使用QPainter在QWidgets之间画一条线

Question

我正在开发一个需要能够在两个 QWidget 对象之间画一条线的应用程序。我已经尝试了很多东西，但我目前的尝试（我认为是在正确的方向我只是觉得我错过了一些东西）是包含小部件（我称之为DrawWidget 并包含添加了 QWidget 对象的 QGridLayout）覆盖 paintEvent 方法并调用 QPainter::drawLine() 函数。

我遇到的问题是：

无论我如何尝试获取小部件的位置，线的端点总是在错误的位置
每当我尝试绘制第二条线时，我绘制的第一条线就会被擦除。

这是包含小部件的 paintEvent 函数：

void paintEvent(QPaintEvent *)
{
    if (!drewSinceUpdate){
        drewSinceUpdate = true;
        QPainter painter(this);
        painter.setPen(QPen(Qt::black));

        painter.drawLine(start->geometry().center(), end->geometry().center());
    }
}

我尝试了很多不同的方法来获得 paintEvent 最后一行中的小部件的正确位置，我将 post 一些方法（我不记得所有的他们):

painter.drawLine(start->pos(), end->pos());
painter.drawLine(start->mapToGlobal(start->geometry().center()), end->mapToGlobal(end->geometry().center()));
painter.drawLine(this->mapToGlobal(start->geometry().center()), this->mapToGlobal(end->geometry().center()));
painter.drawLine(start->mapTo(this, start->pos()), end->mapTo(this, end->pos()));
painter.drawLine(this->mapFrom(start, start->pos()), this->mapFrom(end, end->pos()));

为了让我的问题更清楚，这里是我正在寻找的示例，摘自 QT Diagram Scene Example：但这就是我最终得到的：

感谢您提供的任何帮助。

注意：

-start 和 end 都是 QWidget 我用另一种方法传入的对象

-与DrawWidget相关的层级是：

QMainWindow
->QScrollArea
  ->DrawWidget
    ->QGridLayout
      ->Items       <-- These are the things I want to connect

编辑：为了制作一个完整且可验证的示例，这里是相关代码的全部。

MainWindow.cpp:

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QHBoxLayout>
#include <QVBoxLayout>
#include <QTextEdit>
#include <QPushButton>
#include <QScrollBar>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    // Setting up the relevant hierarchy 
    ui->setupUi(this);
    scrollArea = new QScrollArea();
    setCentralWidget(scrollArea);

    drawWidget = new DrawWidget();
    gridLayout = new QGridLayout();
    gridLayout->setSpacing(300);
    drawWidget->setLayout(gridLayout);

    scrollArea->setWidget(drawWidget);
    scrollArea->setWidgetResizable(true);

    AddItemSlot();

    QApplication::connect(scrollArea->horizontalScrollBar(), SIGNAL(rangeChanged(int,int)), this, SLOT(scrollHorizontal()));
}

// This is just creating a single one of the example widgets which I want to connect
QWidget* MainWindow::CreateNewItem(){
    QWidget* itemWidget = new QWidget();
    itemWidget->setStyleSheet("background-color: lightgray");
    QHBoxLayout* singleItemLayout = new QHBoxLayout();
    itemWidget->setLayout(singleItemLayout);

    QTextEdit* textEdit = new QTextEdit(std::to_string(counter++).c_str());
    textEdit->setStyleSheet("background-color:white;");
    singleItemLayout->addWidget(textEdit);

    QVBoxLayout* rightSidePanel = new QVBoxLayout();
    rightSidePanel->setAlignment(Qt::AlignTop);

    QPushButton* button1 = new QPushButton("Top Button");

    QApplication::connect(button1, SIGNAL(clicked(bool)), this, SLOT(AddItemSlot()));

    rightSidePanel->addWidget(button1);

    QWidget* rightPanelWidget = new QWidget();
    rightSidePanel->setMargin(0);
    rightPanelWidget->setLayout(rightSidePanel);

    singleItemLayout->addWidget(rightPanelWidget);

    itemWidget->setLayout(singleItemLayout);
    itemWidget->setMinimumWidth(400);
    itemWidget->setFixedSize(400,200);

    return itemWidget;
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::scrollHorizontal()
{
    scrollArea->ensureWidgetVisible(noteItems.back());
}

void MainWindow::AddItemSlot()
{
    QWidget* w = CreateNewItem();
    gridLayout->addWidget(w,currRow, currCol++);
    if (!noteItems.empty()){
        drawWidget->updateEndpoints(noteItems.back(), w);
    }
    noteItems.push_back(w);
}

MainWindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QGridLayout>
#include <QWidget>
#include <QMainWindow>
#include <QScrollArea>
#include <drawwidget.h>
#include "drawscrollarea.h"
#include <vector>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

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

public slots:
    void scrollHorizontal();
    void AddItemSlot();

private:
    Ui::MainWindow *ui;
    QWidget* CreateNewItem();
    int counter = 0, currCol = 0, currRow = 0;
    std::vector<QWidget*> noteItems;

    QScrollArea* scrollArea;
    DrawWidget* drawWidget;
    QGridLayout* gridLayout;
};

#endif // MAINWINDOW_H

DrawWidget.cpp:

#include "drawwidget.h"
#include <QDebug>
#include <QRect>

DrawWidget::DrawWidget(QWidget *parent) : QWidget(parent)
{

}

void DrawWidget::paintEvent(QPaintEvent *)
{
    if (!drewSinceUpdate){
        drewSinceUpdate = true;
        QPainter painter(this);
        painter.setPen(QPen(Qt::black));

        for (ConnectedPair pair : items){
            const QWidget* from = pair.from;
            const QWidget* to =pair.to;

            QPoint start =  from->mapToGlobal(from->rect().topRight() +  QPoint(0, from->height()/2));
            QPoint end = to->mapToGlobal(to->rect().topLeft() +  QPoint(0, to->height()/2));

            painter.drawLine(mapFromGlobal(start), mapFromGlobal(end));
        }
    }
}

void DrawWidget::updateEndpoints(QWidget* startIn, QWidget* endIn){
    drewSinceUpdate = false;
    items.push_back(ConnectedPair{startIn, endIn});
}

DrawWidget.h

#ifndef DRAWWIDGET_H
#define DRAWWIDGET_H

#include <QWidget>
#include <QPainter>
#include <QtCore>
#include <vector>

class DrawWidget : public QWidget
{
    Q_OBJECT
public:
    explicit DrawWidget(QWidget *parent = nullptr);
    void updateEndpoints(QWidget* startIn, QWidget* endIn);
    virtual void paintEvent(QPaintEvent *);
signals:

private:
    struct ConnectedPair {
        const QWidget* from;
        const QWidget* to;
    };

    std::vector<ConnectedPair> items;
    bool drewSinceUpdate = true;
};

#endif // DRAWWIDGET_H

Answer 1

对于这种情况，我们使用函数 mapToGlobal() and mapfromGlobal()，因为 pos() returns 相对于父级的位置，如果小部件具有不同的父级，这可能会导致问题。

drawwidget.h

#ifndef DRAWWIDGET_H
#define DRAWWIDGET_H

#include <QWidget>

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

    void addWidgets(const QWidget *from, const QWidget *to);

protected:
    void paintEvent(QPaintEvent *);

private:
    struct WidgetsConnected {
        const QWidget* from;
        const QWidget* to;
    };

    QList<WidgetsConnected> list;

};

#endif // DRAWWIDGET_H

drawwidget.cpp

#include "drawwidget.h"

#include <QPainter>
DrawWidget::DrawWidget(QWidget *parent) : QWidget(parent)
{
}

void DrawWidget::addWidgets(const QWidget * from, const QWidget * to)
{
    list.append(WidgetsConnected{from , to});
    update();
}

void DrawWidget::paintEvent(QPaintEvent *)
{
    QPainter painter(this);
    for(const WidgetsConnected el: list){
        const QWidget* from = el.from;
        const QWidget* to = el.to;

        QPoint start =  from->mapToGlobal(from->rect().topRight() +  QPoint(0, from->height()/2));
        QPoint end = to->mapToGlobal(to->rect().topLeft() +  QPoint(0, to->height()/2));

        painter.drawLine(mapFromGlobal(start), mapFromGlobal(end));
    }
}

_{可以找到完整的示例here。}

使用QPainter在QWidgets之间画一条线

Using QPainter to draw a line between QWidgets

c++

qt

qt5

qpainter