[core] Add DoOnce utility

smack-core/src/main/java/org/jivesoftware/smack/util/DoOnce.java

@@ -0,0 +1,29 @@
+/**
+ *
+ * Copyright 2021 Florian Schmaus.
+ *
+ * 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.jivesoftware.smack.util;
+
+public final class DoOnce {
+
+    private boolean done;
+
+    public void once(Runnable run) {
+        if (done) return;
+
+        done = true;
+        run.run();
+    }
+}