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/app.go

@@ -4,66 +4,14 @@ package app
 
 import (
 	"errors"
-	"image"
 	"math"
 	"os"
 	"strings"
 	"time"
 
-	"gioui.org/op"
 	"gioui.org/unit"
 )
 
-// A FrameEvent asks for a new frame in the form of a list of
-// operations.
-type FrameEvent struct {
-	Config Config
-	// Size is the dimensions of the window.
-	Size image.Point
-	// Insets is the insets to apply.
-	Insets Insets
-	// Frame replaces the window's frame with the new
-	// frame.
-	Frame func(frame *op.Ops)
-	// Whether this draw is system generated and needs a complete
-	// frame before proceeding.
-	sync bool
-}
-
-// DestroyEvent is the last event sent through
-// a window event channel.
-type DestroyEvent struct {
-	// Err is nil for normal window closures. If a
-	// window is prematurely closed, Err is the cause.
-	Err error
-}
-
-// Insets is the space taken up by
-// system decoration such as translucent
-// system bars and software keyboards.
-type Insets struct {
-	Top, Bottom, Left, Right unit.Value
-}
-
-// A StageEvent is generated whenever the stage of a
-// Window changes.
-type StageEvent struct {
-	Stage Stage
-}
-
-// CommandEvent is a system event.
-type CommandEvent struct {
-	Type CommandType
-	// Suppress the default action of the command.
-	Cancel bool
-}
-
-// Stage of a Window.
-type Stage uint8
-
-// CommandType is the type of a CommandEvent.
-type CommandType uint8
-
 type windowRendezvous struct {
 	in   chan windowAndOptions
 	out  chan windowAndOptions
@@ -75,20 +23,6 @@ type windowAndOptions struct {
 	opts   *windowOptions
 }
 
-const (
-	// StagePaused is the Stage for inactive Windows.
-	// Inactive Windows don't receive FrameEvents.
-	StagePaused Stage = iota
-	// StateRunning is for active Windows.
-	StageRunning
-)
-
-const (
-	// CommandBack is the command for a back action
-	// such as the Android back button.
-	CommandBack CommandType = iota
-)
-
 const (
 	inchPrDp = 1.0 / 160
 	mmPrDp   = 25.4 / 160
@@ -109,17 +43,6 @@ const (
 // Set with the go linker flag -X.
 var extraArgs string
 
-func (l Stage) String() string {
-	switch l {
-	case StagePaused:
-		return "StagePaused"
-	case StageRunning:
-		return "StageRunning"
-	default:
-		panic("unexpected Stage value")
-	}
-}
-
 func init() {
 	if extraArgs != "" {
 		args := strings.Split(extraArgs, "|")
@@ -149,8 +72,8 @@ func Main() {
 	main()
 }
 
-// Config implements the layout.Config interface.
+// config implements the system.Config interface.
-type Config struct {
+type config struct {
 	// Device pixels per dp.
 	pxPerDp float32
 	// Device pixels per sp.
@@ -158,11 +81,11 @@ type Config struct {
 	now     time.Time
 }
 
-func (c *Config) Now() time.Time {
+func (c *config) Now() time.Time {
 	return c.now
 }
 
-func (c *Config) Px(v unit.Value) int {
+func (c *config) Px(v unit.Value) int {
 	var r float32
 	switch v.U {
 	case unit.UnitPx:
@@ -202,8 +125,3 @@ func newWindowRendezvous() *windowRendezvous {
 	}()
 	return wr
 }
-
-func (_ FrameEvent) ImplementsEvent()    {}
-func (_ StageEvent) ImplementsEvent()    {}
-func (_ *CommandEvent) ImplementsEvent() {}
-func (_ DestroyEvent) ImplementsEvent()  {}

app/os_android.go

@@ -27,6 +27,7 @@ import (
 	"gioui.org/f32"
 	"gioui.org/io/key"
 	"gioui.org/io/pointer"
+	"gioui.org/io/system"
 	"gioui.org/unit"
 )
 
@@ -37,9 +38,9 @@ type window struct {
 
 	dpi       int
 	fontScale float32
-	insets    Insets
+	insets    system.Insets
 
-	stage   Stage
+	stage   system.Stage
 	started bool
 
 	mu        sync.Mutex
@@ -112,7 +113,7 @@ func onCreateView(env *C.JNIEnv, class C.jclass, view C.jobject) C.jlong {
 	handle := C.jlong(view)
 	views[handle] = w
 	w.loadConfig(env, class)
-	w.setStage(StagePaused)
+	w.setStage(system.StagePaused)
 	return handle
 }
 
@@ -129,7 +130,7 @@ func onDestroyView(env *C.JNIEnv, class C.jclass, handle C.jlong) {
 func onStopView(env *C.JNIEnv, class C.jclass, handle C.jlong) {
 	w := views[handle]
 	w.started = false
-	w.setStage(StagePaused)
+	w.setStage(system.StagePaused)
 }
 
 //export onStartView
@@ -147,7 +148,7 @@ func onSurfaceDestroyed(env *C.JNIEnv, class C.jclass, handle C.jlong) {
 	w.mu.Lock()
 	w.win = nil
 	w.mu.Unlock()
-	w.setStage(StagePaused)
+	w.setStage(system.StagePaused)
 }
 
 //export onSurfaceChanged
@@ -171,7 +172,7 @@ func onLowMemory() {
 func onConfigurationChanged(env *C.JNIEnv, class C.jclass, view C.jlong) {
 	w := views[view]
 	w.loadConfig(env, class)
-	if w.stage >= StageRunning {
+	if w.stage >= system.StageRunning {
 		w.draw(true)
 	}
 }
@@ -182,7 +183,7 @@ func onFrameCallback(env *C.JNIEnv, class C.jclass, view C.jlong, nanos C.jlong)
 	if !exist {
 		return
 	}
-	if w.stage < StageRunning {
+	if w.stage < system.StageRunning {
 		return
 	}
 	w.mu.Lock()
@@ -199,7 +200,7 @@ func onFrameCallback(env *C.JNIEnv, class C.jclass, view C.jlong, nanos C.jlong)
 //export onBack
 func onBack(env *C.JNIEnv, class C.jclass, view C.jlong) C.jboolean {
 	w := views[view]
-	ev := &CommandEvent{Type: CommandBack}
+	ev := &system.CommandEvent{Type: system.CommandBack}
 	w.event(ev)
 	if ev.Cancel {
 		return C.JNI_TRUE
@@ -216,13 +217,13 @@ func onFocusChange(env *C.JNIEnv, class C.jclass, view C.jlong, focus C.jboolean
 //export onWindowInsets
 func onWindowInsets(env *C.JNIEnv, class C.jclass, view C.jlong, top, right, bottom, left C.jint) {
 	w := views[view]
-	w.insets = Insets{
+	w.insets = system.Insets{
 		Top:    unit.Px(float32(top)),
 		Right:  unit.Px(float32(right)),
 		Bottom: unit.Px(float32(bottom)),
 		Left:   unit.Px(float32(left)),
 	}
-	if w.stage >= StageRunning {
+	if w.stage >= system.StageRunning {
 		w.draw(true)
 	}
 }
@@ -233,16 +234,16 @@ func (w *window) setVisible() {
 	if width == 0 || height == 0 {
 		return
 	}
-	w.setStage(StageRunning)
+	w.setStage(system.StageRunning)
 	w.draw(true)
 }
 
-func (w *window) setStage(stage Stage) {
+func (w *window) setStage(stage system.Stage) {
 	if stage == w.stage {
 		return
 	}
 	w.stage = stage
-	w.event(StageEvent{stage})
+	w.event(system.StageEvent{stage})
 }
 
 func (w *window) nativeWindow(visID int) (*C.ANativeWindow, int, int) {
@@ -296,16 +297,18 @@ func (w *window) draw(sync bool) {
 		return
 	}
 	ppdp := float32(w.dpi) * inchPrDp
-	w.event(FrameEvent{
+	w.event(frameEvent{
-		Size: image.Point{
+		FrameEvent: system.FrameEvent{
-			X: int(width),
+			Size: image.Point{
-			Y: int(height),
+				X: int(width),
-		},
+				Y: int(height),
-		Insets: w.insets,
+			},
-		Config: Config{
+			Insets: w.insets,
-			pxPerDp: ppdp,
+			Config: &config{
-			pxPerSp: w.fontScale * ppdp,
+				pxPerDp: ppdp,
-			now:     time.Now(),
+				pxPerSp: w.fontScale * ppdp,
+				now:     time.Now(),
+			},
 		},
 		sync: sync,
 	})

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{
+	w.w.event(frameEvent{
-		Size: image.Point{
+		FrameEvent: system.FrameEvent{
-			X: int(width + .5),
+			Size: image.Point{
-			Y: int(height + .5),
+				X: int(width + .5),
-		},
+				Y: int(height + .5),
-		Insets: Insets{
+			},
-			Top:    unit.Px(float32(top)),
+			Insets: system.Insets{
-			Right:  unit.Px(float32(right)),
+				Top:    unit.Px(float32(top)),
-			Bottom: unit.Px(float32(bottom)),
+				Right:  unit.Px(float32(right)),
-			Left:   unit.Px(float32(left)),
+				Bottom: unit.Px(float32(bottom)),
-		},
+				Left:   unit.Px(float32(left)),
-		Config: Config{
+			},
-			pxPerDp: float32(dpi) * inchPrDp,
+			Config: &config{
-			pxPerSp: float32(sdpi) * inchPrDp,
+				pxPerDp: float32(dpi) * inchPrDp,
-			now:     time.Now(),
+				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

app/os_js.go

@@ -11,6 +11,7 @@ import (
 	"gioui.org/f32"
 	"gioui.org/io/key"
 	"gioui.org/io/pointer"
+	"gioui.org/io/system"
 )
 
 type window struct {
@@ -53,7 +54,7 @@ func createWindow(win *Window, opts *windowOptions) error {
 	go func() {
 		w.w.setDriver(w)
 		w.focus()
-		w.w.event(StageEvent{StageRunning})
+		w.w.event(system.StageEvent{Stage: system.StageRunning})
 		w.draw(true)
 		select {}
 		w.cleanup()
@@ -338,24 +339,26 @@ func (w *window) showTextInput(show bool) {
 
 func (w *window) draw(sync bool) {
 	width, height, scale, cfg := w.config()
-	if cfg == (Config{}) {
+	if cfg == (config{}) {
 		return
 	}
 	w.mu.Lock()
 	w.scale = float32(scale)
 	w.mu.Unlock()
 	cfg.now = time.Now()
-	w.w.event(FrameEvent{
+	w.w.event(frameEvent{
-		Size: image.Point{
+		FrameEvent: system.FrameEvent{
-			X: width,
+			Size: image.Point{
-			Y: height,
+				X: width,
+				Y: height,
+			},
+			Config: &cfg,
 		},
-		Config: cfg,
+		sync: sync,
-		sync:   sync,
 	})
 }
 
-func (w *window) config() (int, int, float32, Config) {
+func (w *window) config() (int, int, float32, config) {
 	rect := w.cnv.Call("getBoundingClientRect")
 	width, height := rect.Get("width").Float(), rect.Get("height").Float()
 	scale := w.window.Get("devicePixelRatio").Float()
@@ -368,7 +371,7 @@ func (w *window) config() (int, int, float32, Config) {
 		w.cnv.Set("height", ih)
 	}
 	const ppdp = 96 * inchPrDp * monitorScale
-	return iw, ih, float32(scale), Config{
+	return iw, ih, float32(scale), config{
 		pxPerDp: ppdp * float32(scale),
 		pxPerSp: ppdp * float32(scale),
 	}

app/os_macos.go

@@ -4,13 +4,6 @@
 
 package app
 
-/*
-#cgo CFLAGS: -DGL_SILENCE_DEPRECATION -Werror -Wno-deprecated-declarations -fmodules -fobjc-arc -x objective-c
-
-#include <AppKit/AppKit.h>
-#include "os_macos.h"
-*/
-import "C"
 import (
 	"errors"
 	"image"
@@ -22,8 +15,17 @@ import (
 	"gioui.org/f32"
 	"gioui.org/io/key"
 	"gioui.org/io/pointer"
+	"gioui.org/io/system"
 )
 
+/*
+#cgo CFLAGS: -DGL_SILENCE_DEPRECATION -Werror -Wno-deprecated-declarations -fmodules -fobjc-arc -x objective-c
+
+#include <AppKit/AppKit.h>
+#include "os_macos.h"
+*/
+import "C"
+
 func init() {
 	// Darwin requires that UI operations happen on the main thread only.
 	runtime.LockOSThread()
@@ -32,7 +34,7 @@ func init() {
 type window struct {
 	view  C.CFTypeRef
 	w     *Window
-	stage Stage
+	stage system.Stage
 	ppdp  float32
 	scale float32
 }
@@ -89,12 +91,12 @@ func (w *window) setAnimating(anim bool) {
 	C.gio_setAnimating(w.view, animb)
 }
 
-func (w *window) setStage(stage Stage) {
+func (w *window) setStage(stage system.Stage) {
 	if stage == w.stage {
 		return
 	}
 	w.stage = stage
-	w.w.event(StageEvent{stage})
+	w.w.event(system.StageEvent{Stage: stage})
 }
 
 // Use a top level func for onFrameCallback to avoid
@@ -197,14 +199,16 @@ func (w *window) draw(sync bool) {
 	height := int(hf*w.scale + .5)
 	cfg := configFor(w.ppdp, w.scale)
 	cfg.now = time.Now()
-	w.setStage(StageRunning)
+	w.setStage(system.StageRunning)
-	w.w.event(FrameEvent{
+	w.w.event(frameEvent{
-		Size: image.Point{
+		FrameEvent: system.FrameEvent{
-			X: width,
+			Size: image.Point{
-			Y: height,
+				X: width,
+				Y: height,
+			},
+			Config: &cfg,
 		},
-		Config: cfg,
+		sync: sync,
-		sync:   sync,
 	})
 }
 
@@ -217,9 +221,9 @@ func getPixelsPerDp(scale float32) float32 {
 	return ppdp / scale
 }
 
-func configFor(ppdp, scale float32) Config {
+func configFor(ppdp, scale float32) config {
 	ppdp = ppdp * scale
-	return Config{
+	return config{
 		pxPerDp: ppdp,
 		pxPerSp: ppdp,
 	}
@@ -230,7 +234,7 @@ func gio_onTerminate(view C.CFTypeRef) {
 	viewDo(view, func(views viewMap, view C.CFTypeRef) {
 		w := views[view]
 		delete(views, view)
-		w.w.event(DestroyEvent{})
+		w.w.event(system.DestroyEvent{})
 	})
 }
 
@@ -238,7 +242,7 @@ func gio_onTerminate(view C.CFTypeRef) {
 func gio_onHide(view C.CFTypeRef) {
 	viewDo(view, func(views viewMap, view C.CFTypeRef) {
 		w := views[view]
-		w.setStage(StagePaused)
+		w.setStage(system.StagePaused)
 	})
 }
 
@@ -246,7 +250,7 @@ func gio_onHide(view C.CFTypeRef) {
 func gio_onShow(view C.CFTypeRef) {
 	viewDo(view, func(views viewMap, view C.CFTypeRef) {
 		w := views[view]
-		w.setStage(StageRunning)
+		w.setStage(system.StageRunning)
 	})
 }
 

app/os_wayland.go

@@ -21,6 +21,7 @@ import (
 	"gioui.org/internal/fling"
 	"gioui.org/io/key"
 	"gioui.org/io/pointer"
+	"gioui.org/io/system"
 	syscall "golang.org/x/sys/unix"
 )
 
@@ -116,7 +117,7 @@ type window struct {
 		dir            f32.Point
 	}
 
-	stage             Stage
+	stage             system.Stage
 	dead              bool
 	pendingErr        error
 	lastFrameCallback *C.struct_wl_callback
@@ -302,7 +303,7 @@ func gio_onXdgSurfaceConfigure(data unsafe.Pointer, wmSurf *C.struct_xdg_surface
 	w.serial = serial
 	w.needAck = true
 	w.mu.Unlock()
-	w.setStage(StageRunning)
+	w.setStage(system.StageRunning)
 	w.draw(true)
 }
 
@@ -772,7 +773,7 @@ loop:
 			break
 		}
 		if w.dead {
-			w.w.event(DestroyEvent{Err: w.pendingErr})
+			w.w.event(system.DestroyEvent{Err: w.pendingErr})
 			break
 		}
 		// Clear poll events.
@@ -979,16 +980,16 @@ func (w *window) updateOutputs() {
 	}
 	w.mu.Unlock()
 	if !found {
-		w.setStage(StagePaused)
+		w.setStage(system.StagePaused)
 	} else {
-		w.setStage(StageRunning)
+		w.setStage(system.StageRunning)
 		w.draw(true)
 	}
 }
 
-func (w *window) config() (int, int, Config) {
+func (w *window) config() (int, int, config) {
 	width, height := w.width*w.scale, w.height*w.scale
-	return width, height, Config{
+	return width, height, config{
 		pxPerDp: w.ppdp * float32(w.scale),
 		pxPerSp: w.ppsp * float32(w.scale),
 	}
@@ -1015,7 +1016,7 @@ func (w *window) draw(sync bool) {
 		return
 	}
 	width, height, cfg := w.config()
-	if cfg == (Config{}) {
+	if cfg == (config{}) {
 		return
 	}
 	if animating && w.lastFrameCallback == nil {
@@ -1024,22 +1025,24 @@ func (w *window) draw(sync bool) {
 		C.gio_wl_callback_add_listener(w.lastFrameCallback, unsafe.Pointer(w.surf))
 	}
 	cfg.now = time.Now()
-	w.w.event(FrameEvent{
+	w.w.event(frameEvent{
-		Size: image.Point{
+		FrameEvent: system.FrameEvent{
-			X: width,
+			Size: image.Point{
-			Y: height,
+				X: width,
+				Y: height,
+			},
+			Config: &cfg,
 		},
-		Config: cfg,
+		sync: sync,
-		sync:   sync,
 	})
 }
 
-func (w *window) setStage(s Stage) {
+func (w *window) setStage(s system.Stage) {
 	if s == w.stage {
 		return
 	}
 	w.stage = s
-	w.w.event(StageEvent{s})
+	w.w.event(system.StageEvent{s})
 }
 
 func (w *window) display() *C.struct_wl_display {

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{
+	w.w.event(frameEvent{
-		Size: image.Point{
+		FrameEvent: system.FrameEvent{
-			X: w.width,
+			Size: image.Point{
-			Y: w.height,
+				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,
 	}

app/window.go

@@ -12,6 +12,7 @@ import (
 	"gioui.org/app/internal/input"
 	"gioui.org/io/event"
 	"gioui.org/io/profile"
+	"gioui.org/io/system"
 	"gioui.org/op"
 	"gioui.org/unit"
 )
@@ -24,6 +25,12 @@ type windowOptions struct {
 	Title         string
 }
 
+type frameEvent struct {
+	system.FrameEvent
+
+	sync bool
+}
+
 // Window represents an operating system window.
 type Window struct {
 	driver    *window
@@ -37,7 +44,7 @@ type Window struct {
 	invalidates chan struct{}
 	frames      chan *op.Ops
 
-	stage        Stage
+	stage        system.Stage
 	animating    bool
 	hasNextFrame bool
 	nextFrame    time.Time
@@ -170,7 +177,7 @@ func (w *Window) updateAnimation() {
 		w.delayedDraw.Stop()
 		w.delayedDraw = nil
 	}
-	if w.stage >= StageRunning && w.hasNextFrame {
+	if w.stage >= system.StageRunning && w.hasNextFrame {
 		if dt := time.Until(w.nextFrame); dt <= 0 {
 			animate = true
 		} else {
@@ -210,10 +217,10 @@ func (w *Window) waitAck() {
 func (w *Window) destroy(err error) {
 	// Ack the current event.
 	w.ack <- struct{}{}
-	w.out <- DestroyEvent{err}
+	w.out <- system.DestroyEvent{Err: err}
 	for e := range w.in {
 		w.ack <- struct{}{}
-		if _, ok := e.(DestroyEvent); ok {
+		if _, ok := e.(system.DestroyEvent); ok {
 			return
 		}
 	}
@@ -223,7 +230,7 @@ func (w *Window) run(opts *windowOptions) {
 	defer close(w.in)
 	defer close(w.out)
 	if err := createWindow(w, opts); err != nil {
-		w.out <- DestroyEvent{err}
+		w.out <- system.DestroyEvent{Err: err}
 		return
 	}
 	for {
@@ -240,9 +247,9 @@ func (w *Window) run(opts *windowOptions) {
 			w.updateAnimation()
 		case e := <-w.in:
 			switch e2 := e.(type) {
-			case StageEvent:
+			case system.StageEvent:
 				if w.gpu != nil {
-					if e2.Stage < StageRunning {
+					if e2.Stage < system.StageRunning {
 						w.gpu.Release()
 						w.gpu = nil
 					} else {
@@ -253,18 +260,18 @@ func (w *Window) run(opts *windowOptions) {
 				w.updateAnimation()
 				w.out <- e
 				w.waitAck()
-			case FrameEvent:
+			case frameEvent:
 				if e2.Size == (image.Point{}) {
 					panic(errors.New("internal error: zero-sized Draw"))
 				}
-				if w.stage < StageRunning {
+				if w.stage < system.StageRunning {
 					// No drawing if not visible.
 					break
 				}
 				w.drawStart = time.Now()
 				w.hasNextFrame = false
 				e2.Frame = w.update
-				w.out <- e2
+				w.out <- e2.FrameEvent
 				var frame *op.Ops
 				// Wait for either a frame or the ack event,
 				// which meant that the client didn't draw.
@@ -303,12 +310,12 @@ func (w *Window) run(opts *windowOptions) {
 						return
 					}
 				}
-			case *CommandEvent:
+			case *system.CommandEvent:
 				w.out <- e
 				w.waitAck()
 			case driverEvent:
 				w.driver = e2.driver
-			case DestroyEvent:
+			case system.DestroyEvent:
 				w.out <- e2
 				w.ack <- struct{}{}
 				return

io/system/system.go

@@ -0,0 +1,102 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+// Package system contains events usually handled at the top-level
+// program level.
+package system
+
+import (
+	"image"
+	"time"
+
+	"gioui.org/op"
+	"gioui.org/unit"
+)
+
+// A FrameEvent asks for a new frame in the form of a list of
+// operations.
+type FrameEvent struct {
+	Config Config
+	// Size is the dimensions of the window.
+	Size image.Point
+	// Insets is the insets to apply.
+	Insets Insets
+	// Frame replaces the window's frame with the new
+	// frame.
+	Frame func(frame *op.Ops)
+	// Whether this draw is system generated and needs a complete
+	// frame before proceeding.
+	sync bool
+}
+
+// Config define the essential properties of
+// the environment.
+type Config interface {
+	// Now returns the current animation time.
+	Now() time.Time
+
+	unit.Converter
+}
+
+// DestroyEvent is the last event sent through
+// a window event channel.
+type DestroyEvent struct {
+	// Err is nil for normal window closures. If a
+	// window is prematurely closed, Err is the cause.
+	Err error
+}
+
+// Insets is the space taken up by
+// system decoration such as translucent
+// system bars and software keyboards.
+type Insets struct {
+	Top, Bottom, Left, Right unit.Value
+}
+
+// A StageEvent is generated whenever the stage of a
+// Window changes.
+type StageEvent struct {
+	Stage Stage
+}
+
+// CommandEvent is a system event.
+type CommandEvent struct {
+	Type CommandType
+	// Suppress the default action of the command.
+	Cancel bool
+}
+
+// Stage of a Window.
+type Stage uint8
+
+// CommandType is the type of a CommandEvent.
+type CommandType uint8
+
+const (
+	// StagePaused is the Stage for inactive Windows.
+	// Inactive Windows don't receive FrameEvents.
+	StagePaused Stage = iota
+	// StateRunning is for active Windows.
+	StageRunning
+)
+
+const (
+	// CommandBack is the command for a back action
+	// such as the Android back button.
+	CommandBack CommandType = iota
+)
+
+func (l Stage) String() string {
+	switch l {
+	case StagePaused:
+		return "StagePaused"
+	case StageRunning:
+		return "StageRunning"
+	default:
+		panic("unexpected Stage value")
+	}
+}
+
+func (_ FrameEvent) ImplementsEvent()    {}
+func (_ StageEvent) ImplementsEvent()    {}
+func (_ *CommandEvent) ImplementsEvent() {}
+func (_ DestroyEvent) ImplementsEvent()  {}

layout/layout.go

@@ -4,9 +4,9 @@ package layout
 
 import (
 	"image"
-	"time"
 
 	"gioui.org/io/event"
+	"gioui.org/io/system"
 	"gioui.org/op"
 	"gioui.org/unit"
 )
@@ -53,20 +53,11 @@ type Context struct {
 	// operation.
 	Dimensions Dimensions
 
-	Config
+	system.Config
 	event.Queue
 	*op.Ops
 }
 
-// Config define the essential properties of
-// the environment.
-type Config interface {
-	// Now returns the current animation time.
-	Now() time.Time
-
-	unit.Converter
-}
-
 const (
 	Start Alignment = iota
 	End
@@ -104,7 +95,7 @@ func (s *Context) Layout(cs Constraints, w Widget) Dimensions {
 
 // Reset the context. The constraints' minimum and maximum values are
 // set to the size.
-func (c *Context) Reset(cfg Config, size image.Point) {
+func (c *Context) Reset(cfg system.Config, size image.Point) {
 	c.Constraints = RigidConstraints(size)
 	c.Dimensions = Dimensions{}
 	c.Config = cfg