用 QT实现拖放（drag-drop）功能

QT的 qdrag 和 qdrop功能在qt的"Puzzle"示例中有介绍。但是那个介绍是以QListWidget为基础的。本文以QWidget为基础，介绍如何实现拖放。拖放可以用QString作为信息载体，也可以使用html等类型。本文采用QByteArray。QByteArray允许程序员使用自定义的类型来作为信息载体。本文定义的类型是ST_INFO。

头文件：

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include 
#include 
#include 
#include 
#include 
#include 


#pragma pack(1)
struct ST_INFO
{
    bool b;
    int  i;
};
#pragma pack()

class MainWindow : public QMainWindow
{
    Q_OBJECT

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

    QPointF         m_pntPress;
    bool            m_bMimeAdded;
    QLineEdit     * m_pText1;
    QLineEdit     * m_pText2;
    QLineEdit     * m_pText3;

protected:
    void    mousePressEvent(QMouseEvent *);
    void    mouseMoveEvent(QMouseEvent *);
    void    dropEvent(QDropEvent *);
    void    dragEnterEvent(QDragEnterEvent *);
    void    dragMoveEvent(QDragMoveEvent *);
    void    resizeEvent(QResizeEvent *);
};

#endif // MAINWINDOW_H

cpp文件：

#include "mainwindow.h"
#include 
#include 
#include 
#include 

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    setAcceptDrops(true);
    m_pText1 = new QLineEdit(this);
    m_pText2 = new QLineEdit(this);
    m_pText3 = new QLineEdit(this);
}

MainWindow::~MainWindow()
{
    delete m_pText1;
    delete m_pText2;
    delete m_pText3;
}

void MainWindow::mouseMoveEvent(QMouseEvent *event)
{
    if ((QLineF(event->screenPos(), m_pntPress)
        .length() < QApplication::startDragDistance()) || m_bMimeAdded)
    {
        return;
    }

    m_bMimeAdded = true;
    QDrag *drag = new QDrag(this);
    QMimeData *mime = new QMimeData;
    drag->setMimeData(mime);

    QImage image("E:\\lena.jpg");

    drag->setPixmap(QPixmap::fromImage(image).scaled(30, 40));
    drag->setHotSpot(QPoint(15, 30));

    QByteArray qba;

    QString qstrTemp1 = m_pText1->text();
    QString qstrTemp2 = m_pText2->text();
    bool bTemp;
    if(qstrTemp1.contains("true"))
    {
        bTemp = true;
    }
    else
    {
        bTemp = false;
    }
    ST_INFO stTemp = {bTemp, qstrTemp2.toInt()};
    qba.resize(sizeof(stTemp));
    memcpy(qba.data(), &stTemp, sizeof(stTemp));
    mime->setData(tr("INFO"), qba);


    drag->exec();
}

void MainWindow::mousePressEvent(QMouseEvent *e)
{
    m_pntPress = e->screenPos();
    m_bMimeAdded = false;
}

void MainWindow::dropEvent(QDropEvent *event)
{
    MainWindow *source = qobject_cast(event->source());
    if(source != this)
    {
        if (event->mimeData()->hasFormat("INFO"))
        {
            QByteArray pieceData = event->mimeData()->data("INFO");

            ST_INFO stTemp;

            memcpy(&stTemp, pieceData.data(), sizeof(stTemp));

            QString qstrTemp;
            QTextStream stream(&qstrTemp, QIODevice::WriteOnly);
            stream<<"b = "<accept();
    else
        event->ignore();
}

void MainWindow::dragMoveEvent(QDragMoveEvent *event)
{
    if (event->mimeData()->hasFormat("INFO")) {
        event->setDropAction(Qt::MoveAction);
        event->accept();
    } else {
        event->ignore();
    }
}

void MainWindow::resizeEvent(QResizeEvent *e)
{
    m_pText1->setGeometry(0,0,width()/2, height()/4);
    m_pText2->setGeometry(0, height()/3, width()/2, height()/4);
    m_pText3->setGeometry(width()/2, height()*2/3, width()/2, height()/4);
}

启动两个一模一样的应用。在第一个窗口中输入如下：

然后在第一个窗口中按下鼠标，拖向第二个窗口，松开：

可见，第一个窗口的信息被正确传入了第二个窗口。