emWin - SEGGER Microcontroller

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19.6 Major changes to the look of the ’Flex’ skin
The drawing mechanism of the default design without skinning is a ’black box’ for the
application designer. The same is true for skinning if no major changes of the default
look are required. If changing the attributes of the default skin is not sufficient to
realize the required look, it is required to understand the details of the drawing
mechanism of skinning.
19.6.1 The skinning callback mechanism
The drawing mechanism for all skinnable widgets is very similar and looks as follows:
int _DrawSkin(const WIDGET_ITEM_DRAW_INFO * pDrawItemInfo) {
switch (pDrawItemInfo->Cmd) {
case WIDGET_ITEM_DRAW_BACKGROUND:
/* Draw the background */
break;
case WIDGET_ITEM_DRAW_TEXT:
/* Draw the text */
break;
case WIDGET_ITEM_CREATE:
/* Additional function calls required to create the widget */
break;
...
}
Elements of WIDGET_ITEM_DRAW_INFO
Data type Element Description
WM_HWIN hWin Handle to the widget.
int Cmd Command to be processed.
int ItemIndex Index of item to be drawn.
int x0 Leftmost coordinate in window coordinates.
int y0 Topmost coordinate in window coordinates.
int x1 Rightmost coordinate in window coordinates.
int y1 Bottommost coordinate in window coordinates.
void * p Data pointer to widget specific information.
This scheme is identical to all skinnable widgets. The callback function receives a
pointer to a WIDGET_ITEM_DRAW_INFO structure. The structure pointed by pDrawItem-
Info contains a command which has to be processed, a handle to the widget and further
information whose meaning may vary by widget. The skinning callback function
has to react with drawing a dedicated detail or with returning a dedicated value.
How to use the drawing information in detail is explained later in this chapter.
19.6.2 Changing the look of the default skin
Understanding the above callback mechanism is important because changing a skin
can easily be done by deriving a new skin from an existing one. A small example
should show how the look of the default skin of a widget can be changed.
UM03001 User & Reference Guide for emWin V5.18 © 1997 - 2012 SEGGER Microcontroller GmbH & Co. KG
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