[REG 6.2 → 6.4]: Connections and signals whose names start with an underscore

Prior to Qt 6.4, a signal whose name starts with an underscore could be connected via a Connections object to a function starting with "on" and ending with the name of the signal. As of Qt 6.4, the first character after the underscore must be uppercased. This is a runtime compatibility breakage; the function that was previously called, is no longer, and there is no warning printed/log output.

The convention is that the first character in the signal name is uppercased in on<Signal> handlers. There is no special rule for underscore, which is the same in uppercase and lowercase.

Consider the below inlined example; the on_foo() function is no longer called.

import QtQuick

Item {
    id: root
    width: 100
    height: 100
    signal _foo

    Connections {
        target: root
        function on_Foo() {
            console.info("Emitted for both Qt 6.2 and 6.4!");
        }
        function on_foo() { // <== Notice that the underscore is itself in uppercase
            console.info("Only emitted for Qt 6.2");
        }
    }

    Timer {
        interval: 100
        onTriggered: root._foo();
        running: true
    }
}