[SOLVED] Connections with null target

[A Former User]

Hi,

I encountered some unexpected behaviour while dealing with the Connections object and its target property. The documentation says:

If set to null, no connection is made and any signal handlers are ignored until the target is not null.

Now have a look at the following snippet:

Item { 
    property var databaseTable: null
    Connections {
        target: databaseTable
        onRowAppended: model.append(row)
    }
}

This leads to the following runtime error:

QML Connections: Cannot assign to non-existent property "onRowAppended"

But the following produces no error:

Item { 
    property var databaseTable: null
    Connections {
        target: databaseTable==null ? null : databaseTable
        onRowAppended: model.append(row)
    }
}

Can anyone please give an explanation for this?

P.S.: I know there is ignoreUnknownSignals: bool, but that's not the point here.

[JKSH]

Interesting! Sounds like a bug, as I'd expect both to behave the same.

You can report this to https://bugreports.qt.io/

[A Former User]

@JKSH Yes, looks like a bug to me, too, but maybe it's just another Javascript craziness?

[JKSH]

@Wieland: I'm not sure. See if anyone with more experience replies. If not, you could ask on the Interest mailing list where the Qt engineers hang out (you need to subscribe first)

[A Former User]

@JKSH Thanks for your help! I just figured out what's going wrong here. It's not a bug, but all my fault:

Item { 
    property var databaseTable: null
    Connections {
        target: databaseTable     // <- tries to access the *global* object "databaseTable"
        onRowAppended: model.append(row)
    }
}

Change it to:

Item { 
    id: myItem
    property var databaseTable: null
    Connections {
        target: myItem.databaseTable   
        onRowAppended: model.append(row)
    }
}

... and it accesses the intended local object.

So, the $500 question: Why the heck does target: databaseTable==null ? null : databaseTable not cause an error?

Here is the answer: The situation in my actual code looks like this:

Item {
    id: someItemUpInTheHierarchy
    property var databaseTable      // not initalized to *null*, so it is *undefined*
    // ... lots of code ...
    Item { 
        property var databaseTable: null
        Connections {
            target: databaseTable==null ? null : databaseTable  // undefined==null evaluates to *true*
            onRowAppended: model.append(row)
        }
    }
}

Stupid me! But Javascript is stupid, too ;-)

[JKSH]

@Wieland said:

Stupid me! But Javascript is stupid, too ;-)

Stupidities make life more interesting ;-) Glad you've solved it! Thanks for sharing your story and solution.

P.S. This is one reason why I like to explicitly state the id (id.propertyName, or at least parent.propertyName) if the property doesn't belong to the current object. Another user had a similar predicament recently: https://forum.qt.io/topic/53997/

[Aleksey_K]

Thanks, had similar bug with target being undefined:

"Unable to assign [undefined] to QObject*"

Fixed according to the Qt doc:
https://doc.qt.io/qt-5/qml-qtqml-connections.html#target-prop

If set to null, no connection is made and any signal handlers are ignored until the target is not null.

Connections {
    // If set to null, no connection is made and
    // any signal handlers are ignored until the target is not null.
    target: myItem ? myItem : null
    onPropertyChanged: {
        ...
    }
}