Started planning uiArea.

doc/area.md

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+# uiArea
+
+uiArea is a uiControl that provides a canvas you can draw on. It receives keyboard and mouse events, supports scrolling, is DPI aware, and has several other useful features. A uiArea consists of the drawing area itself and horizontal and vertical scrollbars.
+
+## The Area Handler
+
+A uiArea is driven by an *area handler*. An area handler is an object with several methods that uiArea calls to do certain tasks. To create an area handler, simply have a structure whose first member is of type `uiAreaHandler`:
+
+```c
+struct uiAreaHandler {
+	void (*Draw)(uiAreaHandler *h, uiArea *a, uiAreaDrawParams *p);
+	void (*HScrollConfig)(uiAreaHandler *h, uiArea *a, uiAreaScrollConfig *c);
+	void (*VScrollConfig)(uiAreaHandler *h, uiArea *a, uiAreaScrollConfig *c);
+}
+```
+
+## Drawing
+
+Unlike drawing canvas controls in other toolkits, uiArea does **not** have a fixed size. The coordinate (0, 0) is always the top-left corner of the drawing area, regardless of how big the uiArea is in the current window or where the scrollbars presently are. Instead, you simulate a size by setting the scrollbar bounds, and you are given the current scrolling positions to base your drawing with.
+
+The visible drawing area is called the *content area* by the drawing machinery.
+
+TODO have a diagram.
+
+When a part of the uiArea needs to be redrawn, the area handler's `Draw()` method is called. It receives the area handler, the uiArea, and a structure of parameters necessary for drawing.
+
+```c
+struct uiAreaDrawParams {
+	uiDrawContext *context;
+
+	intmax_t contentWidth;
+	intmax_t contentHeight;
+
+	intmax_t hscrollpos;
+	intmax_t vscrollpos;
+
+	intmax_t clipX;
+	intmax_t cilpY;
+	intmax_t clipWidth;
+	intmax_t clipHeight;
+
+	TODO dpiX;
+	TODO dpiY;
+};
+```
+
+`context` is the drawing context; see drawing.md for details.
+
+`contentWidth` and `contentHeight` is the current width and height of the content area. `hscrollpos` and `vscrollpos` are the current horizontal and vertical positions of the scrollbars, in units defined by the scrollbar configurations; see below.
+
+`clipX`, `clipY`, `clipWidth`, and `clipHeight` define a rectangle, in content area coordinates, that the OS has requested to be redrawn. You can use this to optimize your drawing by only drawing where drawing is needed; the OS may choose to drop any drawing done outside the clip rectangle.
+
+`dpiX` and `dpiY` are the uiArea's current DPI in the X and Y directions, respectively. Do not save these values; they are not guaranteed to stay the same once `Draw()` returns.