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Details

Type: Bug
Resolution: Done
Priority: P1: Critical
Fix Version/s: 5.15, 6.2, 6.3.0 Beta2, 6.3, 6.4.0 Beta1
Affects Version/s: 5.14.2
Component/s: Quick: Core Declarative QML
Labels:
- Reported_by_support_standard
- earmarked_by_ulf

Commits:
a8c729d839 (qt/qtdeclarative/dev) cb4594718a (qt/qtdeclarative/6.3) cb4594718a (qt/tqtc-qtdeclarative/6.3) a8c729d839 (qt/tqtc-qtdeclarative/dev) c2b6f09d28 (qt/qtdeclarative/6.2) c2b6f09d28 (qt/tqtc-qtdeclarative/6.2) c2b6f09d28 (qt/qtdeclarative/6.2.4)

Description

The following example shows a state group with two defined states, with one when-binding each. It seems well-defined and behaves correctly in Qt 5.12.6, but not in Qt 5.14.2. The issue is that the state string visits the empty string "" in a transition between two well defined states.

Notice how root.number is 2, and is set to 1 with a timer. The state group defines states for when number is 1 and 2.

import QtQuick 2.11

Item {
    id: root
    property int number: 2
    Timer {
        interval: 500
        running: true
        repeat: false
        onTriggered: {
            root.number = 1
        }
    }
    Item {
        id: item
        onStateChanged: console.log("state changed to", state);
        states: [
            State {
                name: "n1"
                when: root.number === 1
            },
            State {
                name: "n2"
                when: root.number === 2
            }
        ]
    }
}

# With Qt 5.12.6:
qml: state changed to n2
qml: state changed to n1

# With Qt 5.14.2:
qml: state changed to n2
qml: state changed to  
qml: state changed to n1

Notice the empty state "". This state change naturally breaks transition animations.

We find this commit suspicious but cannot see what the possible fix is:

commit 64c1fbe96c68b1286a70242ff4922be140128cb2 
Author: Fabian Kosmale <fabian.kosmale@qt.io> 
Date:   Tue Jul 2 13:23:09 2019 +0200 
 
    use boolean for when property

Headsup hhvaal.