Qwt源码解读之QwtPlotItem类（一）
QwtPlotItem类是画布上所有图元的基类。QwtPlotItem是一个抽象基类，所有的实例图元必须实现YourPlotItem::draw()方法（纯虚函数）。具体的看看Qwt文档：
2、设置图元的叠加（绘制）顺序：
Base class for items on the plot canvas.
A plot item is "something", that can be painted on the plot canvas, or only affects the scales of the plot widget. They can be categorized as:
- Representator
A "Representator" is an item that represents some sort of data on the plot canvas. The different representator classes are organized according to the characteristics of the data:- QwtPlotMarker Represents a point or a horizontal/vertical coordinate
- QwtPlotCurve Represents a series of points
- QwtPlotSpectrogram ( QwtPlotRasterItem ) Represents raster data
- ...
- Decorators
A "Decorator" is an item, that displays additional information, that is not related to any data:- QwtPlotGrid
- QwtPlotScaleItem
- QwtPlotSvgItem
- ...
Depending on the QwtPlotItem::ItemAttribute flags, an item is included into autoscaling or has an entry on the legnd.
Before misusing the existing item classes it might be better to implement a new type of plot item ( don't implement a watermark as spectrogram ). Deriving a new type of QwtPlotItemprimarily means to implement the YourPlotItem::draw() method.
- See also:
- The cpuplot example shows the implementation of additional plot items.
QwtPlotItem继承自QwtLegendItemManager类，其目的是为了绑定QwtPlotItem到QwtLegend。即QwtLegendItemManager类主要是被当作接口使用。【Abstract API to bind plot items to the legend.】 继承关系示例图如下：
QwtLegendItemManager类代码分析：
/*!
\brief Abstract API to bind plot items to the legend
*/
class QWT_EXPORT QwtLegendItemManager
{
public:
//! Constructor
QwtLegendItemManager()
{
}
//! Destructor
virtual ~QwtLegendItemManager()
{
}
/*!
Update the widget that represents the item on the legend
\param legend Legend
\sa legendItem()
*/
virtual void updateLegend( QwtLegend *legend ) const = 0;
/*!
Allocate the widget that represents the item on the legend
\return Allocated widget
\sa updateLegend() QwtLegend()
*/
virtual QWidget *legendItem() const = 0;
/*!
QwtLegendItem can display an icon-identifier followed
by a text. The icon helps to identify a plot item on
the plot canvas and depends on the type of information,
that is displayed.
The default implementation paints nothing.
*/
virtual void drawLegendIdentifier( QPainter *, const QRectF & ) const
{
}
};
QwtPlotItem类代码分析：
数据：
class QwtPlotItem::PrivateData
{
public:
PrivateData():
plot( NULL ),
isVisible( true ),
attributes( 0 ),
renderHints( 0 ),
z( 0.0 ),
xAxis( QwtPlot::xBottom ),
yAxis( QwtPlot::yLeft )
{
}
mutable QwtPlot *plot;
bool isVisible;
QwtPlotItem::ItemAttributes attributes;
QwtPlotItem::RenderHints renderHints;
double z;
int xAxis;
int yAxis;
QwtText title;
};1、关于mutable：mutable关键词的作用是可以在常函数中修改被mutable修饰的成员变量的值。但从代码的实现来看，没见过这种需求的必要？
2、QwtPlotItem仍然是一个抽象基类，因此只定义了很少的必须的共有属性。
3、这种共性的提取是在设计一个继承体系时需要重点考虑和验证的，也是在编码过程中重构需要时时关心的地方。
实现：
1、返回运行时类型信息：
virtual int rtti() const; // 运行时类型信息
2、设置图元的叠加（绘制）顺序：
/*!
\brief Set the z value
Plot items are painted in increasing z-order.
\param z Z-value
\sa z(), QwtPlotDict::itemList()
*/
void QwtPlotItem::setZ( double z )
{
if ( d_data->z != z )
{
if ( d_data->plot ) // update the z order
d_data->plot->attachItem( this, false );
d_data->z = z;
if ( d_data->plot )
d_data->plot->attachItem( this, true ); // 重新排列Items的顺序
itemChanged();
}
}3、需要在派生类重新实现的几个虚函数：
virtual void itemChanged(); // 更新legend并会调用QwtPlot的自动更新功能
/*!
\brief Draw the item
\param painter Painter
\param xMap Maps x-values into pixel coordinates.
\param yMap Maps y-values into pixel coordinates.
\param canvasRect Contents rect of the canvas in painter coordinates
*/
virtual void draw( QPainter *painter,
const QwtScaleMap &xMap, const QwtScaleMap &yMap,
const QRectF &canvasRect ) const = 0; // 纯虚函数，绘制Item，会在各派生类中实现
virtual QRectF boundingRect() const; // Item 的外围边界框，默认实现返回一个无效的矩形，会在各派生类中重新实现
virtual void updateLegend( QwtLegend *legend ) const; // 更新（还不存在的话则创建）QwtLegendItem， 并绘制相应的标识图示（drawLegendIdentifier()）
virtual void updateScaleDiv(
const QwtScaleDiv&, const QwtScaleDiv& ); // 该接口的默认的实现是：什么都不做。只是对某些特定的派生类需要重新实现（like QwtPlotGrid()）
virtual QWidget *legendItem() const; // Allocate the widget that represents the item on the legend4、禁用拷贝和赋值：
private:
// Disabled copy constructor and operator=
QwtPlotItem( const QwtPlotItem & );
QwtPlotItem &operator=( const QwtPlotItem & );