don't need an actual binding, but only the value, then you can use

Text {
     id: mytext
     Component.onCompleted: {  if (fooObject) mytext.text = fooObject.name }
}
if the nullishness of fooObject never changes. If it can change, you need to react to it with a Connection element,  a connection, or a property change handler
(depending on where the object is defined/how it is exposed to QML) or you again fall back to Binding.

An alternative would be to take a step back, and  to consider why you don't want to have the write to begin with. If you want to preserve a default value, you
might want to consider exposing that one, too, so that you can write:

// MyText.qml
Text { property string defaultText: "some value"; text: defaultTtext }

// Usage.qml
MyText { id: myText; text: fooObject?.title ?? myText.defaultText }

This of course assumes that you have control over the item setting the default value.

Kind regards,
Fabian Kosmale

________________________________________
Von: Development <development-bounces at qt-project.org> im Auftrag von JiDe Zhang <zccrs at live.com>
Gesendet: Freitag, 19. Juli 2024 07:31
An: Qt邮件列表
Betreff: [Development] How to skip write property in qml's property binding?

For an example:

Text {
    text: fooObject?.title
}

When the fooObject is null, will got a qml error "Unable to assign [undefined] to QString".

I don't want the error, so, how to skip write value for "text" property when fooObject is null?

Text {
    text: fooObject?.title ?? text
}

This is a workaround, but it is a magic, maybe will got a cycle binding?

Text {
    text: fooObject?.title ?? bypass
    objectName: {
       if (fooObject)
            return fooObject.name
       bypass
    }
}

This syntax will more easily understand.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.qt-project.org/pipermail/development/attachments/20240719/e330e4c1/attachment-0001.htm>