如何在 QGraphicsScene 上绘制一个点(鼠标单击)?

How to draw a point (on mouseclick) on a QGraphicsScene?(如何在 QGraphicsScene 上绘制一个点(鼠标单击)?)

问题描述

我有以下代码来设置 QGraphicsScene.我希望单击场景并在我单击的位置绘制一个点.我怎么能这样做?这是我当前的代码:

I have the following code to set up a QGraphicsScene. I wish to click on the scene and draw a point at the location I've clicked. How could I do this? This is my current code:

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    QGraphicsScene *scene;
    QGraphicsView *view = new QGraphicsView(this);

    view->setGeometry(QRect(20, 50, 400, 400));
    scene = new QGraphicsScene(50, 50, 350, 350);
    view->setScene(scene);
}

推荐答案

UPDATE:有一个名为 QGraphicsSceneMouseEvent 的新类，它使这更容易一些.我刚刚在这里完成了一个使用它的例子:

UPDATE: There is a new class called QGraphicsSceneMouseEvent that makes this a little easier. I just finished an example using it here:

https://stackoverflow.com/a/26903599/999943

它与下面的答案的不同之处在于它子类化 QGraphicsScene，而不是 QGraphicsView，并且它使用 mouseEvent->scenePos() 所以无需手动映射坐标.

It differs with the answer below in that it subclasses QGraphicsScene, not QGraphicsView, and it uses mouseEvent->scenePos() so there isn't a need to manually map coordinates.

您走在正确的轨道上，但您还有一些路要走.

You are on the right track, but you still have a little more to go.

您需要子类化 QGraphicsView 才能使用 QMouseEvent 通过鼠标按下或释放鼠标来执行某些操作.

You need to subclass QGraphicsView to be able to do something with mouse presses or with mouse releases using QMouseEvent.

    #include <QGraphicsView>
    #include <QGraphicsScene>
    #include <QGraphicsEllipseItem>
    #include <QMouseEvent>

    class MyQGraphicsView : public QGraphicsView
    {
        Q_OBJECT
    public:
        explicit MyQGraphicsView(QWidget *parent = 0);

    signals:

    public slots:
        void mousePressEvent(QMouseEvent * e);
        // void mouseReleaseEvent(QMouseEvent * e);
        // void mouseDoubleClickEvent(QMouseEvent * e);
        // void mouseMoveEvent(QMouseEvent * e);
    private:
        QGraphicsScene * scene;
    };

QGraphicsView 本身没有无量纲点.您可能希望使用 QGraphicsEllipse 项目或简单地使用半径非常小的 scene->addEllipseItem().

QGraphicsView doesn't natively have dimension-less points. You will probably want to use QGraphicsEllipse item or simply, scene->addEllipseItem() with a very small radius.

    #include "myqgraphicsview.h"
    #include <QPointF>

    MyQGraphicsView::MyQGraphicsView(QWidget *parent) :
        QGraphicsView(parent)
    {
        scene = new QGraphicsScene();
        this->setSceneRect(50, 50, 350, 350);
        this->setScene(scene);
    }

    void MyQGraphicsView::mousePressEvent(QMouseEvent * e)
    {
        double rad = 1;
        QPointF pt = mapToScene(e->pos());
        scene->addEllipse(pt.x()-rad, pt.y()-rad, rad*2.0, rad*2.0, 
            QPen(), QBrush(Qt::SolidPattern));
    }

注意 mapToScene() 的用法，使事件的 pos() 正确映射到鼠标在场景上点击的位置.

Note the usage of mapToScene() to make the pos() of the event map correctly to where the mouse is clicked on the scene.

如果要使用表单，则需要将子类 QGraphicsView 的实例添加到 ui 的 centralWidget 布局中.

You need to add an instance of your subclassed QGraphicsView to the centralWidget's layout of your ui if you are going to use a form.

    QGridLayout * gridLayout = new QGridLayout(ui->centralWidget);
    gridLayout->addWidget( new MyQGraphicsView() );

或者如果您的 ui 已经有布局，它将如下所示:

or if your ui has a layout already it will look like this:

    ui->centralWidget->layout()->addWidget( new MyGraphicsView() );

如果你不使用 QMainWindow 和一个表单，你可以将它添加到一个 QWidget 如果你为它设置一个布局然后添加你的 QGraphicsView 以类似的方式到该布局.如果您不想在 QGraphicsView 周围留出边距，只需调用 show 即可，不要将其放在不同的布局中.

If you don't use a QMainWindow and a form, you can add it to a QWidget if you set a layout for it and then add your QGraphicsView to that layout in a similar manner. If you don't want a margin around your QGraphicsView, just call show on it and don't put it inside a different layout.

    #include <QtGui/QApplication>
    #include "myqgraphicsview.h"

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

        MyQGraphicsView view;
        view.show();

        return a.exec();
    }

就是这样.现在您对 QGraphicsView 及其与鼠标的交互很危险.

And that's it. Now you are dangerous with QGraphicsView's and their interaction with the mouse.

请务必阅读和研究 Qt 的图形视图框架和相关examples在使用QGraphicsView 和 QGraphicsScene.它们是非常强大的 2D 图形工具，可能需要一些学习曲线，但它们值得.