techyourchance · December 28, 2021 13:38 · Aug 15, 2019 · Aug 15, 2019
diff --git a/BaseObservable.java b/BaseObservable.java
@@ -26,7 +26,7 @@ public void unregisterListener(LISTENER_CLASS listener) {
 
     protected Set<LISTENER_CLASS> getListeners() {
         synchronized (MONITOR) {
-            return Collections.unmodifiableSet(mListeners);
+            return Collections.unmodifiableSet(new HashSet<>(mListeners));
         }
     }
 

diff --git a/BaseObservable.java b/BaseObservable.java
@@ -0,0 +1,41 @@
+public abstract class BaseObservable<LISTENER_CLASS> {
+
+    private final Object MONITOR = new Object();
+
+    private final Set<LISTENER_CLASS> mListeners = new HashSet<>();
+
+    public void registerListener(LISTENER_CLASS listener) {
+        synchronized (MONITOR) {
+            boolean hadNoListeners = mListeners.size() == 0;
+            mListeners.add(listener);
+            if (hadNoListeners && mListeners.size() == 1) {
+                onFirstListenerRegistered();
+            }
+        }
+    }
+
+    public void unregisterListener(LISTENER_CLASS listener) {
+        synchronized (MONITOR) {
+            boolean hadOneListener = mListeners.size() == 1;
+            mListeners.remove(listener);
+            if (hadOneListener && mListeners.size() == 0) {
+                onLastListenerUnregistered();
+            }
+        }
+    }
+
+    protected Set<LISTENER_CLASS> getListeners() {
+        synchronized (MONITOR) {
+            return Collections.unmodifiableSet(mListeners);
+        }
+    }
+
+    protected void onFirstListenerRegistered() {
+
+    }
+
+    protected void onLastListenerUnregistered() {
+
+    }
+
+}