Custom Draw不是一个通用控件，而是一个许多控件提供的服务。用于定制控件的外观。
通常以下控件支持Custom Draw功能
- Header controls
- List-view controls
- Rebar controls
- Toolbar controls
- ToolTip controls
- Trackbar controls
- Tree-view controls
- （杯具，我是查找修改CSliderCtrl的办法找到的）
控件在绘制操作的特殊时间发送NM_CUSTOMDRAW通知消息，NM_CUSTOMDRAW通知作为WM_NOTIFY消息发送。
根据系统或者其他应用程序的消息，通用控件周期性地绘制删除自己。
控件绘制或者删除自己的过程叫做paint cycle。
支持custom draw 的控件在每个paint cycle 周期性地发送NM_CUSTOMDRAW通知消息，这个消息一般伴随在
NMCUSTOMDRAW结构体中或者把NMCUSTOMDRAW结构体作为第一个成员的其他结构体中。
NMCUSTOMDRAW结构体的一个成员dwDrawStage代表paint cycle的当前状态。
控件有四个基本绘制状态可以通知它的所属，如下：
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Global draw stage values Description
CDDS_PREPAINT Before the paint cycle begins.
CDDS_POSTPAINT After the paint cycle is complete.
CDDS_PREERASE Before the erase cycle begins.
CDDS_POSTERASE After the erase cycle is complete.
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上面的每个值可以和以下CDDS_ITEM标志组合描述具体item的绘制状态。
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Item-specific draw stage values Description
CDDS_ITEMPREPAINT Before an item is drawn.
CDDS_ITEMPOSTPAINT After an item has been drawn.
CDDS_ITEMPREERASE Before an item is erased.
CDDS_ITEMPOSTERASE After an item has been erased.
CDDS_SUBITEM Shell and Common Controls Versions 4.71. Flag combined with CDDS_ITEMPREPAINTor CDDS_ITEMPOSTPAINT if a subitem is being drawn. This will only be set if CDRF_NOTIFYITEMDRAW is returned from CDDS_PREPAINT.
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应用程序必须处理NM_CUSTOMDRAW通知消息然后回复一个特殊值告诉控件该做什么。
每个paint cycle开始后，控件发送NM_CUSTOMDRAW通知消息，此时NMCUSTOMDRAW结构体
的dwDrawStage成员描述为CDDS_PREPAINT。你的应用程序返回的值指定控件在什么时候如何发送
paint cycle后续的绘制消息。你的应用程序可以发送下面标志位的组合：
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Return value Effect
CDRF_DODEFAULT The control will draw itself. It will not send additional NM_CUSTOMDRAW notifications for this paint cycle. This flag cannot be used with any other flag.
CDRF_NOTIFYITEMDRAW The control will notify the parent of any item-specific drawing operations. It will send NM_CUSTOMDRAW notification messages before and after it draws items.
CDRF_NOTIFYPOSTPAINT The control will send an NM_CUSTOMDRAW notification when the painting cycle for the entire control is complete.
CDRF_SKIPDEFAULT The control will not perform any painting at all.
CDRF_DOERASE The control will only draw the background.
CDRF_SKIPPOSTPAINT The control will not draw the focus rectangle around an item.
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如果你的应用程序为custom draw 的首个prepaint返回CDRF_NOTIFYITEMDRAW，控件会为paint cycle中他绘制的每个item发送通知，这些通知将NMCUSTOMDRAW结构体的dwDrawStage成员设置为CDDS_ITEMPREPAINT。
当控件结束这个item的时候，你可以返回CDRF_NOTIFYPOSTPAINT要求控件发送其他的通知。
返回CDRF_DODEFAULT，控件开始绘制下个item的时候才通知它的父窗口。