allow for non-Window WailsEvent listeners (#3406)

* allow for non-Window WailsEvent listeners

- adds a RegisterListener function on the App struct such
  that code can listen for WailsEvent(s) even if it isn't a Window
- rename dispatchEventToWindows -> dispatchEventToListeners
- add all windows and WailsEventListeners to slice before emitting

* Update v3/pkg/application/application.go

---------

Co-authored-by: Lea Anthony <lea.anthony@gmail.com>

v3/pkg/application/application.go

@@ -76,7 +76,7 @@ func New(appOptions Options) *App {
 	result.logStartup()
 	result.logPlatformInfo()
 
-	result.Events = NewWailsEventProcessor(result.dispatchEventToWindows)
+	result.Events = NewWailsEventProcessor(result.dispatchEventToListeners)
 
 	messageProc := NewMessageProcessor(result.Logger)
 	opts := &assetserver.Options{
@@ -326,6 +326,10 @@ type App struct {
 
 	// signalHandler is used to handle signals
 	signalHandler *signal.SignalHandler
+
+	// Wails Event Listener related
+	wailsEventListenerLock sync.Mutex
+	wailsEventListeners    []WailsEventListener
 }
 
 func (a *App) init() {
@@ -336,6 +340,7 @@ func (a *App) init() {
 	a.keyBindings = make(map[string]func(window *WebviewWindow))
 	a.Logger = a.options.Logger
 	a.pid = os.Getpid()
+	a.wailsEventListeners = make([]WailsEventListener, 0)
 }
 
 func (a *App) getSystemTrayID() uint {
@@ -400,6 +405,12 @@ func (a *App) RegisterHook(eventType events.ApplicationEventType, callback func(
 	}
 }
 
+func (a *App) RegisterListener(listener WailsEventListener) {
+	a.wailsEventListenerLock.Lock()
+	a.wailsEventListeners = append(a.wailsEventListeners, listener)
+	a.wailsEventListenerLock.Unlock()
+}
+
 func (a *App) NewWebviewWindow() *WebviewWindow {
 	return a.NewWebviewWindowWithOptions(WebviewWindowOptions{})
 }
@@ -774,10 +785,16 @@ func SaveFileDialogWithOptions(s *SaveFileDialogOptions) *SaveFileDialogStruct {
 	return result
 }
 
-func (a *App) dispatchEventToWindows(event *WailsEvent) {
+func (a *App) dispatchEventToListeners(event *WailsEvent) {
+	listeners := a.wailsEventListeners
+
 	for _, window := range a.windows {
 		window.DispatchWailsEvent(event)
 	}
+
+	for _, listener := range listeners {
+		listener.DispatchWailsEvent(event)
+	}
 }
 
 func (a *App) IsDarkMode() bool {

v3/pkg/application/events.go

@@ -60,6 +60,12 @@ func (e WailsEvent) ToJSON() string {
 	return string(marshal)
 }
 
+// WailsEventListener is an interface that can be implemented to listen for Wails events
+// It is used by the RegisterListener method of the Application.
+type WailsEventListener interface {
+	DispatchWailsEvent(event *WailsEvent)
+}
+
 type hook struct {
 	callback func(*WailsEvent)
 }