Added initial planning document for the Area Control.

areaplan.md

@@ -0,0 +1,82 @@
+```go
+```go
+type Area struct {		// implements Control
+	// Paint receives requests to redraw from the window system.
+	Paint		chan PaintRequest
+
+	// Keyboard receives keyboard events.
+	Key		chan KeyEvent			// not covered here
+
+	// Mouse receives mouse events.
+	Mouse	chan MouseEvent		// not covered here
+}
+
+// PaintRequest represents a request to redraw an Area.
+// It is sent across Area.Paint.
+type PaintRequest struct {
+	// Rect is the clipping rectangle that needs redraw.
+	Rect		image.Rect
+
+	// Out is a channel on which you send the image to redraw.
+	Out		chan<- *image.NRGBA
+}
+```
+
+and an example of intended use:
+
+```go
+func myAreaGoroutine(area *ui.Area, start <-chan bool) {
+	var img *image.NRGBA
+
+	// initialize img here
+	<-start		// sent after calling Window.Open()
+	area.SetSize(img.Rect.Dx(), img.Rect.Dy())	// sets the internal size; scrollbars and scrolling is handled automatically
+	for {
+		select {
+		case req := <-area.Paint:
+			req.Out <- img.SubImage(req.Rect).(*image.NRGBA)
+		case e := <-area.Mouse:
+			// draw on a mouse click, for instance
+		}
+	}
+}
+```
+
+## Windows
+TODO
+
+## GTK+
+We can use `GtkDrawingArea`. We hook into the `draw` signal; it does something equivalent to
+
+```go
+func draw_callback(widget *C.GtkWidget, cr *C.cairo_t, data C.gpointer) C.gboolean {
+	s := (*sysData)(unsafe.Pointer(data))
+	// TODO get clip rectangle that needs drawing
+	imgret := make(chan *image.NRGBA)
+	defer close(imgret)
+	s.paint <- PaintRequest{
+		Rect:		/* clip rect */,
+		Out:		imgret,
+	}
+	i := <-imgret
+	pixbuf := C.gdk_pixbuf_new_from_data(
+		(*C.guchar)(unsafe.Pointer(&i.Pix[0])),
+		C.GDK_COLORSPACE_RGB,
+		C.TRUE,			// has alpha channel
+		8,				// bits per sample
+		C.int(i.Rect.Dx()),
+		C.int(i.Rect.Dy()),
+		C.int(i.Stride),
+		nil, nil)			// do not free data
+	C.gdk_cairo_set_source_pixbuf(cr,
+		pixbuf,
+		/* x of clip rect */,
+		/* y of clip rect */)
+	return C.FALSE		// TODO what does this return value mean? docs don't say
+}
+```
+
+TODO figure out how scrolling plays into this
+
+## Cocoa
+TODO