vc++技术内幕（第四版）笔记（第５章）(2)

6，当利用CDC对象绘图的时候，所绘制的图形都要依赖于设备环境的状态：如画笔、画刷、字体等GDI绘图对象 和 当前映射模式的选择等。

可以使用CDC成员函数了设置所需要的饿设备环境状态。如：SelectObject()函数可将GDI对象随时选入设备环境中。

7，CPaintDC类：当需要重写OnPaint函数的时候，就需要使用CPaintDC类。默认的OnPaint函数会使用已经设置好的设备环境来调用OnDraw函数

注意：CPaintDC类的构造函数和析构函数所完成的工作都是针对显示用的。

例：
void CMyView::OnPaint()
{
CPaintDC dc(this);
OnPrepareDC(&dc);
dc.TextOut(0,0,"for the display,not the printer");
OnDraw(&dc);
}

说明：

1）CPaintDC( CWnd* pWnd );

//CPaintDC::CPaintDC Constructs a CPaintDC object, prepares the application window for painting, and stores the PAINTSTRUCT structure in the m_ps member variable.

2）CPaintDC类的构造函数自动调用BeginPaint,而它的析构函数会自动调用EndPaint.

3）CDC* BeginPaint( LPPAINTSTRUCT lpPaint );

//CWnd::BeginPaint Prepares CWnd for painting and fills a PAINTSTRUCT data structure with information about the painting.
void EndPaint( LPPAINTSTRUCT lpPaint );
//CWnd::EndPaint Marks the end of painting in the given window. The EndPaint member function is required for each call to the BeginPaint member function, but only after painting is complete.

问题：

1）在View类中一旦定义了OnPaint()函数，则WM_PAINT消息由OnPaint函数来响应。OnDraw函数不再被调用。原因？

8，GDI对象：

Class   Windows handle type

CPen   HPEN
CBrush   HBRUSH
CFont   HFONT
CBitmap  HBITMAP
CPalette  HPALETTE
CRgn   HRGN

说明：

Each graphic-object class in the class library has a constructor that allows you to create graphic objects of that class, which you must then initialize with the appropriate create function, such as CreatePen.

Each graphic-object class in the class library has a cast operator that will cast an MFC object to the associated Windows handle. The resulting handle is valid until the associated object detaches it. Use the object’s Detach member function to detach the handle.