Wrote up the initial uiButton implementation. It (mostly; just needs text and for that I need to add a uiControlHandle()) works!

button_windows.c

@@ -0,0 +1,73 @@
+// 7 april 2015
+#include "uipriv_windows.h"
+
+struct button {
+	uiControl *c;
+	void (*onClicked)(uiControl *, void *);
+	void *onClickedData;
+};
+
+#define B(x) ((struct button *) (x))
+
+static BOOL onWM_COMMAND(uiControl *c, WPARAM wParam, LPARAM lParam, void *data, LRESULT *lResult)
+{
+	if (HIWORD(wParam) != BN_CLICKED)
+		return FALSE;
+	(*(B(data)->onClicked))(c, B(data)->onClickedData);
+	*lResult = 0;
+	return TRUE;
+}
+
+static BOOL onWM_NOTIFY(uiControl *c, WPARAM wParam, LPARAM lParam, void *data, LRESULT *lResult)
+{
+	return FALSE;
+}
+
+static void preferredSize(uiControl *c, int baseX, int baseY, LONG internalLeading, intmax_t *width, intmax_t *height)
+{
+	// TODO
+}
+
+static void defaultOnClicked(uiControl *c, void *data)
+{
+	// do nothing
+}
+
+uiControl *uiNewButton(const char *text)
+{
+	struct button *b;
+	uiWindowsNewControlParams p;
+	WCHAR *wtext;
+
+	b = uiNew(struct button);
+
+	p.dwExStyle = 0;
+	p.lpClassName = L"button";
+	p.dwStyle = BS_PUSHBUTTON;
+	p.hInstance = hInstance;
+	p.onWM_COMMAND = onWM_COMMAND;
+	p.onWM_NOTIFY = onWM_NOTIFY;
+	p.onCommandNotifyData = b;
+	p.preferredSize = preferredSize;
+	p.data = b;
+	b->c = uiWindowsNewControl(&p);
+
+	wtext = toUTF16(text);
+	// TODO set text
+	uiFree(wtext);
+
+	b->onClicked = defaultOnClicked;
+
+	return b->c;
+}
+
+// TODO text
+
+void uiButtonOnClicked(uiControl *c, void (*f)(uiControl *, void *), void *data)
+{
+	struct button *b;
+
+	b = (struct button *) uiWindowsControlData(c);
+	b->onClicked = f;
+	b->onClickedData = data;
+}

newcontrol_windows.c

@@ -10,6 +10,7 @@ struct uiSingleHWNDControl {
 	BOOL (*onWM_NOTIFY)(uiControl *, WPARAM, LPARAM, void *, LRESULT *);
 	void *onCommandNotifyData;
 	void (*preferredSize)(uiControl *, int, int, LONG, intmax_t *, intmax_t *);
+	void *data;
 };
 
 #define S(c) ((uiSingleHWNDControl *) (c))
@@ -100,8 +101,15 @@ uiControl *uiWindowsNewControl(uiWindowsNewControlParams *p)
 	c->onCommandNotifyData = p->onCommandNotifyData;
 	c->preferredSize = p->preferredSize;
 
+	c->data = p->data;
+
 	if ((*fv_SetWindowSubclass)(c->hwnd, singleSubclassProc, 0, (DWORD_PTR) c) == FALSE)
 		logLastError("error subclassing Windows control in uiWindowsNewControl()");
 
 	return (uiControl *) c;
 }
+
+void *uiWindowsControlData(uiControl *c)
+{
+	return S(c)->data;
+}

test.c

@@ -9,10 +9,16 @@ int onClosing(uiWindow *w, void *data)
 	return 1;
 }
 
+void onClicked(uiControl *b, void *data)
+{
+	printf("button clicked!\n");
+}
+
 int main(void)
 {
 	uiInitError *err;
 	uiWindow *w;
+	uiControl *button;
 
 	err = uiInit(NULL);
 	if (err != NULL) {
@@ -25,6 +31,10 @@ int main(void)
 	uiWindowOnClosing(w, onClosing, NULL);
 	uiWindowShow(w);
 
+	button = uiNewButton("Click Me");
+	uiButtonOnClicked(button, onClicked, NULL);
+	uiWindowSetChild(w, button);
+
 	uiMain();
 	printf("after uiMain()\n");
 	return 0;

ui.h

@@ -15,6 +15,8 @@ void uiInitErrorFree(uiInitError *);
 void uiMain(void);
 void uiQuit(void);
 
+typedef struct uiControl uiControl;
+
 typedef struct uiWindow uiWindow;
 uiWindow *uiNewWindow(char *, int, int);
 void uiWindowDestroy(uiWindow *);
@@ -23,7 +25,10 @@ uintptr_t uiWindowHandle(uiWindow *);
 void uiWindowShow(uiWindow *);
 void uiWindowHide(uiWindow *);
 void uiWindowOnClosing(uiWindow *, int (*)(uiWindow *, void *), void *);
+void uiWindowSetChild(uiWindow *, uiControl *);
 
-typedef struct uiControl uiControl;
+uiControl *uiNewButton(const char *);
+// TODO text
+void uiButtonOnClicked(uiControl *, void (*)(uiControl *, void *), void *);
 
 #endif

ui_windows.h

@@ -30,8 +30,12 @@ struct uiWindowsNewControlParams {
 	// baseX and baseY are the base units used to convert between dialog units and pixels.
 	// internalLeading is the internal leading of the control font.
 	void (*preferredSize)(uiControl *c, int baseX, int baseY, LONG internalLeading, intmax_t *width, intmax_t *height);
+
+	// Data you can get with uiWindowsControlData()
+	void *data;
 };
 uiControl *uiWindowsNewControl(uiWindowsNewControlParams *);
+void *uiWindowsControlData(uiControl *);
 
 // use these in your preferredSize() implementation with baseX and baseY
 #define uiDlgUnitToX(dlg, baseX) MulDiv((dlg), baseX, 4)

window_windows.c

@@ -133,3 +133,9 @@ void uiWindowOnClosing(uiWindow *w, int (*f)(uiWindow *, void *), void *data)
 	w->onClosing = f;
 	w->onClosingData = data;
 }
+
+void uiWindowSetChild(uiWindow *w, uiControl *c)
+{
+	w->child = c;
+	(*(w->child->setParent))(w->child, (uintptr_t) (w->hwnd));
+}