1. Redraw events

• The reason for the redrawing event
  • repaint() Function or update() Function called ;
  • Hidden parts are now redisplayed ;
• Most components can simply redraw the entire interface , However, some parts that are slow to draw need to be optimized and only the needed areas need to be drawn （ have access to **QPaintEvent::region()** To get the area ）, This speed optimization does not affect the results .
• Qt It will speed up drawing by merging multiple redrawing events into one event , When update() The function is called many times , Or when the window system sends multiple redrawing Events ,Qt These events will be merged into one event , And this time has the largest area that needs to be redrawn .
• **update()** The function does not redraw immediately , Wait until the Qt Only after returning to the dwell time cycle , So multiple calls update() Functions usually cause only one paintEvent() Function call .
• repaint() The function immediately calls paintEvent() Function to redraw parts , Only if you must redraw immediately （ For example, in animation ）, Only use repaint() function .
• update() Function allows Qt Optimize speed and reduce flicker , however repaint() Functions don't support such optimizations .
• Never in **paintEvent()** Call in function update() and repaint()
• Here is the code to make the interface display darker colors ：

	setBackgroundRole(QPalette::Dark);
	setAutoFillBackground(true);

2. shear

• QPainter You can cut any drawing operation , It can cut a rectangle 、 Content in an area or a path , This can be used separately **setClipRect()、setClipRegion()、setClipPath()** Function to implement .
• The following code cuts the text in a rectangle ：

	QPainter painter(this);
	painter.setClipRect(10,0,20,10);
	painter.drawText(10,10,tr("10086"));

3. Reading and writing images

• The easiest way to read an image is to use QImage and QPixmap Constructor for , Or call QImage::load() and QPixmap::load() function .
• Qt Medium QImageReader Class provides a format independent interface , You can read images from files or other devices . More control can be provided when reading images , If you use setScaledSize() Function to read the image in the specified size , You can also use setClipRect() Read an area of the image .
• QImageWriter Class is used to store images , It supports specific options for formatting images , Such as gamma level 、 Compression grade and quality, etc . If you don't need to set these options , Use it directly QImage::save() and **QImagePixmap::save()** Function .