Checkbox: checked property can not be assigned to root property

[JaberwockySeamonstah]

Bug Description

I try to connect the checked property of a CheckBox to a variable in the root of my component. Despite the value of checked changing, my variable always shows false.

Reproducible Code (if applicable)

in-out property <bool> append_project_elements;
CheckBox {
    text:    "Append elements from project";
    checked: root.append_project_elements;
    toggled() => {
        debug("CheckBox checked: ", self.checked);
        debug("CheckBox checked: ", root.append_project_elements);
        // "root.append_project_elements" always shows "false"
    }
}

Environment Details

• Slint Version: 1.9.2 (Have not been brave enough to update)
• Platform/OS: Linux, KDE, Wayland
• Programming Language: Rust
• Backend/Renderer: No idea, probably the default (How do I know this?!)

Product Impact

I'm building an open-source application with slint, it is my first slint project. This issue is a minor one, because there are simple workarounds.

[FloVanGH]

Good Morning,

thank you for the question. To get this working you need a two way binding <=>. What you have used is a one way binding : and works only in one direction.

A short example

import { CheckBox, VerticalBox } from "std-widgets.slint";

export component Demo {
    property <bool> checked;
    
    VerticalBox {
        alignment: start;
       
        CheckBox {
            text: "Check me";
            checked <=> root.checked;

            toggled => {
                debug.text = "";
                debug.text += self.checked ? "s.c: true; " : "s.c: false";
                debug.text += root.checked ? "r.c: true; " : "r.c: false";
            }
        }

        debug := Text {}
    }
}

If you want to know more about two-way bindings this is the link to the documentation https://docs.slint.dev/latest/docs/slint/guide/language/concepts/reactivity/#two-way-bindings

[JaberwockySeamonstah]

Hey again,

Thank you for explaining this as well! I had no idea it was possible to use <=> in this context.
I mostly used LineEdits where the use of : was enough to get this to work.

Thank you

[FloVanGH]

I'm glad I could help

[FloVanGH]

I have moved the issue in discussions because it's not a bug.