Add SingleLiveData utility class

app/src/main/java/org/mercury_im/messenger/android/ui/base/SingleLiveEvent.java

@@ -0,0 +1,75 @@
+/*
+ *  Copyright 2017 Google Inc.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+package org.mercury_im.messenger.android.ui.base;
+
+import android.util.Log;
+
+import androidx.annotation.MainThread;
+import androidx.annotation.NonNull;
+import androidx.lifecycle.LifecycleOwner;
+import androidx.lifecycle.MutableLiveData;
+import androidx.lifecycle.Observer;
+
+import java.util.concurrent.atomic.AtomicBoolean;
+
+/**
+ * A lifecycle-aware observable that sends only new updates after subscription, used for events like
+ * navigation and Snackbar messages.
+ * <p>
+ * This avoids a common problem with events: on configuration change (like rotation) an update
+ * can be emitted if the observer is active. This LiveData only calls the observable if there's an
+ * explicit call to setValue() or call().
+ * <p>
+ * Note that only one observer is going to be notified of changes.
+ */
+public class SingleLiveEvent<T> extends MutableLiveData<T> {
+
+    private static final String TAG = "SingleLiveEvent";
+
+    private final AtomicBoolean pending = new AtomicBoolean(false);
+
+    @Override
+    @MainThread
+    public void observe(@NonNull LifecycleOwner owner, @NonNull Observer<? super T> observer) {
+        if (hasActiveObservers()) {
+            Log.w(TAG, "Multiple observers registered but only one will be notified of changes.");
+        }
+
+        super.observe(owner, new Observer<T>() {
+            @Override
+            public void onChanged(T t) {
+                if (pending.compareAndSet(true, false)) {
+                    observer.onChanged(t);
+                }
+            }
+        });
+    }
+
+    @Override
+    @MainThread
+    public void setValue(T value) {
+        pending.set(true);
+        super.setValue(value);
+    }
+
+    /**
+     * Used for cases where T is Void, to make calls cleaner.
+     */
+    @MainThread
+    public void call() {
+        setValue(null);
+    }
+}