Qt 5：用于缩放图像的鼠标滚轮事件行为

我有一个窗口，其中是一个QGraphicsView，它将显示一个图像。我已经实现了wheelEvent（）。我的图像大多会比窗口大，所以我会在窗口中设置滚动条

在Windows Photo Viewer中查看图像时，当我们旋转滚轮时通常会看到，当我们向上移动滚轮（朝向其导线）时，图像会放大，而当我们向下移动滚轮（朝向外体）时，图像会缩小

取而代之的是，当我向自己移动滚轮（缩小）图像而不是缩小图像时，首先向下滚动，并且仅当滚动条触及其最底部点时才开始缩小

最好通过尝试代码来理解问题。我想我无法用英语解释。

我想要标准的行为。怎么办

代码

#include "viewer.h"
#include "ui_viewer.h"
#include <QGraphicsView>
#include <QGraphicsItem>
#include <QGraphicsPixmapItem>
#include <QWheelEvent>
#include <QDebug>
#include <QImage>
#include <QImageReader>
#include <QApplication>
#include <QDesktopWidget>

viewer::viewer(QWidget *parent) : QWidget(parent),ui2(new Ui::viewer)
{
     ui2->setupUi(this);
} 

viewer::~viewer()
{
    delete ui2;
}

int viewer::show_changes(QString folder)
{
    QDesktopWidget *desktop = QApplication::desktop();
    int screenWidth  = desktop->width();
    int screenHeight = desktop->height();
    QString filename = "image_bigger_than_window.jpg";
    QPixmap pixmap = QPixmap(filename);
    QImageReader reader(filename);
    QImage image = reader.read();
    QSize size = image.size();
    int width = 800;
    int height = (width * size.height()) / size.width();
    int x = (screenWidth - width) / 2;
    int y = (screenHeight - height) / 2 - 30;
    setGeometry(x,y,width, height);
    setWindowTitle("OUTPUT");
    ui2->graphicsView->setGeometry(0,0,width,height);
    QGraphicsScene* viewScene = new QGraphicsScene(QRectF(0, 0,width,  height), 0);
    QGraphicsPixmapItem *item = viewScene->addPixmap(pixmap.scaled(QSize((int)viewScene->width(), (int)viewScene->height()),
                             Qt::IgnoreAspectRatio, Qt::SmoothTransformation));
    viewScene->addItem(item);
    ui2->graphicsView->fitInView(QRectF(0, 0, width,     height),Qt::IgnoreAspectRatio);
    ui2->graphicsView->setScene(viewScene);
    ui2->graphicsView->show();
    return 0;
}

void viewer::wheelEvent(QWheelEvent * event)
{
    const int degrees = event->delta() / 8;
    qDebug() << degrees;
    int steps = degrees / 15;        
    double scaleFactor = 1.0; 
    const qreal minFactor = 1.0;
    const qreal maxFactor = 10.0;
    qreal h11 = 1.0, h22 = 0;
    if(steps > 0)
    {
        h11 = (h11 >= maxFactor) ? h11 : (h11 + scaleFactor);
        h22 = (h22 >= maxFactor) ? h22 : (h22 + scaleFactor);
    }
    else
    {
        h11 = (h11 <= minFactor) ? minFactor : (h11 - scaleFactor);
        h22 = (h22 <= minFactor) ? minFactor : (h22 - scaleFactor);
    }
    ui2->graphicsView->setTransformationAnchor(QGraphicsView::AnchorUnderMouse);
    ui2->graphicsView->setTransform(QTransform(h11, 0, 0,0, h22, 0, 0,0,1));

}

---

代码表明，您没有子类化

QGraphicsView

，而是在自己的小部件中使用了一个

轮子事件将首先发送到实际的图形视图小部件。在那里，它是用Qt的默认行为处理的，即滚动。只有当您滚动到底部时，图形视图才能处理控制盘事件，并且它会传播到其父类，即您的类。这就是为什么只有滚动到边框时才能缩放

要解决这个问题，您应该。这允许您截取控制盘事件并在类中处理它：

// Outline, not tested
viewer::viewer(QWidget *parent) : QWidget(parent),ui2(new Ui::viewer)
{
    ui2->setupUi(this);
    // Let me handle your events
    ui2->graphicsView->installEventFilter(this);
} 

// should be protected
bool viewer::eventFilter(QObject *obj, QEvent *event) {
  if (event->type() == QEvent::GraphicsSceneWheel) {
      // Your implementation. 
      // You can't use QWheelEvent, as Graphicscene works with its own events...
     handleWheelOnGraphicsScene(static_cast<QGraphicsSceneWheelEvent*> (event));

     // Don't propagate
     return true;
  }

  // Other events should propagate
  return false;
}

---

还要注意的是，

handleHeelongraphicscene

或任何你想调用它的东西，应该是一个私有方法，而不必是插槽。

我对代码做了一些更改。现在我的图像根本没有滚动。它只在滚动滚轮时上下移动。出什么事了？一切都按照我的要求进行。谢谢你的帮助。我的意思是，事件是在一个链中处理的。当事件被发送到不处理它的小部件（例如按钮上的滚轮事件）时，它将被传播到其父小部件，以便它可以处理它（例如包含按钮的滚动视图）。事件筛选器也是如此。另请参见：

// Outline, not tested
viewer::viewer(QWidget *parent) : QWidget(parent),ui2(new Ui::viewer)
{
    ui2->setupUi(this);
    // Let me handle your events
    ui2->graphicsView->installEventFilter(this);
} 

// should be protected
bool viewer::eventFilter(QObject *obj, QEvent *event) {
  if (event->type() == QEvent::GraphicsSceneWheel) {
      // Your implementation. 
      // You can't use QWheelEvent, as Graphicscene works with its own events...
     handleWheelOnGraphicsScene(static_cast<QGraphicsSceneWheelEvent*> (event));

     // Don't propagate
     return true;
  }

  // Other events should propagate
  return false;
}

// In Constructor, or where appropriate
ui2->graphicsView->scene()->installEventFilter(this);


bool viewer::eventFilter(QObject *obj, QEvent *event) {
    if (event->type() == QEvent::GraphicsSceneWheel) {
        handleWheelOnGraphicsScene(static_cast<QGraphicsSceneWheelEvent*> (event));

        // Don't propagate
        event->accept();
        return true;
    }
    return false;
}