问题
`I retrieved the position of the mouse cursor and I want to draw a line by paintevent but is not called by update so nothing is happening
.cpp
void MainWindow::mousePressEvent(QMouseEvent * event)
{
if(event->button() == Qt::LeftButton)
{
if(mFirstClick)
{
mStartX = event->x();
mStartY = event->y();
mFirstClick = false;
}
else if(!mFirstClick)
{
mEndX = event->x();
mEndY = event->y();
mFirstClick = true;
mPaintFlag = true;
update();
}
}
ui->label_7->setText(QString::number(mStartX)+ ", "+QString::number(mStartY) +"//"+QString::number(mEndX)+ ", "+QString::number(mEndY));
}
void MainWindow::paintEvent(QPaintEvent * event)
{
QMainWindow::paintEvent(event);
if(mPaintFlag=true)
{
QPainter painter(this);
QPen paintpen(Qt::red);
paintpen.setWidth(4);
QPen linepen(Qt::red);
linepen.setWidth(4);
QPoint p1;
p1.setX(mStartX);
p1.setY(mStartY);
painter.setPen(paintpen);
painter.drawPoint(p1);
QPoint p2;
p2.setX(mEndX);
p2.setY(mEndY);
painter.setPen(paintpen);
painter.drawPoint(p2);
painter.setPen(linepen);
painter.drawLine(p1, p2);
}
}
.h
protected:
void mousePressEvent(QMouseEvent * e);
virtual void paintEvent(QPaintEvent * e) override;
I did repaint instead of update but nothing is displayed ,how can i make the call to paintevent to draw the line .
回答1:
Not sure what OP did wrong. At least, OP didn't expose an MCVE.
Out of curiosity, I did one on my own – testQMainWindowPaint.cc:
#include <QtWidgets>
class MainWindow: public QMainWindow {
private:
QPoint start, end;
bool firstClick;
public:
MainWindow();
virtual ~MainWindow() = default;
MainWindow(const MainWindow&) = delete;
MainWindow& operator=(const MainWindow&) = delete;
protected:
virtual void mousePressEvent(QMouseEvent *pQEvent) override;
virtual void paintEvent(QPaintEvent *pQEvent) override;
};
MainWindow::MainWindow():
QMainWindow(),
start(0, 0), end(0, 0), firstClick(true)
{ }
void MainWindow::mousePressEvent(QMouseEvent *pQEvent)
{
if (pQEvent->button() == Qt::LeftButton) {
(firstClick ? start : end) = pQEvent->pos();
firstClick = !firstClick;
update();
pQEvent->accept();
}
}
void MainWindow::paintEvent(QPaintEvent *pQEvent)
{
QMainWindow::paintEvent(pQEvent);
if (!firstClick) return;
QPainter painter(this);
QPen pen(Qt::red);
pen.setWidth(4);
painter.setPen(pen);
painter.drawLine(start, end);
}
int main(int argc, char **argv)
{
qDebug() << "Qt Version:" << QT_VERSION_STR;
QApplication app(argc, argv);
// init GUI
MainWindow winMain;
winMain.show();
// runtime loop
return app.exec();
}
Compile with testQMainWindowPaint.pro:
SOURCES = testQMainWindowPaint.cc
QT += widgets
Compiled in cygwin64 on Windows 10:
$ qmake-qt5 testQMainWindowPaint.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 testQMainWindowPaint.o testQMainWindowPaint.cc
g++ -o testQMainWindowPaint.exe testQMainWindowPaint.o -lQt5Widgets -lQt5Gui -lQt5Core -lGL -lpthread
$ ./testQMainWindowPaint
Qt Version: 5.9.4
After clicking twice, I made the following snapshot:
Hence, I came to the conclusion: The mistake of OP must be in the non-exposed code. Although, I reproduced code of OP not exactly, I didn't see any critical part which might've prevented rendering of that red line.
来源：https://stackoverflow.com/questions/56481098/update-does-not-call-paintevent