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>

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()  {}