IDisplay.StartDrawing和EndDrawing

StartDrawing and FinishDrawing are used to manage clipping, symbols, and caching. Call StartDrawing and FinishDrawing anytime you want to draw to a device such as a display, printer, or cache (bitmap).  However, if you are drawing in response to IActiveViewEvents::AfterDraw, the Map object automatically makes these calls for you.

 StartDrawing fires the IDisplayEvents::DisplayStarted event.

When FinishDrawing is called, all the caches get flushed to the screen.

FinishDrawing must be called before StartDrawing can be called again.

比如:执行了FinishDrawing之后,图形才会显示在屏幕上.在执行这个方法之前,所有的东西是画在内存中的.