QT update和repaint的区别

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;

}

}

http://blog.csdn.net/songjinshi/article/details/6569910