如何使用复选框更改QGraphicsView背景

Question

在这个代码中，它改变了QGraphicsView背景。现在我需要更改背景，当我检查真正的checkBox。当我设置为checkBox检查true我需要设置背景像这个代码。当我设置checkBox检查false。我需要将QGraphicsView设置为正常的默认方式。我怎样才能做到这一点。

这是我的代码：

mainwindow.cpp

#include "mainwindow.h"
#include <QGraphicsTextItem>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent)
{
    scene = new Scene(this);
    scene->setSceneRect(10,10,260,200);
    view = new QGraphicsView(scene);
    setCentralWidget(view);
}

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QGraphicsView>
#include "scene.h"
#include "customrectitem.h"

class MainWindow : public QMainWindow
{
public:
    explicit MainWindow(QWidget *parent = 0);
private:
    QGraphicsView* view;
    QGraphicsScene* scene;
};

#endif // MAINWINDOW_H

Scene.h

#ifndef SCENE_H
#define SCENE_H

#include <QGraphicsScene>
#include <QPainter>
#include <QApplication>

class Scene : public QGraphicsScene
{
    Q_OBJECT
public:
    explicit Scene(QObject *parent = 0);
    int getGridSize() const {return this->gridSize;}

protected:
    void drawBackground (QPainter* painter, const QRectF &rect);
private:
    int gridSize;

};

#endif // SCENE_H

Scene.cpp

#include "scene.h"

Scene::Scene(QObject *parent) : QGraphicsScene(parent), gridSize(20)
{
    Q_ASSERT(gridSize > 0);
}

void Scene::drawBackground(QPainter *painter, const QRectF &rect)
{
        QColor c (10,140,255,155);
        painter->setPen(c);
        qreal left = int(rect.left()) - (int(rect.left()) % gridSize);
        qreal top = int(rect.top()) - (int(rect.top()) % gridSize);
        QVarLengthArray<QLineF,100> lines;
        for (qreal x = left; x < rect.right(); x += gridSize)
            lines.append(QLineF(x,rect.top(),x,rect.bottom()));

        for (qreal y = top; y < rect.bottom(); y += gridSize)
            lines.append(QLineF(rect.left(),y,rect.right(),y));
        painter->drawLines(lines.data(),lines.size());
}

Answer 1

虽然@Scheff的解决方案很好，但它有以下问题：

• 第一个解决方案使它只覆盖一个小空间，如果改变QGraphicsView的大小，它将无法覆盖所有空间。
• 第二个解决方案的问题是你使用viewport()坐标，而不是场景，如果你添加一个项目，然后放大QGraphicsView而不移动项目，那么你会看到一个滚动。

初始：

改变大小后：

纠正这两个错误的解决方案是覆盖QGraphicsScene的drawBackground方法：

#ifndef GRAPHICSSCENE_H
#define GRAPHICSSCENE_H

#include <QGraphicsScene>
#include <QPainter>


class GraphicsScene: public QGraphicsScene{

    int mGridSize;
    QColor mGridColor;
    bool mGridVisible;

    Q_OBJECT
public:
    GraphicsScene(QObject *parent = Q_NULLPTR):QGraphicsScene(parent),
        mGridSize(20), mGridColor(0x0a8affu), mGridVisible(true)
    {

    }
    bool gridVisible() const {
        return mGridVisible;
    }
    void setGridVisible(bool gridVisible){
        if(mGridVisible != gridVisible){
            mGridVisible = gridVisible;
            update();
        }
    }
protected:
    void drawBackground(QPainter *painter, const QRectF &rect){
        if(mGridVisible) {
            QRect r = rect.toRect();
            int xmin  =r.left() - r.left()%mGridSize - mGridSize;
            int ymin = r.top() - r.top()%mGridSize - mGridSize;
            int xmax = r.right() - r.right()%mGridSize + mGridSize;
            int ymax = r.bottom() - r.bottom()%mGridSize + mGridSize;
            for(int x=xmin ; x <= xmax; x += mGridSize ){
                painter->drawLine(x, r.top(), x, r.bottom());
            }

            for(int y=ymin ; y <= ymax; y+= mGridSize ){
                painter->drawLine(r.left(), y, r.right(), y);
            }
        }
    }
};

#endif // GRAPHICSSCENE_H

这种解决方案的优点是它不会加载任何新对象，除了图形位于场景的坐标中，因此没有第二个解决方案的位移。

初始：

改变大小后：

完整的例子可以在以下link找到

Answer 2

我有两个机会画一个网格：

1. 从QGraphicsView派出一个类并重载paintEvent()并绘制网格然后回落到QGraphicsView::paintEvent()
2. 将网格线添加为QGraphicsItemGroup下的场景项，以便控制可见性。

第一种方法的优点是它可以无缝地适应视口的任何大小调整。

这是我的示例代码testQGraphicsView-BgGrid.cc：

#include <QtWidgets>

class Canvas: public QGraphicsView {
  private:
    int _gridSize;
    QColor _gridColor;
    bool _gridVisible;

  public:
    explicit Canvas(QWidget *pQParent = nullptr):
      QGraphicsView(pQParent),
      _gridSize(20), _gridColor(0x0a8affu), _gridVisible(true)
    { }

    bool gridVisible() const { return _gridVisible; }
    void setGridVisible(bool gridVisible)
    {
      _gridVisible = gridVisible;
      viewport()->update();
    }

  protected:
    virtual void paintEvent(QPaintEvent *pQEvent) override
    {
      QPainter qPainter(viewport());
      if (_gridVisible) {
        const int wView = viewport()->width(), hView = viewport()->height();
        qPainter.setPen(_gridColor);
        for (int x = _gridSize / 2; x < wView; x += _gridSize) {
          qPainter.drawLine(x, 0, x, hView - 1);
        }
        for (int y = _gridSize / 2; y < hView; y += _gridSize) {
          qPainter.drawLine(0, y, wView - 1, y);
        }
      }
      QGraphicsView::paintEvent(pQEvent);
    }
};

void makeGrid(
  QGraphicsItemGroup &qItemGrp, const QSize &size,
  int gridSize = 20, const QColor &gridColor = 0x0a8affu)
{
  const int wView = size.width(), hView = size.height();
  for (int x = gridSize / 2; x < wView; x += gridSize) {
    QGraphicsLineItem *pQItem = new QGraphicsLineItem(x, 0, x, hView - 1);
    pQItem->setPen(gridColor);
    qItemGrp.addToGroup(pQItem);
  }
  for (int y = gridSize / 2; y < hView; y += gridSize) {
    QGraphicsLineItem *pQItem = new QGraphicsLineItem(0, y, wView - 1, y);
    pQItem->setPen(gridColor);
    qItemGrp.addToGroup(pQItem);
  }
}

int main(int argc, char **argv)
{
  QApplication app(argc, argv);
  // setup GUI
  QWidget qWnd;
  QGridLayout qGrid;
  QLabel qLblL(QString::fromUtf8("Grid as part of scene"));
  qGrid.addWidget(&qLblL, 0, 0);
  QCheckBox qTglGridL(QString::fromUtf8("Show Grid"));
  qGrid.addWidget(&qTglGridL, 0, 1);
  QGraphicsView qGraphView;
  qGrid.addWidget(&qGraphView, 1, 0, 1, 2);
  QLabel qLblR(QString::fromUtf8("Grid painted as background"));
  qGrid.addWidget(&qLblR, 0, 2);
  QCheckBox qTglGridR(QString::fromUtf8("Show Grid"));
  qGrid.addWidget(&qTglGridR, 0, 3);
  Canvas canvas;
  qGrid.addWidget(&canvas, 1, 2, 1, 2);
  qWnd.setLayout(&qGrid);
  qWnd.show();
  // init GUI
  QGraphicsScene qGraphSceneL;
  QGraphicsItemGroup qItemGrid;
  makeGrid(qItemGrid, qGraphView.viewport()->size());
  qGraphSceneL.addItem(&qItemGrid);
  qGraphView.setScene(&qGraphSceneL);
  qTglGridL.setCheckState(
    qItemGrid.isVisible() ? Qt::Checked : Qt::Unchecked);
  qTglGridR.setCheckState(
    canvas.gridVisible() ? Qt::Checked : Qt::Unchecked);
  // install signal handlers
  QObject::connect(&qTglGridL, &QCheckBox::stateChanged,
    [&qItemGrid](int state)
    {
      qItemGrid.setVisible(state != Qt::Unchecked);
    });
  QObject::connect(&qTglGridR, &QCheckBox::stateChanged,
    [&canvas](int state)
    {
      canvas.setGridVisible(state != Qt::Unchecked);
    });
  // runtime loop
  return app.exec();
}

使用VS2013和Qt 5.9.2编译，在Window 10（64位）上测试：

---

更新：

QMake脚本testQGraphicsView-BgGrid.pro：

SOURCES = testQGraphicsView-BgGrid.cc

QT += widgets

在cygwin上建立和测试bash：

$ qmake-qt5 testQGraphicsView-BgGrid.pro

$ make
g++ -c -fno-keep-inline-dllexport -D_GNU_SOURCE -pipe -O2 -Wall -W -D_REENTRANT -DQT_NO_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I. -isystem /usr/include/qt5 -isystem /usr/include/qt5/QtWidgets -isystem /usr/include/qt5/QtGui -isystem /usr/include/qt5/QtCore -I. -I/usr/lib/qt5/mkspecs/cygwin-g++ -o testQGraphicsView-BgGrid.o testQGraphicsView-BgGrid.cc
g++  -o testQGraphicsView-BgGrid.exe testQGraphicsView-BgGrid.o   -lQt5Widgets -lQt5Gui -lQt5Core -lGL -lpthread 

$ ./testQGraphicsView-BgGrid 

与X11应用程序相同的程序：

---

另一个更新：

我承认，在我的第一个版本中，我专注于如何使用复选框更改QGraphicsView背景，强调复选框忽略QGraphicsScene::drawBackground()（我不知道）并忽略网格渲染本身的问题（我用来代替）。正如eyllanesc's answer所指出的，两者都表明解决方案提供了一些弱点，至少我应该记录这些弱点。

1. 左视图（涉及网格到场景）不一定填充viewport()。恕我直言，如果网格填充场景使用的相关范围，这是可以接受的。 （这可能是为了想象这个场景只覆盖了viewport()的一部分。）然而，const QSize &size的论点makeGrid()是关于这个意图的不幸选择。我用const QRect &rect替换它修复了这个设计问题。
2. 右视图（重载paintEvent()）没有考虑可能的场景滚动。我通过将左上角映射到场景坐标（const QPointF offs = mapToScene(0, 0);）来修复此问题，并将偏移量视为循环中的起始值。

更新示例代码testQGrapicsView-BgGrid.cc：

#include <QtWidgets>

class Canvas: public QGraphicsView {
  private:
    int _gridSize;
    QColor _gridColor;
    bool _gridVisible;

  public:
    explicit Canvas(QWidget *pQParent = nullptr):
      QGraphicsView(pQParent),
      _gridSize(20), _gridColor(0x0a8affu), _gridVisible(true)
    { }

    bool gridVisible() const { return _gridVisible; }
    void setGridVisible(bool gridVisible)
    {
      _gridVisible = gridVisible;
      viewport()->update();
    }

  protected:
    virtual void paintEvent(QPaintEvent *pQEvent) override
    {
      QPainter qPainter(viewport());
      if (_gridVisible) {
        const int wView = viewport()->width(), hView = viewport()->height();
        const QPointF offs = mapToScene(0, 0);
        qPainter.setPen(_gridColor);
        for (int x = (int)offs.x() % _gridSize; x < wView; x += _gridSize) {
          qPainter.drawLine(x, 0, x, hView - 1);
        }
        for (int y = (int)offs.y() % _gridSize; y < hView; y += _gridSize) {
          qPainter.drawLine(0, y, wView - 1, y);
        }
      }
      QGraphicsView::paintEvent(pQEvent);
    }
};

void makeGrid(
  QGraphicsItemGroup &qItemGrp, const QRect &rect,
  int gridSize = 20, const QColor &gridColor = 0x0a8affu)
{
  for (int x = rect.x(), xE = x + rect.width(); x < xE; x += gridSize) {
    QGraphicsLineItem *pQItem
      = new QGraphicsLineItem(x, rect.y(), x, rect.height() - 1);
    pQItem->setPen(gridColor);
    qItemGrp.addToGroup(pQItem);
  }
  for (int y = rect.y(), yE = y + rect.height(); y < yE; y += gridSize) {
    QGraphicsLineItem *pQItem
      = new QGraphicsLineItem(rect.x(), y, rect.width() - 1, y);
    pQItem->setPen(gridColor);
    qItemGrp.addToGroup(pQItem);
  }
}

int main(int argc, char **argv)
{
  QApplication app(argc, argv);
  // setup GUI
  QWidget qWnd;
  QGridLayout qGrid;
  QLabel qLblL(QString::fromUtf8("Grid as part of scene"));
  qGrid.addWidget(&qLblL, 0, 0);
  QCheckBox qTglGridL(QString::fromUtf8("Show Grid"));
  qGrid.addWidget(&qTglGridL, 0, 1);
  QGraphicsView qGraphView;
  qGrid.addWidget(&qGraphView, 1, 0, 1, 2);
  QLabel qLblR(QString::fromUtf8("Grid painted as background"));
  qGrid.addWidget(&qLblR, 0, 2);
  QCheckBox qTglGridR(QString::fromUtf8("Show Grid"));
  qGrid.addWidget(&qTglGridR, 0, 3);
  Canvas canvas;
  qGrid.addWidget(&canvas, 1, 2, 1, 2);
  qWnd.setLayout(&qGrid);
  qWnd.show();
  // init GUI
  QGraphicsScene qGraphSceneL;
  QGraphicsItemGroup qItemGrid;
  makeGrid(qItemGrid, QRect(0, 0, 320, 240));
  qGraphSceneL.addItem(&qItemGrid);
  qGraphView.setScene(&qGraphSceneL);
  qTglGridL.setCheckState(
    qItemGrid.isVisible() ? Qt::Checked : Qt::Unchecked);
  qTglGridR.setCheckState(
    canvas.gridVisible() ? Qt::Checked : Qt::Unchecked);
  // install signal handlers
  QObject::connect(&qTglGridL, &QCheckBox::stateChanged,
    [&qItemGrid](int state)
    {
      qItemGrid.setVisible(state != Qt::Unchecked);
    });
  QObject::connect(&qTglGridR, &QCheckBox::stateChanged,
    [&canvas](int state)
    {
      canvas.setGridVisible(state != Qt::Unchecked);
    });
  // runtime loop
  return app.exec();
}

以下快照显示启动后和调整大小后的应用程序：