io/event: move event types from package ui to its own package

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

io/event/event.go

@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+/*
+Package event contains the types for event handling.
+
+The Queue interface is the protocol for receiving external events.
+
+For example:
+
+	var queue event.Queue = ...
+
+	for _, e := range queue.Events(h) {
+		switch e.(type) {
+			...
+		}
+	}
+
+In general, handlers must be declared before events become
+available. Other packages such as pointer and key provide
+the means for declaring handlers for specific event types.
+
+The following example declares a handler ready for key input:
+
+	import gioui.org/io/key
+
+	ops := new(ui.Ops)
+	var h *Handler = ...
+	key.InputOp{Key: h}.Add(ops)
+
+*/
+package event
+
+// Queue maps an event handler key to the events
+// available to the handler.
+type Queue interface {
+	// Events returns the available events for a
+	// Key.
+	Events(k Key) []Event
+}
+
+// Key is the stable identifier for an event handler.
+// For a handler h, the key is typically &h.
+type Key interface{}
+
+// Event is the marker interface for events.
+type Event interface {
+	ImplementsEvent()
+}

io/key/key.go

@@ -10,8 +10,9 @@ events.
 package key
 
 import (
-	"gioui.org/ui"
 	"gioui.org/internal/opconst"
+	"gioui.org/io/event"
+	"gioui.org/ui"
 )
 
 // InputOp declares a handler ready for key events.
@@ -19,7 +20,7 @@ import (
 // focused key handler. Set the Focus flag to request
 // the focus.
 type InputOp struct {
-	Key   ui.Key
+	Key   event.Key
 	Focus bool
 }
 

io/pointer/doc.go

@@ -6,7 +6,7 @@ A pointer is either a mouse controlled cursor or a touch
 object such as a finger.
 
 The InputOp operation is used to declare a handler ready for pointer
-events. Use a ui.Queue to receive events.
+events. Use an event.Queue to receive events.
 
 Areas
 

io/pointer/pointer.go

@@ -7,9 +7,10 @@ import (
 	"image"
 	"time"
 
-	"gioui.org/ui"
 	"gioui.org/f32"
 	"gioui.org/internal/opconst"
+	"gioui.org/io/event"
+	"gioui.org/ui"
 )
 
 // Event is a pointer event.
@@ -63,7 +64,7 @@ type areaOp struct {
 // InputOp declares an input handler ready for pointer
 // events.
 type InputOp struct {
-	Key ui.Key
+	Key event.Key
 	// Grab, if set, request that the handler get
 	// Grabbed priority.
 	Grab bool

io/profile/profile.go

@@ -6,13 +6,14 @@ package profile
 
 import (
 	"gioui.org/internal/opconst"
+	"gioui.org/io/event"
 	"gioui.org/ui"
 )
 
 // Op registers a handler for receiving
 // Events.
 type Op struct {
-	Key ui.Key
+	Key event.Key
 }
 
 // Event contains profile data from a single