widget: add drag and drop support

This patch adds internal Drag and Drop support to app.Windows.

The new package io/transfer adds the ability to
define draggable and droppable targets, which
are leveraged by the new widget.Draggable type.

The API is generic and could handle future use
cases, such as external Drag and Drop.

Updates gio#153

Signed-off-by: Pierre Curto <pierre.curto@gmail.com>

widget/dnd_test.go

@@ -0,0 +1,86 @@
+package widget
+
+import (
+	"image"
+	"testing"
+
+	"gioui.org/f32"
+	"gioui.org/io/pointer"
+	"gioui.org/io/router"
+	"gioui.org/io/transfer"
+	"gioui.org/layout"
+	"gioui.org/op"
+	"gioui.org/op/clip"
+)
+
+func TestDraggable(t *testing.T) {
+	var r router.Router
+	gtx := layout.Context{
+		Constraints: layout.Exact(image.Pt(100, 100)),
+		Queue:       &r,
+		Ops:         new(op.Ops),
+	}
+
+	drag := &Draggable{
+		Type: "file",
+	}
+	defer pointer.PassOp{}.Push(gtx.Ops).Pop()
+	dims := drag.Layout(gtx, func(gtx layout.Context) layout.Dimensions {
+		return layout.Dimensions{Size: gtx.Constraints.Min}
+	}, nil)
+	stack := clip.Rect{Max: dims.Size}.Push(gtx.Ops)
+	transfer.TargetOp{
+		Tag:  drag,
+		Type: drag.Type,
+	}.Add(gtx.Ops)
+	stack.Pop()
+
+	r.Frame(gtx.Ops)
+	r.Queue(
+		pointer.Event{
+			Position: f32.Pt(10, 10),
+			Type:     pointer.Press,
+		},
+		pointer.Event{
+			Position: f32.Pt(20, 10),
+			Type:     pointer.Move,
+		},
+		pointer.Event{
+			Position: f32.Pt(20, 10),
+			Type:     pointer.Release,
+		},
+	)
+	ofr := &offer{data: "hello"}
+	drag.Offer(gtx.Ops, "file", ofr)
+	r.Frame(gtx.Ops)
+
+	evs := r.Events(drag)
+	if len(evs) != 1 {
+		t.Fatalf("expected 1 event, got %d", len(evs))
+	}
+	ev := evs[0].(transfer.DataEvent)
+	ev.Open = nil
+	if got, want := ev.Type, "file"; got != want {
+		t.Errorf("expected %v; got %v", got, want)
+	}
+	if ofr.closed {
+		t.Error("offer closed prematurely")
+	}
+	r.Frame(gtx.Ops)
+	if !ofr.closed {
+		t.Error("offer was not closed")
+	}
+}
+
+// offer satisfies io.ReadCloser for use in data transfers.
+type offer struct {
+	data   string
+	closed bool
+}
+
+func (*offer) Read([]byte) (int, error) { return 0, nil }
+
+func (o *offer) Close() error {
+	o.closed = true
+	return nil
+}