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void QWidget::repaint ( int x, int y, int w, int h, bool erase = TRUE ) [槽]
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; } }
参考：http://www.cnblogs.com/SkylineSoft/articles/2046303.html
http://www.informit.com/articles/article.aspx?p=1405227&seqNum=4
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原文地址：http://www.cnblogs.com/findumars/p/4733606.html
