widget: added example for Clickable

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

widget/example_test.go

@@ -0,0 +1,71 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package widget_test
+
+import (
+	"fmt"
+	"image"
+
+	"gioui.org/f32"
+	"gioui.org/io/pointer"
+	"gioui.org/io/router"
+	"gioui.org/layout"
+	"gioui.org/op"
+	"gioui.org/widget"
+)
+
+func ExampleClickable_passthrough() {
+	// When laying out clickable widgets on top of each other,
+	// pointer events can be passed down for the underlying
+	// widgets to pick them up.
+	var button1, button2 widget.Clickable
+	var r router.Router
+	gtx := layout.Context{
+		Ops:         new(op.Ops),
+		Constraints: layout.Exact(image.Pt(100, 100)),
+		Queue:       &r,
+	}
+
+	// widget lays out two buttons on top of each other.
+	widget := func() {
+		// button2 completely covers button1, but PassOp allows pointer
+		// events to pass through to button1.
+		button1.Layout(gtx)
+		// PassOp is applied to the area defined by button1.
+		pointer.PassOp{Pass: true}.Add(gtx.Ops)
+		button2.Layout(gtx)
+	}
+
+	// The first layout and call to Frame declare the Clickable handlers
+	// to the input router, so the following pointer events are propagated.
+	widget()
+	r.Frame(gtx.Ops)
+	// Simulate one click on the buttons by sending a Press and Release event.
+	r.Add(
+		pointer.Event{
+			Source:   pointer.Mouse,
+			Buttons:  pointer.ButtonLeft,
+			Type:     pointer.Press,
+			Position: f32.Pt(50, 50),
+		},
+		pointer.Event{
+			Source:   pointer.Mouse,
+			Buttons:  pointer.ButtonLeft,
+			Type:     pointer.Release,
+			Position: f32.Pt(50, 50),
+		},
+	)
+	// The second layout ensures that the click event is registered by the buttons.
+	widget()
+
+	if button1.Clicked() {
+		fmt.Println("button1 clicked!")
+	}
+	if button2.Clicked() {
+		fmt.Println("button2 clicked!")
+	}
+
+	// Output:
+	// button1 clicked!
+	// button2 clicked!
+}