Set up restrictions tracking. Added a restriction that a window and its controls are fixed to the window once it has been open. Started accounting for parent windows in controls.

control.go

@@ -9,6 +9,6 @@ import (
 // A Control represents an UI control. Note that Control contains unexported members; this has the consequence that you can't build custom controls that interface directly with the system-specific code (fo rinstance, to import an unsupported control), or at least not without some hackery. If you want to make your own controls, embed Area and provide its necessities.
 type Control interface {
 	apply() error
-	unapply() error
 	setParent(c Control)
+	setParentWindow(w *Window)
 }

restrictions.md

@@ -0,0 +1,5 @@
+This is a file to keep track of API restrictions that simplify the implementation of the package. I would like to eliminate them, but...
+
+- Once you open a window, the controls are finalized: you cannot change the window's control or add/remove controls to layouts.
+- Once you open a window, you cannot change its event channels or its controls's event channels.
+- [Windows] At most 65535 controls can be made, period. This is because child window IDs are alloted by the UI library application-global, not window-local, and BN_CLICKED only stores the control ID in a word (and I start counting at 1 to be safe). If I keep the first restriction and amend it such that you can only set the control of a window at the time of first open (somehow; split create and open?), I can easily make them window-local.

window.go

@@ -32,23 +32,17 @@ func NewWindow(title string) *Window {
 	}
 }
 
-// SetControl sets the window's central control to control.
+// SetControl sets the window's central control to control. This function cannot be called once the window has been opened.
 func (w *Window) SetControl(control Control) (err error) {
 	w.lock.Lock()
 	defer w.lock.Unlock()
 
-	w.control = control
-	err = w.control.unapply()
-	if err != nil {
-		return err
-	}
-	w.control.setParent(w)
 	if w.created {
-		err = w.control.apply()
-		if err != nil {
-			return err
-		}
+		panic("cannot set window control after window has been opened")
 	}
+	w.control = control
+	w.control.setParent(w)
+	w.control.setParentWindow(w)
 	return nil
 }
 
@@ -85,9 +79,9 @@ func (w *Window) Close() (err error) {
 func (w *Window) apply() error {
 	panic("Window.apply() should never be called")
 }
-func (w *Window) unapply() error {
-	panic("Window.unapply() should never be called")
-}
 func (w *Window) setParent(c Control) {
 	panic("Window.setParent() should never be called")
 }
+func (w *Window) setParentWindow(w *Window) {
+	panic("Window.setParent() should never be called")
+}