CheckBox needs to track external properties

[Tim Peeters]

Often a CheckBox/Switch is used to show the value of a property outside of the CheckBox. For example:

Switch {
    id: hiddenSwitch
    checked: !header.exposed
    onClicked: header.exposed = !header.exposed
}

At first look, this code may seem fine, but clicking the switch will trigger checked = !checked, which breaks the binding, so if header.exposed updates because of events happening outside of the switch, the switch will be out-of-sync.

Perhaps we need to add some properties to the CheckBox:

CheckBox {
  property Object bindTarget
  property string bindProperty
}

and then the behavior of this:
Switch {
    id: exposedSwitch
    bindTarget: header
    bindProperty: "exposed"
}

can do what we want: the switch shows the value of "exposed", and clicking the switch should update the "exposed" value (which in turn will update the state of the switch).

Note that in this case we could not track "!exposed", so if we need that, some more complicated solution is needed.

--- update 2015-09-11 (thanks Christian) ---
It appears to work like this:
Switch {
    id: hiddenSwitch
    checked: header.exposed
    function trigger() { header.exposed = !header.exposed; }
}

so DON'T use the onClicked or onTriggered signal as you would expect, but override the trigger() function. We need to document this.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Cris Dywan (kalikiana)]

I can only make a guess since you're not providing a complete example, but I don't see a problem with my code doing what you're describing. The binding isn't broken, the only problem I can see is that the state gets reversed because the value is changed twice. To achieve the desired result you need to override trigger().

[Tim Peeters (tpeeters)]

Christian, you are right. Overriding the trigger() function works for me!

However, we do not have this documented, and if I am not an exception here, many developers may run into this issue, so I keep the bug open, but I will update its description to say that we need documentation.

[Tim Peeters (tpeeters)]

More people run into problems here: http://pastebin.ubuntu.com/12417204/