Implemented Tab on Mac OS X. Woo! I'll need to add justification for what I'm doing with the whole recursive call thing; when I get confirmation from the GTK+ camp I will.

redo/containers_darwin.go

@@ -0,0 +1,52 @@
+// 25 july 2014
+
+package ui
+
+import (
+	"unsafe"
+)
+
+// #include "objc_darwin.h"
+import "C"
+
+type tab struct {
+	*widgetbase
+
+	containers	[]*container
+}
+
+func newTab() Tab {
+	t := new(tab)
+	id := C.newTab(unsafe.Pointer(t))
+	t.widgetbase = newWidget(id)
+	return t
+}
+
+func (t *tab) Append(name string, control Control) {
+	// TODO isolate and standardize
+	c := new(container)
+	// don't set beginResize; this container's resize() will be a recursive call
+	t.containers = append(t.containers, c)
+	cname := C.CString(name)
+	defer C.free(unsafe.Pointer(cname))
+	tabview := C.tabAppend(t.id, cname)
+	c.child = control
+	c.child.setParent(tabview)
+}
+
+func (t *tab) allocate(x int, y int, width int, height int, d *sizing) []*allocation {
+	// set up the recursive calls
+	for _, c := range t.containers {
+		c.d = d
+	}
+	// and prepare the tabbed control itself
+	return t.widgetbase.allocate(x, y, width, height, d)
+}
+
+//export tabResized
+func tabResized(data unsafe.Pointer, width C.intptr_t, height C.intptr_t) {
+	t := (*tab)(unsafe.Pointer(data))
+	for _, c := range t.containers {
+		c.resize(int(width), int(height))
+	}
+}

redo/containers_darwin.m

@@ -0,0 +1,46 @@
+// 25 july 2014
+
+#import "objc_darwin.h"
+#import "_cgo_export.h"
+#import <Cocoa/Cocoa.h>
+
+#define toNSTabView(x) ((NSTabView *) (x))
+
+@interface goTabView : NSTabView {
+@public
+	void *gotab;
+}
+@end
+
+@implementation goTabView
+
+- (void)setFrame:(NSRect)r
+{
+	NSRect content;
+
+	[super setFrame:r];
+	content = [self contentRect];
+	tabResized(self->gotab, (intptr_t) content.size.width, (intptr_t) content.size.height);
+}
+
+@end
+
+id newTab(void *gotab)
+{
+	goTabView *t;
+
+	t = [[goTabView alloc] initWithFrame:NSMakeRect(0, 0, 100, 100)];
+	// TODO font
+	t->gotab = gotab;
+	return (id) t;
+}
+
+id tabAppend(id t, char *name)
+{
+	NSTabViewItem *i;
+
+	i = [[NSTabViewItem alloc] initWithIdentifier:nil];
+	[i setLabel:[NSString stringWithUTF8String:name]];
+	[toNSTabView(t) addTabViewItem:i];
+	return (id) [i view];
+}

redo/containers_unix.go

@@ -1,3 +1,5 @@
+// +build !windows,!darwin
+
 // 25 july 2014
 
 package ui

redo/controls_darwin.go

@@ -96,7 +96,3 @@ func (c *checkbox) Checked() bool {
 func (c *checkbox) SetChecked(checked bool) {
 	C.checkboxSetChecked(c.id, toBOOL(checked))
 }
-
-//TODO
-func newTab() Tab{return newButton("tab")}
-func(*button)Append(string,Control){}

redo/controls_darwin.m

@@ -11,7 +11,7 @@
 
 void parent(id control, id parentid)
 {
-	[[toNSWindow(parentid) contentView] addSubview:toNSView(control)];
+	[toNSView(parentid) addSubview:toNSView(control)];
 }
 
 void controlSetHidden(id control, BOOL hidden)

redo/objc_darwin.h

@@ -28,6 +28,7 @@ extern void windowSetTitle(id, const char *);
 extern void windowShow(id);
 extern void windowHide(id);
 extern void windowClose(id);
+extern id windowContentView(id);
 extern void windowRedraw(id);
 
 /* controls_darwin.m */
@@ -44,4 +45,8 @@ extern void checkboxSetChecked(id, BOOL);
 /* sizing_darwin.m */
 extern void moveControl(id, intptr_t, intptr_t, intptr_t, intptr_t);
 
+/* containers_darwin.m */
+extern id newTab(void *);
+extern id tabAppend(id, char *);
+
 #endif

redo/window_darwin.go

@@ -35,7 +35,7 @@ func newWindow(title string, width int, height int, control Control) *window {
 	w.container.beginResize = w.beginResize
 	C.windowSetDelegate(id, unsafe.Pointer(w))
 	w.child = control
-	w.child.setParent(w.id)
+	w.child.setParent(C.windowContentView(w.id))
 	return w
 }
 

redo/window_darwin.m

@@ -80,6 +80,11 @@ void windowClose(id win)
 	[toNSWindow(win) close];
 }
 
+id windowContentView(id win)
+{
+	return (id) [toNSWindow(win) contentView];
+}
+
 // fake a resize event under certain conditions; see each invocation for details
 void windowRedraw(id win)
 {