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_ios.go

@@ -25,6 +25,7 @@ import (
 	"gioui.org/f32"
 	"gioui.org/io/key"
 	"gioui.org/io/pointer"
+	"gioui.org/io/system"
 	"gioui.org/unit"
 )
 
@@ -61,7 +62,7 @@ func onCreate(view C.CFTypeRef) {
 	w.layer = C.CFTypeRef(layerFactory())
 	C.gio_addLayerToView(view, w.layer)
 	views[view] = w
-	w.w.event(StageEvent{StagePaused})
+	w.w.event(system.StageEvent{Stage: system.StagePaused})
 }
 
 //export onDraw
@@ -74,27 +75,29 @@ func onDraw(view C.CFTypeRef, dpi, sdpi, width, height C.CGFloat, sync C.int, to
 	w.visible.Store(true)
 	C.gio_updateView(view, w.layer)
 	if !wasVisible {
-		w.w.event(StageEvent{StageRunning})
+		w.w.event(system.StageEvent{Stage: system.StageRunning})
 	}
 	isSync := false
 	if sync != 0 {
 		isSync = true
 	}
-	w.w.event(FrameEvent{
-		Size: image.Point{
-			X: int(width + .5),
-			Y: int(height + .5),
-		},
-		Insets: Insets{
-			Top:    unit.Px(float32(top)),
-			Right:  unit.Px(float32(right)),
-			Bottom: unit.Px(float32(bottom)),
-			Left:   unit.Px(float32(left)),
-		},
-		Config: Config{
-			pxPerDp: float32(dpi) * inchPrDp,
-			pxPerSp: float32(sdpi) * inchPrDp,
-			now:     time.Now(),
+	w.w.event(frameEvent{
+		FrameEvent: system.FrameEvent{
+			Size: image.Point{
+				X: int(width + .5),
+				Y: int(height + .5),
+			},
+			Insets: system.Insets{
+				Top:    unit.Px(float32(top)),
+				Right:  unit.Px(float32(right)),
+				Bottom: unit.Px(float32(bottom)),
+				Left:   unit.Px(float32(left)),
+			},
+			Config: &config{
+				pxPerDp: float32(dpi) * inchPrDp,
+				pxPerSp: float32(sdpi) * inchPrDp,
+				now:     time.Now(),
+			},
 		},
 		sync: isSync,
 	})
@@ -104,14 +107,14 @@ func onDraw(view C.CFTypeRef, dpi, sdpi, width, height C.CGFloat, sync C.int, to
 func onStop(view C.CFTypeRef) {
 	w := views[view]
 	w.visible.Store(false)
-	w.w.event(StageEvent{StagePaused})
+	w.w.event(system.StageEvent{Stage: system.StagePaused})
 }
 
 //export onDestroy
 func onDestroy(view C.CFTypeRef) {
 	w := views[view]
 	delete(views, view)
-	w.w.event(DestroyEvent{})
+	w.w.event(system.DestroyEvent{})
 	C.gio_removeLayer(w.layer)
 	C.CFRelease(w.layer)
 	w.layer = 0