[QT Operation XML] QT reads and writes XML files, QT modifies XML files

Introduction to XML

Concept: Extensible Markup Language Extensible Markup Language (extensible: tags are customized)
Function: Store data (1. Configuration file, 2. Transmission in the network)
Basic XML syntax:
1. The suffix of the xml document.xml
2. The first line of xml must be defined as a document declaration
3. There is one and only one root tag in the xml document
4. The attribute value must be enclosed in quotation marks (either single or double)
5. The tag must be closed correctly
6. The name of the xml tag is distinguished Capitalization
Document declaration format: <?xml version='1.0' encoding='UTF-8'?>
version: version number, required attribute
encoding: encoding method, tells the parsing engine the character set encoding method used by the current document, the default value : ISO-8859-1
standalone: ​​Whether it is independent (value, yes: do not rely on other files; no: rely on other files)

QT operation XML, write, read, modify

Pro file adds xml module: QT += xml
includes header file: #include

//写入XML
void MainWindow::writeXML()
{
    
    
    QFile file("test.xml");//打开或新建xml文件
    if(!file.open(QIODevice::WriteOnly|QIODevice::Truncate))//Truncate表示清空原来的内容
    {
    
    
        QMessageBox::warning(this,"错误","文件打开失败");
        return;
    }

    QDomDocument doc;
    //写入xml头部
    QDomProcessingInstruction instruction;//添加处理指令
    instruction = doc.createProcessingInstruction("xml","version=\"1.0\" encoding=\"UTF-8\"");
    doc.appendChild(instruction);
    //添加根节点
    QDomElement root = doc.createElement("library");
    doc.appendChild(root);
    //添加第一个子节点，及其子元素
    QDomElement book = doc.createElement("book");
    book.setAttribute("id",1);//方法1，创建属性，键值对可以是各种类型
    QDomAttr time = doc.createAttribute("time");//方法2，创建属性，值必须是字符串
    time.setValue("2020/6/3");
    book.setAttributeNode(time);

    QDomElement title = doc.createElement("title");//创建子元素
    QDomText text = doc.createTextNode("C++ primer");//设置括号标签中间的值
    book.appendChild(title);
    title.appendChild(text);
    QDomElement author = doc.createElement("author");//创建子元素
    text = doc.createTextNode("Stanley B.Lippman");
    author.appendChild(text);
    book.appendChild(author);
    root.appendChild(book);

    //添加第二个子节点，部分变量只需重新赋值
    book=doc.createElement("book");
    book.setAttribute("id",2);
    time = doc.createAttribute("time");
    time.setValue("2007/5/25");
    book.setAttributeNode(time);
    title = doc.createElement("title");
    text = doc.createTextNode("Thinking in Java");
    book.appendChild(title);
    title.appendChild(text);
    author = doc.createElement("author");
    text = doc.createTextNode("Bruce Eckel");
    author.appendChild(text);
    book.appendChild(author);
    root.appendChild(book);

    //输出文件
    QTextStream out_stream(&file);
    doc.save(out_stream,4);//缩进4格
    file.close();
}

//读XML文件
void MainWindow::readXML()
{
    
    
    QFile file("test.xml");
    if(!file.open(QIODevice::ReadOnly))
    {
    
    
        QMessageBox::warning(this,"错误","读XML，文件打开失败");
        return;
    }
    QDomDocument doc;
    if(!doc.setContent(&file))//从字节数组中解析XML文档，并将其设置为文档的内容
    {
    
    
        file.close();
        return;
    }
    file.close();
    QDomElement root = doc.documentElement();//返回根节点
    qDebug()<<"["<<__FILE__<<"]"<<__LINE__<<__FUNCTION__<<"root.nodeName "<<root.nodeName();//打印根节点
    QDomNode node = root.firstChild();//获得第一个子节点
    while(!node.isNull())//如果节点不为空
    {
    
    
        if(node.isElement())//如果节点是元素
        {
    
    
            QDomElement e= node.toElement();//节点转换为元素
            //打印键值对，tagName和nodeName相同
            qDebug() << e.tagName()<< " " <<e.attribute("id")<<" "<<e.attribute("time");
            QDomNodeList list = e.childNodes();//子节点列表
            for(int i=0;i<list.count();i++)//遍历子节点
            {
    
    
                QDomNode n = list.at(i);
                if(n.isElement())
                    qDebug() << n.nodeName()<<":"<<n.toElement().text();
            }
        }
        node=node.nextSibling();//下一个兄弟节点
    }
}

//增加XML内容
//先把文件读进来，再重写
void MainWindow::addXML()
{
    
    
    QFile file("test.xml");
    if(!file.open(QIODevice::ReadOnly))
    {
    
    
        QMessageBox::warning(this,"错误","增加XML，文件打开失败1");
        return;
    }
    QDomDocument doc;
    if(!doc.setContent(&file))//从字节数组中解析XML文档，并将其设置为文档的内容
    {
    
    
        file.close();
        return;
    }
    file.close();

    QDomElement root=doc.documentElement();
    QDomElement book=doc.createElement("book");
    book.setAttribute("id",3);
    book.setAttribute("time","1813/1/27");
    QDomElement title=doc.createElement("title");
    QDomText text;
    text=doc.createTextNode("Pride and Prejudice");
    title.appendChild(text);
    book.appendChild(title);
    QDomElement author=doc.createElement("author");
    text=doc.createTextNode("Jane Austen");
    author.appendChild(text);
    book.appendChild(author);
    root.appendChild(book);

    if(!file.open(QFile::WriteOnly|QFile::Truncate))//重写文件，如果不用truncate就是在后面追加内容，就无效了
    {
    
    
        QMessageBox::warning(this,"错误","增加XML，文件打开失败2");
        return;
    }
    QTextStream out_stream(&file);
    doc.save(out_stream,4);
    file.close();
}

//删减XML内容
void MainWindow::removeXML()
{
    
    
    QFile file("test.xml");
    if(!file.open(QIODevice::ReadOnly))
    {
    
    
        QMessageBox::warning(this,"错误","增加XML，文件打开失败1");
        return;
    }
    QDomDocument doc;
    if(!doc.setContent(&file))//从字节数组中解析XML文档，并将其设置为文档的内容
    {
    
    
        file.close();
        return;
    }
    file.close();
    QDomElement root=doc.documentElement();
    QDomNodeList list = doc.elementsByTagName("book");//指定名称的节点列表
    for(int i=0;i<list.count();i++)
    {
    
    
        QDomElement e = list.at(i).toElement();
        if(e.attribute("time")=="2007/5/25")
            root.removeChild(list.at(i));
    }
    if(!file.open(QFile::WriteOnly|QFile::Truncate))//重写文件，如果不用truncate就是在后面追加内容，就无效了
    {
    
    
        QMessageBox::warning(this,"错误","删减XML内容，文件打开失败");
        return;
    }
    QTextStream out_stream(&file);
    doc.save(out_stream,4);
    file.close();
}

//更新XML内容
//如果了解XML结构，可以直接定位到指定标签上更新
//或者用遍历的方法去匹配tagname或者attribut，value来更新
void MainWindow::updateXML()
{
    
    
    QFile file("test.xml");
    if(!file.open(QIODevice::ReadOnly))
    {
    
    
        QMessageBox::warning(this,"错误","更新XML，文件打开失败1");
        return;
    }
    QDomDocument doc;
    if(!doc.setContent(&file))//从字节数组中解析XML文档，并将其设置为文档的内容
    {
    
    
        file.close();
        return;
    }
    file.close();

    QDomElement root = doc.documentElement();//获得根节点
    QDomNodeList list = root.elementsByTagName("book");//指定名称的节点列表
    QDomNode node = list.at(list.count()-1).firstChild();//定位到第三个一级子节点的子元素
    QDomNode oldNode = node.firstChild();//标签之间的内容作为节点的子节点出现,当前是Pride and Projudice
    oldNode.setNodeValue("dalao");//修改元素内容
//    node.firstChild().setNodeValue("diannao");
//    QDomNode newNode = node.firstChild();
//    node.replaceChild(newNode,oldNode);

    if(!file.open(QFile::WriteOnly|QFile::Truncate))//重写文件，如果不用truncate就是在后面追加内容，就无效了
    {
    
    
        QMessageBox::warning(this,"错误","更新XML内容，文件打开失败2");
        return;
    }
    QTextStream out_stream(&file);
    doc.save(out_stream,4);
    file.close();
}

XML effect demonstration

<?xml version='1.0' encoding='UTF-8'?>
<library>
    <book id="1" time="2020/6/3">
        <title>C++ primer</title>
        <author>Stanley B.Lippman</author>
    </book>
    <book id="2" time="2007/5/25">
        <title>Thinking in Java</title>
        <author>Bruce Eckel</author>
    </book>
    <book id="3" time="1813/1/27">
        <title>diannao</title>
        <author>Jane Austen</author>
    </book>
</library>