Add Application.SetBeforeFocusFunc and Application.SetAfterFocusFunc

CHANGELOG

@@ -2,6 +2,7 @@ v1.4.5 (WIP)
 - Add multithreading support
 - Add ContextMenu (initially supported by List)
 - Add List.Transform and TreeView.Transform
+- Add Application.SetBeforeFocusFunc and Application.SetAfterFocusFunc
 - Merge upstream mouse support
 
 v1.4.4 (2020-02-24)

application.go

@@ -90,6 +90,14 @@ type Application struct {
 	// is drawn.
 	afterResize func(width int, height int)
 
+	// An optional callback function which is invoked before the application's
+	// focus changes.
+	beforeFocus func(p Primitive) bool
+
+	// An optional callback function which is invoked after the application's
+	// focus changes.
+	afterFocus func(p Primitive)
+
 	// An optional callback function which is invoked just before the root
 	// primitive is drawn.
 	beforeDraw func(screen tcell.Screen) bool
@@ -738,14 +746,34 @@ func (a *Application) GetAfterResizeFunc() func(width int, height int) {
 // called on the new primitive.
 func (a *Application) SetFocus(p Primitive) *Application {
 	a.Lock()
+
+	if a.beforeFocus != nil {
+		a.Unlock()
+		ok := a.beforeFocus(p)
+		if !ok {
+			return a
+		}
+		a.Lock()
+	}
+
 	if a.focus != nil {
 		a.focus.Blur()
 	}
+
 	a.focus = p
+
 	if a.screen != nil {
 		a.screen.HideCursor()
 	}
-	a.Unlock()
+
+	if a.afterFocus != nil {
+		a.Unlock()
+
+		a.afterFocus(p)
+	} else {
+		a.Unlock()
+	}
+
 	if p != nil {
 		p.Focus(func(p Primitive) {
 			a.SetFocus(p)
@@ -764,6 +792,30 @@ func (a *Application) GetFocus() Primitive {
 	return a.focus
 }
 
+// SetBeforeFocusFunc installs a callback function which is invoked before the
+// application's focus changes. Return false to maintain the current focus.
+//
+// Provide nil to uninstall the callback function.
+func (a *Application) SetBeforeFocusFunc(handler func(p Primitive) bool) *Application {
+	a.Lock()
+	defer a.Unlock()
+
+	a.beforeFocus = handler
+	return a
+}
+
+// SetAfterFocusFunc installs a callback function which is invoked after the
+// application's focus changes.
+//
+// Provide nil to uninstall the callback function.
+func (a *Application) SetAfterFocusFunc(handler func(p Primitive)) *Application {
+	a.Lock()
+	defer a.Unlock()
+
+	a.afterFocus = handler
+	return a
+}
+
 // QueueUpdate queues a function to be executed as part of the event loop.
 //
 // Note that Draw() is not implicitly called after the execution of f as that

go.mod

@@ -7,6 +7,6 @@ require (
 	github.com/lucasb-eyer/go-colorful v1.0.3
 	github.com/mattn/go-runewidth v0.0.9
 	github.com/rivo/uniseg v0.1.0
-	golang.org/x/sys v0.0.0-20200413165638-669c56c373c4 // indirect
+	golang.org/x/sys v0.0.0-20200420163511-1957bb5e6d1f // indirect
 	golang.org/x/text v0.3.2 // indirect
 )

go.sum

@@ -16,8 +16,8 @@ github.com/rivo/uniseg v0.1.0 h1:+2KBaVoUmb9XzDsrx/Ct0W/EYOSFf/nWTauy++DprtY=
 github.com/rivo/uniseg v0.1.0/go.mod h1:J6wj4VEh+S6ZtnVlnTBMWIodfgj8LQOQFoIToxlJtxc=
 golang.org/x/sys v0.0.0-20190626150813-e07cf5db2756 h1:9nuHUbU8dRnRRfj9KjWUVrJeoexdbeMjttk6Oh1rD10=
 golang.org/x/sys v0.0.0-20190626150813-e07cf5db2756/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
-golang.org/x/sys v0.0.0-20200413165638-669c56c373c4 h1:opSr2sbRXk5X5/givKrrKj9HXxFpW2sdCiP8MJSKLQY=
-golang.org/x/sys v0.0.0-20200413165638-669c56c373c4/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20200420163511-1957bb5e6d1f h1:gWF768j/LaZugp8dyS4UwsslYCYz9XgFxvlgsn0n9H8=
+golang.org/x/sys v0.0.0-20200420163511-1957bb5e6d1f/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/text v0.3.0 h1:g61tztE5qeGQ89tm6NTjjM9VPIm088od1l6aSorWRWg=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 golang.org/x/text v0.3.2 h1:tW2bmiBqwgJj/UpqtC8EpXEZVYOwU0yG4iWbprSVAcs=