feat: implement android notification cancellation

android/NotificationHelper.java

@@ -31,4 +31,8 @@ public class NotificationHelper {
         NotificationManager notificationManager = ctx.getSystemService(NotificationManager.class);
         notificationManager.notify(notificationID, builder.build());
     }
+    public static void cancelNotification(Context ctx, int notificationID) {
+        NotificationManager notificationManager = ctx.getSystemService(NotificationManager.class);
+        notificationManager.cancel(notificationID);
+    }
 }

android/notify_android.go

@@ -8,6 +8,10 @@ import (
 	"github.com/tailscale/tailscale-android/jni"
 )
 
+const (
+	helperClass = "ht/sr/git/whereswaldon/niotify/NotificationHelper"
+)
+
 var (
 	idlock             sync.Mutex
 	nextNotificationID int32
@@ -21,15 +25,20 @@ func nextID() int32 {
 	return id
 }
 
+// NotificationChannel represents a stream of notifications that an application
+// provisions on android. Such streams can be selectively enabled and disabled
+// by the user, and should be used for different purposes.
 type NotificationChannel struct {
 	id string
 }
 
+// NewChannel creates a new notification channel identified by the provided id
+// and with the given user-visible name and description.
 func NewChannel(id, name, description string) (*NotificationChannel, error) {
 	if err := jni.Do(jni.JVMFor(app.JavaVM()), func(env jni.Env) error {
 		appCtx := jni.Object(app.AppContext())
 		classLoader := jni.ClassLoaderFor(env, appCtx)
-		notifyClass, err := jni.LoadClass(env, classLoader, "ht/sr/git/whereswaldon/niotify/NotificationHelper")
+		notifyClass, err := jni.LoadClass(env, classLoader, helperClass)
 		if err != nil {
 			return err
 		}
@@ -51,16 +60,19 @@ func NewChannel(id, name, description string) (*NotificationChannel, error) {
 	return nc, nil
 }
 
+// Notification represents a notification that has been requested to be shown to the user.
+// This type provides methods to cancel or update the contents of the notification.
 type Notification struct {
 	id int32
 }
 
+// Send creates a new Notification and requests that it be displayed on this channel.
 func (nc *NotificationChannel) Send(title, text string) (*Notification, error) {
 	notificationID := nextID()
 	if err := jni.Do(jni.JVMFor(app.JavaVM()), func(env jni.Env) error {
 		appCtx := jni.Object(app.AppContext())
 		classLoader := jni.ClassLoaderFor(env, appCtx)
-		notifyClass, err := jni.LoadClass(env, classLoader, "ht/sr/git/whereswaldon/niotify/NotificationHelper")
+		notifyClass, err := jni.LoadClass(env, classLoader, helperClass)
 		if err != nil {
 			return err
 		}
@@ -74,9 +86,27 @@ func (nc *NotificationChannel) Send(title, text string) (*Notification, error) {
 		}
 		return nil
 	}); err != nil {
-		return nil, fmt.Errorf("failed creating notification channel: %w", err)
+		return nil, fmt.Errorf("failed sending notification: %w", err)
 	}
 	return &Notification{
 		id: notificationID,
 	}, nil
 }
+
+// Cancel removes a previously created notification from display.
+func (n *Notification) Cancel() error {
+	notificationID := n.id
+	if err := jni.Do(jni.JVMFor(app.JavaVM()), func(env jni.Env) error {
+		appCtx := jni.Object(app.AppContext())
+		classLoader := jni.ClassLoaderFor(env, appCtx)
+		notifyClass, err := jni.LoadClass(env, classLoader, helperClass)
+		if err != nil {
+			return err
+		}
+		newChannelMethod := jni.GetStaticMethodID(env, notifyClass, "cancelNotification", "(Landroid/content/Context;I)V")
+		return jni.CallStaticVoidMethod(env, notifyClass, newChannelMethod, jni.Value(app.AppContext()), jni.Value(notificationID))
+	}); err != nil {
+		return fmt.Errorf("failed cancelling notification: %w", err)
+	}
+	return nil
+}

example/hello.go

@@ -7,6 +7,7 @@ package main
 import (
 	"image/color"
 	"log"
+	"time"
 
 	"gioui.org/app"
 	"gioui.org/io/system"
@@ -62,6 +63,10 @@ func loop(w *app.Window) error {
 						log.Printf("notification send failed: %v", err)
 					}
 					log.Println(notif)
+					time.Sleep(time.Second * 10)
+					if err := notif.Cancel(); err != nil {
+						log.Printf("failed cancelling: %v", err)
+					}
 				}()
 			}
 		}