app,io/system: extract system events to separate package

Package app is the only package that depends on native libraries and
Cgo. Minimize its API, thereby minimizing Gio clients' dependency on
it. In the future, a headless, testing or remote "Window" should be
very easy to replace app.Window.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/os_windows.go

@@ -17,6 +17,7 @@ import (
 	"gioui.org/f32"
 	"gioui.org/io/key"
 	"gioui.org/io/pointer"
+	"gioui.org/io/system"
 )
 
 var winMap = make(map[syscall.Handle]*window)
@@ -60,7 +61,7 @@ type window struct {
 	w      *Window
 	width  int
 	height int
-	stage  Stage
+	stage  system.Stage
 	dead   bool
 
 	mu        sync.Mutex
@@ -183,7 +184,7 @@ func createWindow(window *Window, opts *windowOptions) error {
 		defer delete(winMap, w.hwnd)
 		w.w = window
 		w.w.setDriver(w)
-		defer w.w.event(DestroyEvent{})
+		defer w.w.event(system.DestroyEvent{})
 		showWindow(w.hwnd, _SW_SHOWDEFAULT)
 		setForegroundWindow(w.hwnd)
 		setFocus(w.hwnd)
@@ -326,9 +327,9 @@ func windowProc(hwnd syscall.Handle, msg uint32, wParam, lParam uintptr) uintptr
 	case _WM_SIZE:
 		switch wParam {
 		case _SIZE_MINIMIZED:
-			w.setStage(StagePaused)
+			w.setStage(system.StagePaused)
 		case _SIZE_MAXIMIZED, _SIZE_RESTORED:
-			w.setStage(StageRunning)
+			w.setStage(system.StageRunning)
 			w.draw(true)
 		}
 	}
@@ -395,9 +396,9 @@ func (w *window) postRedraw() {
 	}
 }
 
-func (w *window) setStage(s Stage) {
+func (w *window) setStage(s system.Stage) {
 	w.stage = s
-	w.w.event(StageEvent{s})
+	w.w.event(system.StageEvent{Stage: s})
 }
 
 func (w *window) draw(sync bool) {
@@ -407,13 +408,15 @@ func (w *window) draw(sync bool) {
 	w.height = int(r.bottom - r.top)
 	cfg := configForDC(w.hdc)
 	cfg.now = time.Now()
-	w.w.event(FrameEvent{
-		Size: image.Point{
-			X: w.width,
-			Y: w.height,
+	w.w.event(frameEvent{
+		FrameEvent: system.FrameEvent{
+			Size: image.Point{
+				X: w.width,
+				Y: w.height,
+			},
+			Config: &cfg,
 		},
-		Config: cfg,
-		sync:   sync,
+		sync: sync,
 	})
 }
 
@@ -474,14 +477,14 @@ func convertKeyCode(code uintptr) (rune, bool) {
 	return r, true
 }
 
-func configForDC(hdc syscall.Handle) Config {
+func configForDC(hdc syscall.Handle) config {
 	dpi := getDeviceCaps(hdc, _LOGPIXELSX)
 	ppdp := float32(dpi) * inchPrDp * monitorScale
 	// Force a minimum density to keep text legible and to handle bogus output geometry.
 	if ppdp < minDensity {
 		ppdp = minDensity
 	}
-	return Config{
+	return config{
 		pxPerDp: ppdp,
 		pxPerSp: ppdp,
 	}