Qt中update()和repaint()的区别
解释1
voidQWidget::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
#include
#include
#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
#include
#include
#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
#include
#include
#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;
}
}
解释2
Qt里面的重绘和Windows编程里面的重绘差不多。但是Qt的重绘更有特色，更加智能。
在讲之前，先说说paintEvent()
paintEvent()是一个虚函数槽(slot)，子类可以对父类的paintEvent进行重写。当调用update()，repaint()的时候，paintEvent()会被调用，另外，当界面有任何改变的时候，paintEvent()也会被调用，这种界面的改变包括界面从隐藏到显示，界面尺寸改变，当然还包括界面内容改变的时候会被调用。paintEvent()是已经被高度优化过的函数，它本身已经自动开启并实现了双缓冲(X11系统需要手动去开启双缓冲)，因此Qt中重绘不会引起任何闪烁。
X11系列系统手动开启双缓冲的方法如下：
[cpp] view plaincopyprint?
1. ...
2. extern void qt_x11_set_global_double_buffer(bool);
3. qt_x11_set_global_double_buffer(false);
4. ...
有了paintEvent的知识，现在再来看看update()和repaint()
update和repaint是一类的，需要重绘的对象主动去调用，然后重绘。update和repaint调用之后，都回去调用paintEvent().
repaint()，被调用之后，立即执行重绘，因此repaint是最快的，紧急情况下需要立刻重绘的可以使用repaint()。但是调用repaint的函数不能放到paintEvent中调用。举个例子：有一个继承于QWidget的子类MyWidget，并在子类中对paintEvent进行了重写。我们在MyWidget::myrepaint()中调用repaint()。但是，myrepaint()又被重写的paintEvent()调用。这样调用repaint()的函数又被paintEvent()调用，由于repaint()是立即重绘，而且repaint()在调用paintEvent之前几乎不做任何优化操作，而是会造成死循环，即：先调用repaint()，继而调用paintEvent()，paintEvent()反过来有调用repaint()...如此死循环。
update()跟repaint()比较，update则更加有优越性。update()调用之后并不是立即重绘，而是将重绘事件放入主消息循环中，由main的event loop来统一调度的(其实也是比较快的)。update在调用paintEvent之前，还做了很多优化，如果update被调用了很多次，最后这些update会合并到一个大的重绘事件加入到消息队列，最后只有这个大的update被执行一次。同时也避免了repaint()中所提到的死循环。因此，一般情况下，我们调用update就够了，跟repaint()比起来，update是推荐使用的。