Need help understanding the difference between one-way and two-way bindings.

[coderedart]

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

export component Demo inherits VerticalBox {
    alignment: start;
    property <float> foo;
    a := Slider {
        minimum: 0;
        maximum: 100;
        value <=> foo;
        changed value => {
            debug("a: ", self.value)
        }
    }
    changed foo => {
        debug("foo", foo)
    }
    b := Slider {
        minimum: 0;
        maximum: 100;
        value: foo;
        changed value => {
            debug("b: ", self.value)
        }
    }
}

Load this snippet on to slintpad and follow the steps:

1. move slider a
2. move slider b
3. move slider a again.

I have two questions:

1. when you move slider a, it doesn't debug a. It only debugs foo and b. why?
2. After moving slider b once, moving slider a will assign to foo, but that won't change b.value. Slider b stops following a and when moving a, we don't see debug printing of b anymore. what happened to the value: foo binding in b?

[ogoffart]

Basically, the two way binding means the two properties are kind of "merged" in one: change to one impact the other and vice-versa.
But normal binding only means that value: foo is the initial value, and changes in foo are tracked, until value is set externally. So the binding is kind of the initial value, but once the slider is changed, that value is overwritten, and the binding is "broken".

[coderedart]

1. why is changed not triggering for a.value? Is it because a.value and foo are the same, which causes only foo changed to trigger?
2. So, is changing slider b like setting b.value = 5?

[ogoffart]

is changing slider b like setting b.value = 5?

Yes, exactly that.

why is changed not triggering for a.value? Is it because a.value and foo are the same, which causes only foo changed to trigger?

That is really strange. I thin there is a bug there. Maybe the same as #7747