io/clipboard,app: add WriteOp, ReadOp

Previously, the only way to manipulate the clipboard (read or write) is
using the `app.Window`.

The new `clipboard.ReadOp` and `clipboard.WriteOp`makes possible to
read/write from the widget.

Signed-off-by: Inkeliz <inkeliz@inkeliz.com>

io/router/clipboard.go

@@ -0,0 +1,66 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package router
+
+import (
+	"gioui.org/internal/opconst"
+	"gioui.org/internal/ops"
+	"gioui.org/io/event"
+)
+
+type clipboardQueue struct {
+	receivers map[event.Tag]struct{}
+	// request avoid read clipboard every frame while waiting.
+	requested bool
+	text      *string
+	reader    ops.Reader
+}
+
+// WriteClipboard returns the most recent text to be copied
+// to the clipboard, if any.
+func (q *clipboardQueue) WriteClipboard() (string, bool) {
+	if q.text == nil {
+		return "", false
+	}
+	text := *q.text
+	q.text = nil
+	return text, true
+}
+
+// ReadClipboard reports if any new handler is waiting
+// to read the clipboard.
+func (q *clipboardQueue) ReadClipboard() bool {
+	if len(q.receivers) <= 0 || q.requested {
+		return false
+	}
+	q.requested = true
+	return true
+}
+
+func (q *clipboardQueue) Push(e event.Event, events *handlerEvents) {
+	for r := range q.receivers {
+		events.Add(r, e)
+		delete(q.receivers, r)
+	}
+}
+
+func (q *clipboardQueue) ProcessWriteClipboard(d []byte, refs []interface{}) {
+	if opconst.OpType(d[0]) != opconst.TypeClipboardWrite {
+		panic("invalid op")
+	}
+	q.text = refs[0].(*string)
+}
+
+func (q *clipboardQueue) ProcessReadClipboard(d []byte, refs []interface{}) {
+	if opconst.OpType(d[0]) != opconst.TypeClipboardRead {
+		panic("invalid op")
+	}
+	if q.receivers == nil {
+		q.receivers = make(map[event.Tag]struct{})
+	}
+	tag := refs[0].(event.Tag)
+	if _, ok := q.receivers[tag]; !ok {
+		q.receivers[tag] = struct{}{}
+		q.requested = false
+	}
+}