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为什么 QMouseEvent 为 QWidget 上的单个移动传递多个事件？

我正在实现简单的拖动效果，但结果不是我所期望的。

以下代码会将小部件移动到新位置，但会立即将其移回原始位置。

自定义小部件.h

#ifndef CUSTOMWIDGET_H
#define CUSTOMWIDGET_H

#include <QWidget>
#include <fstream>

class CustomWidget : public QWidget
{
    Q_OBJECT
public:
    explicit CustomWidget(QWidget *parent = nullptr);
    ~CustomWidget();

protected:
    // define the painting agorithm to see the area of this widget
    void paintEvent(QPaintEvent* ev);

    // handle the pressing event to initialize the dragging algorithm
    // and to track the start of moving event
    void mousePressEvent(QMouseEvent* ev);

    // implement the dragging algorithm
    void mouseMoveEvent(QMouseEvent* ev);

    // handle the releasing event to track the end of moving event
    void mouseReleaseEvent(QMouseEvent* ev);

private:
    std::ofstream fout; // open file "debug.txt"
    QPoint prev; // to save the previous point of cursor.
};

#endif // CUSTOMWIDGET_H

自定义小部件.cpp

#include "customwidget.h"
#include <QMouseEvent>
#include <QPaintEvent>
#include <QPainter>
#include <QBrush>

CustomWidget::CustomWidget(QWidget *parent) : QWidget(parent)
{
    // open file for output
    fout.open("debug.txt");

    // set the widget size and position
    setGeometry(0, 0, 100, 100);
}

CustomWidget::~CustomWidget()
{
    // close file when program ended
    fout.close();
}

void CustomWidget::paintEvent(QPaintEvent *ev)
{
    // draw the area with blue color
    QPainter painter(this);
    QBrush brush(Qt::GlobalColor::blue);
    painter.setBrush(brush);
    painter.setBackground(brush);
    painter.drawRect(ev->rect());
}

void CustomWidget::mousePressEvent(QMouseEvent *ev)
{
    ev->accept();

    // debug output
    fout << "pressed at (" << ev->x() << ',' << ev->y() << ')' << std::endl;

    // initialize the dragging start point
    prev = ev->pos();
}

void CustomWidget::mouseMoveEvent(QMouseEvent *ev)
{
    ev->accept();

    // get the cursor position of this event
    const QPoint& pos = ev->pos();

    // debug output
    fout << "moved from (" << prev.x() << ',' << prev.y() << ") to ("
         << pos.x() << ',' << pos.y() << ')' << std::endl;

    // calculate the cursor movement
    int dx = pos.x() - prev.x();
    int dy = pos.y() - prev.y();

    // move the widget position to match the direction of the cursor.
    move(geometry().x() + dx, geometry().y() + dy);

    // update the cursor position for the next event
    prev = pos;
}

void CustomWidget::mouseReleaseEvent(QMouseEvent *ev)
{
    ev->accept();
    fout << "released at (" << ev->x() << ',' << ev->y() << ')' << std::endl;
}

主文件

#include "customwidget.h"
#include <QApplication>
#include <QMainWindow>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    // generate simple main window.
    QMainWindow w;

    // set the size of the window.
    w.setGeometry(0, 0, 800, 800);

    // generate the CustomWidget
    CustomWidget *widget = new CustomWidget(&w);

    // display the window containing the widget
    w.show();

    return a.exec();
}

并且 debug.txt 对光标单次移动的结果是

CustomWidget pressed at (79,83)
CustomWidget moved from (79,83) to (79,83)
CustomWidget moved from (79,83) to (80,83)
CustomWidget moved from (80,83) to (79,83)
CustomWidget released at (80,83)

结果是将小部件移动到新位置一段时间，然后将其移回其原始位置。

无论您如何拖动小部件，该程序的外观几乎看起来都不会移动小部件。

我的理论是事件管理器在您移动光标时传递事件。但是在处理完第一个事件之后，管理器会传递另一个与小部件的新位置和光标当前位置相关的事件。然后该过程会将小部件移回原来的位置。

虽然我可以更改获取光标位置的方法

ev->pos()

至

ev->globalPos()

解决问题。

但仍然想知道为什么事件管理器会那样做。

4

1 回答 1

0

您必须执行以下操作：

  • 在鼠标按下事件时，存储鼠标光标相对于小部件的偏移量，
  • 移动您的小部件，使鼠标光标始终保持初始的非零偏移，
  • 重置鼠标释放事件的偏移量。

代码（草稿）可能如下所示：

void CustomWidget::mousePressEvent(QMouseEvent* event)
{
    // m_offset is a member variable of CustomWidget
    m_offset = event->globalPos() - pos();
    QWidget::mousePressEvent(event);
}

void CustomWidget::mouseMoveEvent(QMouseEvent* event)
{
    if (!m_offset.isNull()) {
        move(event->globalPos() - m_offset);
    }
    QWidget::mouseMoveEvent(event);
}

void CustomWidget::mouseReleaseEvent(QMouseEvent* event)
{
    // Reset the offset value to prevent the movement.
    m_offset = QPoint();
    QWidget::mouseReleaseEvent(event);
}
于 2018-10-22T08:15:50.100 回答