Qt ListView没有'；t在其模型数据更改时更新

我有一个

QList

，它从

ListView

模块中读取到

模型中。首先，QList是一个空对象。我希望在将新列表分配给
QList
对象时更新
indexBackup

import QtQuick          2.5
import QtQuick.Controls 2.0
import QtQuick.Layouts  1.2
import QtQuick.Controls.Styles 1.0

ListView {
    id: layersListView

    model: settingsList   //QList<QObject*> object
    int indexBackup: 0

    onModelChanged: {
        if(indexBackup && indexBackup < model.length) 
        {
            indexBackup = model.length
        }
        else 
        {
            indexBackup = - 1
        }
    }
}    

导入QtQuick 2.5
导入QtQuick.Controls 2.0
导入QtQuick.Layouts 1.2
导入QtQuick.Controls.Styles 1.0
列表视图{
id:LayerListView
模型：settingsList//QList对象
int indexBackup:0
在模型更改时：{
if（indexBackup&&indexBackup
对于每个有类似问题的人，都可以找到答案