wrong connected line with multi-touch in iOS using my code

[mkdh]

I deploy my code on android.
Then, it will draw separable line by each finger.
Question 1:
But when I deploy my code on iOS(iPad),
it will connect the line between the different finger. ,That is not what I want.
Why ? And how to correct it?

Question 2:
I put a pushbutton on my code.
The below text is my current code.
But I can't click the button on my android and iOS.
Should I change some thing in my code?

Question 3:
In fact, I just want to ZoomIn or ZoomOut my graph by finger.
I watch some video in Qt studio. But I don't find any example....

TouchTest::TouchTest(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::TouchTest)
{
    ui->setupUi(this);
    this->setAttribute(Qt::WA_AcceptTouchEvents, true);
    connect(ui->pushButton,SIGNAL(clicked()),this,SLOT(onClear_clicked()));
}

bool TouchTest::event(QEvent *e){
    switch (e->type()) {
    case QEvent::TouchBegin: {
        return true;
    }
    case QEvent::TouchUpdate:{
    }
    case QEvent::TouchEnd:
    {
        QTouchEvent *te = static_cast<QTouchEvent *>(e);
        this->addLine(te);
        return true;
    }
    case QEvent::Paint:
    {
        paintEvent((QPaintEvent *)e);
    }
    default:;
    }
    return QWidget::event(e);
}

void TouchTest::addLine(QTouchEvent *e)
{
    QLineF line;
    QList<QTouchEvent::TouchPoint> touchPoints = e->touchPoints();
    foreach (QTouchEvent::TouchPoint i , touchPoints){
        qDebug()<<"i.pos()"<<i.pos()<<"i.lastPos()"<<i.lastPos();
        line.setP1(i.lastPos());
        line.setP2(i.pos());
        this->lines.push_back(line);
    }
    this->update();
}

void TouchTest::paintEvent(QPaintEvent *event){
    QPainter painter(this);
    painter.drawLines(this->lines);
    painter.end();
}

void TouchTest::onClear_clicked()
{
    this->lines.clear();
    this->update();
}

TouchTest::~TouchTest()
{
    delete ui;
}

PS:
QVector<QLineF> lines;