Changed Area to use a delegate handler object that implements the new AreaHandler interface for handling events. Also updated the GTK+ backend with this change, and made a few more tweaks to the documentation in area.go.

area.go

@@ -10,42 +10,40 @@ import (
 // Area represents a blank canvas upon which programs may draw anything and receive arbitrary events from the user.
 // An Area has an explicit size, represented in pixels, that may be different from the size shown in its Window; scrollbars are placed automatically should they be needed.
 // The coordinate system of an Area always has an origin of (0,0) which maps to the top-left corner; all image.Points and image.Rectangles sent across Area's channels conform to this.
+// 
+// To handle events to the Area, an Area must be paired with an AreaHandler.
+// See AreaHandler for details.
+// 
 // To facilitate development and debugging, for the time being, Areas have a fixed size of 320x240 and only work on GTK+.
 type Area struct {
-	// Paint is signaled when the Area needs to be redrawn.
-	// You MUST handle Paint signals; failure to do so will result in the UI task hanging.
-	// See the documentation of PaintRequest for details.
-	Paint		chan PaintRequest
-
-	// Mouse is signaled when the Area receives a mouse event.
-	// See MouseEvent for details.
-	Mouse	chan MouseEvent
-
 	lock		sync.Mutex
 	created	bool
 	sysData	*sysData
+	handler	AreaHandler
 }
 
-// PaintRequest contains the information needed to redraw an Area.
-// On each Paint event, an Area will receive a full request on its Paint channel.
-// It must send something back on Out in order to complete the painting.
-// Example:
-// 	imgFromFile, _, err := image.Decode(file)
-// 	if err != nil { panic(err) }
-// 	img := image.NewNRGBA(imgFromFile.Rect)
-// 	draw.Draw(img, img.Rect, imgFromFile, image.ZP, draw.Over)
-// 	for req := range area.Paint {
-// 		req.Out <- img.SubImage(req.Rect).(*image.NRGBA)
-// 	}
-type PaintRequest struct {
-	// Rect is the clip rectangle of the whole Area that needs to be redrawn.
-	// The image sent on Out must have the same size as Rect (but does not have to have the same Rect.Min/Rect.Max points).
-	Rect		image.Rectangle
+// AreaHandler represents the events that an Area should respond to.
+// You are responsible for the thread safety of any members of the actual type that implements ths interface.
+// (Having to use this interface does not strike me as being particularly Go-like, but the nature of Paint makes channel-based event handling a non-option; in practice, deadlocks occur.)
+type AreaHandler interface {
+	// Paint is called when the Area needs to be redrawn.
+	// You MUST handle this event, and you MUST return a valid image, otherwise deadlocks and panicking will occur.
+	// The image returned must have the same size as rect (but does not have to have the same origin points).
+	// Example:
+	// 	imgFromFile, _, err := image.Decode(file)
+	// 	if err != nil { panic(err) }
+	// 	img := image.NewNRGBA(imgFromFile.Rect)
+	// 	draw.Draw(img, img.Rect, imgFromFile, image.ZP, draw.Over)
+	// 	// ...
+	// 	func (h *myAreaHandler) Paint(rect image.Rectangle) *image.NRGBA {
+	// 		return img.SubImage(rect).(*image.NRGBA)
+	// 	}
+	Paint(rect image.Rectangle) *image.NRGBA
 
-	// Out is where you send the image to draw.
-	// Only one image per PaintRequest may be sent; you must send an image.
-	// Do not close Out; the package will do this itself.
-	Out		chan<- *image.NRGBA
+	// Mouse is called when the Area receives a mouse event.
+	// You are allowed to do nothing in this handler (to ignore mouse events).
+	// See MouseEvent for details.
+	Mouse(e MouseEvent)
 }
 
 // MouseEvent contains all the information for a mous event sent by Area.Mouse.
@@ -63,6 +61,7 @@ type MouseEvent struct {
 	// If the event was generated by a mouse button being released, Up contains the ID of that button.
 	// Otherwise, Up contains 0.
 	// If both Down and Up are 0, the event represents mouse movement (with optional held buttons; see below).
+	// Down and Up shall not both be nonzero.
 	Up			uint
 
 	// If Down is nonzero, Count indicates the number of clicks: 1 for single-click, 2 for double-click.
@@ -98,11 +97,14 @@ const (
 )
 
 // NewArea creates a new Area.
-func NewArea() *Area {
+// It panics if handler is nil.
+func NewArea(handler AreaHandler) *Area {
+	if handler == nil {
+		panic("handler passed to NewArea() must not be nil")
+	}
 	return &Area{
 		sysData:	mksysdata(c_area),
-		Paint:	make(chan PaintRequest),
-		Mouse:	make(chan MouseEvent),
+		handler:	handler,
 	}
 }
 
@@ -110,8 +112,7 @@ func (a *Area) make(window *sysData) error {
 	a.lock.Lock()
 	defer a.lock.Unlock()
 
-	a.sysData.paint = a.Paint
-	a.sysData.mouse = a.Mouse
+	a.sysData.handler = a.handler
 	err := a.sysData.make("", window)
 	if err != nil {
 		return err

area_unix.go

@@ -45,13 +45,7 @@ func our_area_draw_callback(widget *C.GtkWidget, cr *C.cairo_t, data C.gpointer)
 	// thanks to desrt in irc.gimp.net/#gtk+
 	C.cairo_clip_extents(cr, &x, &y, &w, &h)
 	cliprect := image.Rect(int(x), int(y), int(w), int(h))
-	imgret := make(chan *image.NRGBA)
-	defer close(imgret)
-	s.paint <- PaintRequest{
-		Rect:		cliprect,
-		Out:		imgret,
-	}
-	i := <-imgret
+	i := s.handler.Paint(cliprect)
 	// pixel order is [R G B A] (see Example 1 on https://developer.gnome.org/gdk-pixbuf/2.26/gdk-pixbuf-The-GdkPixbuf-Structure.html) so we don't have to convert anything
 	// gdk-pixbuf is not alpha-premultiplied (thanks to desrt in irc.gimp.net/#gtk+)
 	pixbuf := C.gdk_pixbuf_new_from_data(
@@ -111,13 +105,7 @@ func finishMouseEvent(data C.gpointer, me MouseEvent, mb uint, x C.gdouble, y C.
 		me.Held = append(me.Held, 5)
 	}
 	me.Pos = image.Pt(int(x), int(y))
-	// see cSysData.signal() in sysdata.go
-	go func() {
-		select {
-		case s.mouse <- me:
-		default:
-		}
-	}()
+	s.handler.Mouse(me)
 }
 
 //export our_area_button_press_event_callback

sysdata.go

@@ -18,11 +18,8 @@ type cSysData struct {
 	ctype	int
 	event	chan struct{}
 	resize	func(x int, y int, width int, height int, winheight int) error
-	alternate	bool		// editable for Combobox, multi-select for listbox, password for lineedit
-
-	// for Area
-	paint		chan PaintRequest
-	mouse	chan MouseEvent
+	alternate	bool			// editable for Combobox, multi-select for listbox, password for lineedit
+	handler	AreaHandler	// for Areas
 }
 func (c *cSysData) make(initText string, window *sysData) error {
 	panic(runtime.GOOS + " sysData does not define make()")