Allow updating the callback function on an existing button instance.

CHANGELOG.md

@@ -60,6 +60,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   main states (up, focused and up, down).
 - the `button` widget allows specifying separate fill color values for each of
   its main states (up, focused and up, down).
+- the `button` widget now has a method `SetCallback` that allows updating the
+  callback function on an existing `button` instance.
 
 #### Updates to the `textinput` widget
 

widgets/button/button.go

@@ -154,6 +154,13 @@ func NewFromChunks(chunks []*TextChunk, cFn CallbackFn, opts ...Option) (*Button
 	}, nil
 }
 
+// SetCallback replaces the callback function of the button with the one provided.
+func (b *Button) SetCallback(cFn CallbackFn) {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	b.callback = cFn
+}
+
 // Vars to be replaced from tests.
 var (
 	// Runes to use in cells that contain the button.

widgets/button/button_test.go

@@ -45,6 +45,10 @@ type callbackTracker struct {
 	// count is the number of times the callback was called.
 	count int
 
+	// useSetCallback when set to true instructs the test to set the callback
+	// via button.SetCallback instead of button.New or button.NewFromChunks.
+	useSetCallback bool
+
 	// mu protects the tracker.
 	mu sync.Mutex
 }
@@ -411,7 +415,7 @@ func TestButton(t *testing.T) {
 			wantCallback: &callbackTracker{},
 		},
 		{
-			desc:     "mouse triggered the callback",
+			desc:     "mouse triggered a callback set via the constructor",
 			callback: &callbackTracker{},
 			text:     "hello",
 			canvas:   image.Rect(0, 0, 8, 4),
@@ -451,6 +455,50 @@ func TestButton(t *testing.T) {
 				count:  1,
 			},
 		},
+		{
+			desc: "mouse triggered a callback set via SetCallback",
+			callback: &callbackTracker{
+				useSetCallback: true,
+			},
+			text:   "hello",
+			canvas: image.Rect(0, 0, 8, 4),
+			meta:   &widgetapi.Meta{Focused: false},
+			events: []*event{
+				{
+					ev:   &terminalapi.Mouse{Position: image.Point{0, 0}, Button: mouse.ButtonLeft},
+					meta: &widgetapi.EventMeta{},
+				},
+				{
+					ev:   &terminalapi.Mouse{Position: image.Point{0, 0}, Button: mouse.ButtonRelease},
+					meta: &widgetapi.EventMeta{},
+				},
+			},
+			want: func(size image.Point) *faketerm.Terminal {
+				ft := faketerm.MustNew(size)
+				cvs := testcanvas.MustNew(ft.Area())
+
+				// Shadow.
+				testcanvas.MustSetAreaCells(cvs, image.Rect(1, 1, 8, 4), 's', cell.BgColor(cell.ColorNumber(240)))
+
+				// Button.
+				testcanvas.MustSetAreaCells(cvs, image.Rect(0, 0, 7, 3), 'x', cell.BgColor(cell.ColorNumber(117)))
+
+				// Text.
+				testdraw.MustText(cvs, "hello", image.Point{1, 1},
+					draw.TextCellOpts(
+						cell.FgColor(cell.ColorBlack),
+						cell.BgColor(cell.ColorNumber(117))),
+				)
+
+				testcanvas.MustApply(cvs, ft)
+				return ft
+			},
+			wantCallback: &callbackTracker{
+				called:         true,
+				count:          1,
+				useSetCallback: true,
+			},
+		},
 		{
 			desc:     "draws button in down state due to a keyboard event, callback triggered",
 			callback: &callbackTracker{},
@@ -1373,6 +1421,10 @@ func TestButton(t *testing.T) {
 			var cFn CallbackFn
 			if gotCallback == nil {
 				cFn = nil
+			} else if gotCallback.useSetCallback {
+				// Set an no-op callback via the constructor.
+				// It will be updated to the real one via SetCallback.
+				cFn = func() error { return nil }
 			} else {
 				cFn = gotCallback.callback
 			}
@@ -1402,6 +1454,10 @@ func TestButton(t *testing.T) {
 				btn = b
 			}
 
+			if gotCallback.useSetCallback {
+				btn.SetCallback(gotCallback.callback)
+			}
+
 			{
 				// Draw once which initializes the mouse state machine with the current canvas area.
 				c, err := canvas.New(tc.canvas)