gesture: support multiple click actions

Double click or tap actions are common in user interfaces and this
commit adds support for it in the gesture package.
ClickEvent has now a new field NumClicks that contains the number of
successive clicks that occurred within 200ms.

Fixes gio#101

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

gesture/gesture_test.go

@@ -0,0 +1,87 @@
+package gesture
+
+import (
+	"testing"
+	"time"
+
+	"gioui.org/io/event"
+	"gioui.org/io/pointer"
+	"gioui.org/io/router"
+	"gioui.org/op"
+)
+
+func TestMouseClicks(t *testing.T) {
+	for _, tc := range []struct {
+		label  string
+		events []event.Event
+		clicks []int // number of combined clicks per click (single, double...)
+	}{
+		{
+			label:  "single click",
+			events: mouseClickEvents(200 * time.Millisecond),
+			clicks: []int{1},
+		},
+		{
+			label: "double click",
+			events: mouseClickEvents(
+				100*time.Millisecond,
+				100*time.Millisecond+doubleClickDuration-1),
+			clicks: []int{1, 2},
+		},
+		{
+			label: "two single clicks",
+			events: mouseClickEvents(
+				100*time.Millisecond,
+				100*time.Millisecond+doubleClickDuration+1),
+			clicks: []int{1, 1},
+		},
+	} {
+		t.Run(tc.label, func(t *testing.T) {
+			var click Click
+			var ops op.Ops
+			click.Add(&ops)
+
+			var r router.Router
+			r.Frame(&ops)
+			r.Add(tc.events...)
+
+			events := click.Events(&r)
+			clicks := filterMouseClicks(events)
+			if got, want := len(clicks), len(tc.clicks); got != want {
+				t.Fatalf("got %d mouse clicks, expected %d", got, want)
+			}
+
+			for i, click := range clicks {
+				if got, want := click.NumClicks, tc.clicks[i]; got != want {
+					t.Errorf("got %d combined mouse clicks, expected %d", got, want)
+				}
+			}
+		})
+	}
+}
+
+func mouseClickEvents(times ...time.Duration) []event.Event {
+	press := pointer.Event{
+		Type:    pointer.Press,
+		Source:  pointer.Mouse,
+		Buttons: pointer.ButtonLeft,
+	}
+	events := make([]event.Event, 0, 2*len(times))
+	for _, t := range times {
+		release := press
+		release.Type = pointer.Release
+		release.Time = t
+		events = append(events, press, release)
+	}
+	return events
+}
+
+func filterMouseClicks(events []ClickEvent) []ClickEvent {
+	var clicks []ClickEvent
+	for _, ev := range events {
+		if ev.Type == TypeClick {
+			clicks = append(clicks, ev)
+		}
+	}
+	return clicks
+}