Question

关于Qt 中update和repaint的区别

Answer 1

通过立即调用paintEvent()来直接重新绘制窗口部件，如果erase为真，Qt在paintEvent()调用之前擦除区域(x,y,w,h)。
如果w是负数，它被width()-x替换，并且如果h是负数，它被height()-y替换。 如果你需要立即重新绘制，建议使用repaint()，
比如在动画期间。在绝大多数情况下，update()更好，因为它允许Qt来优化速度并且防止闪烁。
警告：如果你在一个函数中调用repaint()，而它自己又被paintEvent()调用，你也许会看到无线循环。
update()函数从来不会产生循环。
void QWidget::update () [槽]
更新窗口部件，当Qt回到主事件中时，它规划了所要处理的绘制事件。这样允许Qt进行优化从而得到比调用repaint()更快的速度和
更少的闪烁。 几次调用update()的结果通常仅仅是一次paintEvent()调用。 Qt通常在paintEvent()调用之前擦除这个窗口部件的
区域，仅仅只有在WRepaintNoErase窗口部件标记被设置的时候才不会。
在这区别中关键点是：repaint()是立即调用paintEvent()，而update()是几次执行才调用一次paintEvent()。
这样update()会造成这样的结果：paintEvent()中的任务没有执行完，就又被update().paintEvent()中被积压的任务越来越多。
程序例子：
(1)问题出现时候的情况(10毫秒每次，用update()。paintEvent()积累了很多处理任务)：
#include<QPainter>
#include<QDebug>
#include<QMessageBox>
#include "mainwindow.h"
#include "ui_mainwindow.h"
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
this->showMaximized();
i = 0;
realWidth = this->width();
realHeight = this->height();
pixmap = QPixmap(realWidth,realHeight);
connect(this,SIGNAL(haveData(QPoint)),this,SLOT(getPointAndDraw(QPoint)));
connect(&timer,SIGNAL(timeout()),this,SLOT(getPoint()));
timer.start(10);
}
MainWindow::~MainWindow()
{
delete ui;
}
void MainWindow::getPoint()
{
if(i < realWidth)
{
point = QPoint(i,(uint(qrand())) % realHeight);
i++;
}
else
{
i = i % realWidth;
point = QPoint(i,(uint(qrand())) % realHeight);
i++;
}
emit haveData(point);
}
void MainWindow::getPointAndDraw(QPoint point)
{
index = point.x();
QPainter painter(&pixmap);
painter.setPen(Qt::green);
painter.drawLine(lastPoint,point);
painter.setPen(Qt::black);
painter.setBrush(Qt::red);
painter.drawRect(index+1,0,5,realHeight);
if(point.x() < realWidth-1)
lastPoint = point;
else
lastPoint = QPoint(0,0);
update();
// this->repaint(index-1,0,5,realHeight);
}
void MainWindow::paintEvent(QPaintEvent *e)
{
//return ;
QPainter painter(this);
QRect target1(0, 0, realWidth, realHeight/5);
QRect target2(0, realHeight/5, realWidth, realHeight/5);
QRect target3(0, 2*realHeight/5, realWidth, realHeight/5);
QRect target4(0, 3*realHeight/5, realWidth, realHeight/5);
QRect target5(0, 4*realHeight/5, realWidth, realHeight/5);
QRect source(0, 0, realWidth, realHeight);
painter.drawPixmap(target1,pixmap,source);
painter.drawPixmap(target2,pixmap,source);
painter.drawPixmap(target3,pixmap,source);
painter.drawPixmap(target4,pixmap,source);
painter.drawPixmap(target5,pixmap,source);
}
void MainWindow::resizeEvent(QResizeEvent *e)
{
realWidth = this->width();
realHeight = this->height();
}
void MainWindow::changeEvent(QEvent *e)
{
QMainWindow::changeEvent(e);
switch (e->type()) {
case QEvent::LanguageChange:
ui->retranslateUi(this);
break;
default:
break;
}
}
(2)每隔1000毫秒刷新一次，用update().一秒种有足够的时间处理paintEvent()，无积累。
#include<QPainter>
#include<QDebug>
#include<QMessageBox>
#include "mainwindow.h"
#include "ui_mainwindow.h"
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
this->showMaximized();
i = 0;
realWidth = this->width();
realHeight = this->height();
pixmap = QPixmap(realWidth,realHeight);
connect(this,SIGNAL(haveData(QPoint)),this,SLOT(getPointAndDraw(QPoint)));
connect(&timer,SIGNAL(timeout()),this,SLOT(getPoint()));
timer.start(1000);
}
MainWindow::~MainWindow()
{
delete ui;
}
void MainWindow::getPoint()
{
if(i < realWidth)
{
point = QPoint(i,(uint(qrand())) % realHeight);
i++;
}
else
{
i = i % realWidth;
point = QPoint(i,(uint(qrand())) % realHeight);
i++;
}
emit haveData(point);
}
void MainWindow::getPointAndDraw(QPoint point)
{
index = point.x();
QPainter painter(&pixmap);
painter.setPen(Qt::green);
painter.drawLine(lastPoint,point);
painter.setPen(Qt::black);
painter.setBrush(Qt::red);
painter.drawRect(index+1,0,5,realHeight);
if(point.x() < realWidth-1)
lastPoint = point;
else
lastPoint = QPoint(0,0);
update();
// this->repaint(index-1,0,5,realHeight);
}
void MainWindow::paintEvent(QPaintEvent *e)
{
//return ;
QPainter painter(this);
QRect target1(0, 0, realWidth, realHeight/5);
QRect target2(0, realHeight/5, realWidth, realHeight/5);
QRect target3(0, 2*realHeight/5, realWidth, realHeight/5);
QRect target4(0, 3*realHeight/5, realWidth, realHeight/5);
QRect target5(0, 4*realHeight/5, realWidth, realHeight/5);
QRect source(0, 0, realWidth, realHeight);
painter.drawPixmap(target1,pixmap,source);
painter.drawPixmap(target2,pixmap,source);
painter.drawPixmap(target3,pixmap,source);
painter.drawPixmap(target4,pixmap,source);
painter.drawPixmap(target5,pixmap,source);
}
void MainWindow::resizeEvent(QResizeEvent *e)
{
realWidth = this->width();
realHeight = this->height();
}
void MainWindow::changeEvent(QEvent *e)
{
QMainWindow::changeEvent(e);
switch (e->type()) {
case QEvent::LanguageChange:
ui->retranslateUi(this);
break;
default:
break;
}
}
(3)继续改进(10毫秒每次，用repaint()。一次repaint(),一次paintEvent(),无积累).
#include<QPainter>
#include<QDebug>
#include<QMessageBox>
#include "mainwindow.h"
#include "ui_mainwindow.h"
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
this->showMaximized();
i = 0;
realWidth = this->width();
realHeight = this->height();
pixmap = QPixmap(realWidth,realHeight);
connect(this,SIGNAL(haveData(QPoint)),this,SLOT(getPointAndDraw(QPoint)));
connect(&timer,SIGNAL(timeout()),this,SLOT(getPoint()));
timer.start(10);
}
MainWindow::~MainWindow()
{
delete ui;
}
void MainWindow::getPoint()
{
if(i < realWidth)
{
point = QPoint(i,(uint(qrand())) % realHeight);
i++;
}
else
{
i = i % realWidth;
point = QPoint(i,(uint(qrand())) % realHeight);
i++;
}
emit haveData(point);
}
void MainWindow::getPointAndDraw(QPoint point)
{
index = point.x();
QPainter painter(&pixmap);
painter.setPen(Qt::green);
painter.drawLine(lastPoint,point);
painter.setPen(Qt::black);
painter.setBrush(Qt::red);
painter.drawRect(index+1,0,5,realHeight);
if(point.x() < realWidth-1)
lastPoint = point;
else
lastPoint = QPoint(0,0);
this->repaint(index-1,0,5,realHeight);
}
void MainWindow::paintEvent(QPaintEvent *e)
{
//return ;
QPainter painter(this);
QRect target1(0, 0, realWidth, realHeight/5);
QRect target2(0, realHeight/5, realWidth, realHeight/5);
QRect target3(0, 2*realHeight/5, realWidth, realHeight/5);
QRect target4(0, 3*realHeight/5, realWidth, realHeight/5);
QRect target5(0, 4*realHeight/5, realWidth, realHeight/5);
QRect source(0, 0, realWidth, realHeight);
painter.drawPixmap(target1,pixmap,source);
painter.drawPixmap(target2,pixmap,source);
painter.drawPixmap(target3,pixmap,source);
painter.drawPixmap(target4,pixmap,source);
painter.drawPixmap(target5,pixmap,source);
}
void MainWindow::resizeEvent(QResizeEvent *e)
{
realWidth = this->width();
realHeight = this->height();
}
void MainWindow::changeEvent(QEvent *e)
{
QMainWindow::changeEvent(e);
switch (e->type()) {
case QEvent::LanguageChange:
ui->retranslateUi(this);
break;
default:
break;
}
}