您正在使用onCompleted()。因此，您应该使用onCurrentIndexChanged()。完整示例如下所示：

import QtQuick 2.15
import QtQuick.Window 2.15
import QtQuick.Controls 2.12

Window {
    width: 640
    height: 480
    visible: true
    title: qsTr("Hello World")
    function formatText(c,t){
        return t;
    }

    Component {
        id: delegateComponent

        Label {
            text: formatText(Tumbler.tumbler.count, modelData)
            opacity: 1.0 - Math.abs(Tumbler.displacement) / (Tumbler.tumbler.visibleItemCount / 2)
            horizontalAlignment: Text.AlignHCenter
            verticalAlignment: Text.AlignVCenter
//            font.pixelSize: fontMetrics.font.pixelSize * 1.25 
//            Component.onCompleted: {
//                console.log("log ", Tumbler.tumbler.currentIndex)
//            }

        }
    }

    Frame {
        id: frame
        padding: 0
        anchors.centerIn: parent

        Row {
            id: row

            Tumbler {
                id: hoursTumbler
                model: 3
                delegate: delegateComponent
                onCurrentIndexChanged: {
                    if(currentIndex !== undefined) console.log("log ", currentIndex)
                }
            }
        }
    }

}