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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.
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