ok,let's to see the function readNext.

TokenType QXmlStreamReader::readNext()
Reads the next token and returns its type.

1. what is the next token ?

you can print the token to string.

QString QXmlStreamReader::tokenString() const
Returns the reader's current token as string.

2. what is the type?

QXmlStreamReader::NoToken
QXmlStreamReader::Invalid
QXmlStreamReader::StartDocument
QXmlStreamReader::EndDocument
QXmlStreamReader::StartElement
QXmlStreamReader::EndElement
QXmlStreamReader::Characters
QXmlStreamReader::Comment
QXmlStreamReader::DTD
QXmlStreamReader::EntityReference
QXmlStreamReader::ProcessingInstruction

you should to check every type whether is you want, after you call readNext.

from the code, readNext return Characters token type.

QXmlStreamReader::Characters The reader reports characters in text(). If the characters are all white-space, isWhitespace() returns true. If the characters stem from a CDATA section, isCDATA() returns true.

3. How to understanding xml tokens?

go to here

And I can't find the output of qDebug. It should be in the output window, right? But my output window is constantly emitting such kind of informations.

I write the code in my way. it maybe safer than your code.

void ParseXml::ReadBndbox()
{
    if(reader.name() == "bndbox"){
        reader.readNext();
    }else{
       /// maybe return false at here.
    }

    while(!reader.isStartElement()){
        reader.readNext();
    }

    m_xmin = ReadItemPos("xmin");
    m_ymin = ReadItemPos("ymin");
    m_xmax = ReadItemPos("xmax");
    m_ymax = ReadItemPos("ymax");
    Q_ASSERT(m_xmin >= 0 && m_ymin >= 0);
    Q_ASSERT(m_xmax >= 0 && m_ymax >= 0);
    qDebug("%d %d %d %d",m_xmin,m_ymin,m_xmax,m_ymax);
}

int ParseXml::ReadItemPos(QString itemName)
{
    int nVal = -1;
    bool bOk;
    while(!reader.isStartElement()){
        reader.readNext();
    }

    if(reader.name() == itemName){
        nVal = reader.readElementText().toInt(&bOk);
        Q_ASSERT(bOk == true);
        reader.readNext();
    }

    return nVal;
}

Hi, I found your error. below the code, you will know where are you wrong.

void ParseXml::ReadBndbox()
{
    bool bOk;
    int points[4];

    reader.readNext(); ///< Note: here, skip the '\n' some characters
    for (int i=0;i<4;i++)
    {
        bOk = false;
        reader.readNext();
        points[i] = reader.readElementText().toInt(&bOk);
        if(!bOk){
            qDebug("toInt data failed");
        }
        reader.readNext();
    }

    m_xmin = points[0];
    m_ymin = points[1];
    m_xmax = points[2];
    m_ymax = points[3];
    qDebug("%d %d %d %d\n",m_xmin,m_ymin,m_xmax,m_ymax);
}

If i modified the xml file format, the code has error.

        <bndbox><xmin>144</xmin> ///< not has '\n' ,will has error
            <ymin>485</ymin>
            <xmax>164</xmax>
            <ymax>515</ymax>
        </bndbox>

QXmlStream* => io stream

The class ParseXml is used in ImageWidget.

//parsexml.h

#pragma once
#include <QXmlStreamReader>
#include <QMessageBox>
#include <QString>
#include <QFile>
class ParseXml
{
public:
	ParseXml(void);
	ParseXml(QString fileName);
	~ParseXml(void);

	void ReadFile(const QString &fileName);
	void ReadAnnotation();
	void ReadObject();
	void ReadBndbox();
	void Skip();

	QPoint get_start_point();
	QPoint get_end_point();

private:
	QXmlStreamReader reader;
	int m_xmin;
	int m_ymin;
	int m_xmax;
	int m_ymax;
};

//parsexml.cpp

#include "ParseXml.h"


ParseXml::ParseXml(void)
{
}

ParseXml::ParseXml(QString fileName)
{
	ReadFile(fileName);
}

ParseXml::~ParseXml(void)
{
}

void ParseXml::ReadFile(const QString &fileName)
{
	QFile file(fileName);

	if (file.open(QIODevice::ReadOnly|QIODevice::Text))
	{
		reader.setDevice(&file);

		while (!reader.atEnd())
		{
			if (reader.isStartElement())
			{
				if (reader.name()=="annotation")
				{
					ReadAnnotation();
				}
				else
				{
					reader.raiseError(QObject::tr("Not a valid annotation file"));
				}
			}
			else
			{
				reader.readNext();
			}
		}

		file.close();
	}
}

void ParseXml::ReadAnnotation()
{
	reader.readNext();

	while(!reader.atEnd())
	{
		if (reader.isEndElement())
		{
			reader.readNext();
			break;
		}

		if (reader.isStartElement())
		{
			if (reader.name()=="object")
			{
				ReadObject();
			}
			else
			{
				Skip();
			}
		}
		else
		{
			reader.readNext();
		}
	}
}

void ParseXml::ReadObject()
{
	reader.readNext();

	while (!reader.atEnd())
	{
		if (reader.isEndElement())
		{
			reader.readNext();
			break;
		}

		if (reader.isStartElement())
		{
			if (reader.name()=="bndbox")
			{
				ReadBndbox();
			}
			else
			{
				Skip();
			}
		}
		else
		{
			reader.readNext();
		}
	}
}

void ParseXml::ReadBndbox()
{
	int points[4];

	for (int i=0;i<4;i++)
	{
		reader.readNext();
		points[i] = reader.readElementText().toInt();
		reader.readNext();
	}

	m_xmin = points[0];
	m_ymin = points[1];
	m_xmax = points[2];
	m_ymax = points[3];
}

void ParseXml::Skip()
{
	reader.readNext();

	while(!reader.atEnd())
	{
		if (reader.isEndElement())
		{
			reader.readNext();
			break;
		}

		if (reader.isStartElement())
		{
			Skip();
		}
		else
		{
			reader.readNext();
		}
	}
}

QPoint ParseXml::get_start_point()
{
	return QPoint(m_xmin,m_ymin);
}

QPoint ParseXml::get_end_point()
{
	return QPoint(m_xmax,m_ymax);
}

//imagewidget.h

#ifndef IMAGEWIDGET_H
#define IMAGEWIDGET_H

#include <QWidget>
#include <QImage>
#include <vector>
#include <QPainter>
#include <QFileDialog>
#include <QMouseEvent>
#include <QMessageBox>
#include <QXmlStreamReader>
#include <QToolTip>
#include "ui_imagewidget.h"
#include "Rect.h"
#include "ParseXml.h"

using namespace std;

class ImageWidget : public QWidget
{
	Q_OBJECT

public:
	ImageWidget(QWidget *parent = 0);
	~ImageWidget();

public slots:
	void Open();
	void Save();
	void SaveAnnotation();
	void ReadXml();
	void SaveXml();
	void Delete();
	void paintEvent(QPaintEvent *);
	void mousePressEvent(QMouseEvent *event);
	void mouseMoveEvent(QMouseEvent *event);
	void mouseReleaseEvent(QMouseEvent *event);

private:
	Ui::ImageWidget ui;

	
	QImage *ptr_image;
	QImage *ptr_save;
	QPoint m_Start;
	QPoint m_End;
	bool draw_status;
	bool load_status;
	vector<Rect*> rect_array;


};

#endif // IMAGEWIDGET_H

//imagewidget.cpp

#include "imagewidget.h"

ImageWidget::ImageWidget(QWidget *parent)
	: QWidget(parent)
{
	ui.setupUi(this);

	ptr_image = new QImage();
	ptr_save = new QImage();
	draw_status = false;
	load_status = false;
	setMouseTracking(true);
	
}

ImageWidget::~ImageWidget()
{
	for (int i=0;i<rect_array.size();i++)
	{
		if (rect_array[i])
		{
			delete rect_array[i];
			rect_array[i] = NULL;
		}
	}
}

void ImageWidget::Open()
{
	QString fileName = QFileDialog::getOpenFileName(this,tr("Read Image"),".",tr("Image(*.bmp *.png *.jpg)"));

	if (!fileName.isEmpty())
	{
		ptr_image->load(fileName);
		load_status = true;
		 
	}

	update();
}

void ImageWidget::Save()
{
	QString fileName = QFileDialog::getSaveFileName(this,tr("Save Image"),".",tr("Image(*.bpm *.png *.jpg)"));

	if (!fileName.isEmpty())
	{
		ptr_save->save(fileName);
		
	}
}

void ImageWidget::SaveAnnotation()
{

}

void ImageWidget::ReadXml()
{
	QString fileName = QFileDialog::getOpenFileName(this,tr("Read Xml File"),".",tr("Xml(*.xml)"));
	ParseXml parsexml_(fileName);
	m_Start = parsexml_.get_start_point();
	m_End = parsexml_.get_end_point();
}

void ImageWidget::SaveXml()
{
	QString fileName = QFileDialog::getSaveFileName(this,tr("Save Xml File"),".",tr("Xml(*.xml)"));
}

void ImageWidget::Delete()
{
	if (!rect_array.empty())
	{
		rect_array.pop_back();
	}
}

void ImageWidget::mousePressEvent(QMouseEvent *event)
{
	if (Qt::LeftButton==event->button()&&load_status)
	{
		draw_status = true;
		m_Start = m_End = event->pos();
	}
}

void ImageWidget::mouseMoveEvent(QMouseEvent *event)
{
	QToolTip::showText(event->globalPos(),QString::number( event->pos().x() ) + ", " +QString::number( event->pos().y() ),this, rect() );
	QWidget::mouseMoveEvent(event);
	
	if (draw_status)
	{
		m_End = event->pos();
	}
}

void ImageWidget::mouseReleaseEvent(QMouseEvent *event)
{
	Rect *current_rect = NULL;
	current_rect = new Rect(m_Start,m_End);
	rect_array.push_back(current_rect);
	draw_status = false;
}

void ImageWidget::paintEvent(QPaintEvent *)
{
	*ptr_save = *ptr_image;
	QPen pen;
	QPainter painter(this);
	QPainter paintImage(ptr_save);
	QRect image_rect(0,0,ptr_image->width(),ptr_image->height());

	
	pen.setWidth(5);
	pen.setColor(Qt::red);
	painter.setPen(pen);
	paintImage.setPen(pen);

	painter.begin(this);
	paintImage.begin(ptr_save);
	painter.drawImage(image_rect,*ptr_image);

	painter.drawRect(m_Start.rx(),m_Start.ry(),m_End.rx()-m_Start.rx(),m_End.ry()-m_End.ry());

	for (int i=0;i<rect_array.size();i++)
	{
		rect_array[i]->Draw(painter);
		rect_array[i]->Draw(paintImage);
	}
	
	if (draw_status)
	{
		painter.drawRect(m_Start.rx(),m_Start.ry(),m_End.rx()-m_Start.rx(),m_End.ry()-m_Start.ry());
		paintImage.drawRect(m_Start.rx(),m_Start.ry(),m_End.rx()-m_Start.rx(),m_End.ry()-m_Start.ry());
	}

	painter.end();
	update();

}

1. you can search key words of Qt QXml, you will get some Qt class about xml .
2. here are some demos about how to process xml file in qt. link