We create another custom window class that does `WM_PAINT` and handles input events thereof.
For this mockup, I'll extract the message handling into its own function and assume I can call Windows API functions and use their types and constants as normal. For `WM_PAINT` both `wparam` and `lparam` are unused.
```go
func repaint(s *sysData) HRESULT {
var xrect RECT
var ps PAINTSTRUCT
// TODO send TRUE if we want to erase the clip area
if GetUpdateRect(s.hwnd, &xrect, FALSE) == 0 {
// no update rect, so we're done
return 0
}
hdc, err := BeginPaint(s.hwnd, &ps)
if hdc == 0 { // failure
panic(fmt.Errorf("error beginning Area repaint: %v", err))
(`GpBitmap` extends `GpImage`.) The only problem is the pixel format: the most appropriate one is `PixelFormat32bppARGB`, which is not premultiplied, but the components are in the wrong order... (there is no RGBA pixel format in any bit width) (TODO `GdipDisposeImage` seems wrong since it bypasses `~Bitmap()` and goes right for `~Image()` but I don't see an explicit `~Bitmap`...)
Disregarding the RGBA issue, the draw code would be
TODO note http://msdn.microsoft.com/en-us/library/windows/desktop/bb775501%28v=vs.85%29.aspx#win_class for information on handling some key presses, tab switch, etc. (need to do this for the ones below too)
TODO figure out scrolling
- windows can have either standard or custom scrollbars; need to figure out how the standard scrollbars work
- standard scrollbars cannot be controlled by the keyboard; either we must provide an option for doing that or allow scrolling ourselves (the `myAreaGoroutine` would read the keyboard events and scroll manually, in the same way)
For this one we **must** create a subclass of `NSView` that overrides the drawing and keyboard/mouse event messages.
The drawing message is `-[NSView drawRect:]`, which just takes the `NSRect` as an argument. So we already need to use `bleh_darwin.m` to grab the actual `NSRect` and convert it into something with a predictable data type before passing it back to Go. If we do this:
```go
//export our_drawRect
func our_drawRect(self C.id, rect C.struct_xrect) {
```
we can call `our_drawRect()` from this C wrapper:
```objective-c
extern void our_drawRect(id, struct xrect);
void _our_drawRect(id self, SEL sel, NSRect r)
{
struct xrect t;
t.x = (int64_t) s.origin.x;
t.y = (int64_t) s.origin.y;
t.width = (int64_t) s.size.width;
t.height = (int64_t) s.size.height;
our_drawRect(self, t);
}
```
This just leaves `our_drawRect` itself. For this mockup, I will use "Objective-Go":
```go
//export our_drawRect
func our_drawRect(self C.id, rect C.struct_xrect) {
bitmapFormat:NSAlphaNonpremultipliedBitmapFormat // this is where the flag for placing alpha first would go if alpha came first; the default is alpha last, which is how we're doing things
Due to the size of the `NSBitmapImageRep` constructor, I might just have another C function that performs the `NSBitmapImageRep` constructor using the `image.NRGBA` fields.
