[Core] Add callback and internal returning runnable (aws-amplify#701)

to support sync and async methods

aws-android-sdk-core/src/main/java/com/amazonaws/async/Callback.java

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2018-2019 Amazon.com, Inc. or its affiliates.
+ * All Rights Reserved.
+ *
+ * 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 com.amazonaws.async;
+
+/**
+ * Callback for async operations.
+ * @param <R> the type of the result
+ */
+public interface Callback<R> {
+
+    /**
+     * If the underlying operation succeeds this method will be called with the result.
+     *
+     * @param result the result
+     */
+    void onResult(R result);
+
+    /**
+     * If the underlying operation fails this method will be called with the exception.
+     *
+     * @param e the error
+     */
+    void onError(Exception e);
+}

aws-android-sdk-core/src/main/java/com/amazonaws/internal/ReturningRunnable.java

@@ -0,0 +1,89 @@
+/*
+ * Copyright 2019-2019 Amazon.com, Inc. or its affiliates.
+ * All Rights Reserved.
+ *
+ * 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 com.amazonaws.internal;
+
+import com.amazonaws.async.Callback;
+
+/**
+ * Internal class to allow easy surfacing of synchronous and asynchronous operations.
+ * This class may change without notice and may not be reflected in the SDK versioning scheme.
+ *
+ * @param <R> the type of the result/return
+ */
+public abstract class ReturningRunnable<R> {
+
+    private final String operationDescription;
+
+    /**
+     * Default constructor. When an exception is thrown, it will not be wrapped with another
+     * exception.
+     */
+    public ReturningRunnable() {
+        this.operationDescription = null;
+    }
+
+    /**
+     * When an exception is thrown, it will be wrapped with another exception with a message
+     * specified by operationDescription.
+     *
+     * @param operationDescription The message of the exception that wraps the underlying exception.
+     */
+    public ReturningRunnable(final String operationDescription) {
+        this.operationDescription = operationDescription;
+    }
+
+    /**
+     * This method contains the underlying operation.
+     *
+     * @return Any result object of type R
+     * @throws Exception Any error from performing the underlying operation
+     */
+    public abstract R run() throws Exception;
+
+    /**
+     * Runs the code inside {@link #run()} synchronously on the same thread this method is called.
+     *
+     * @return the result from the {@link #run()} method
+     * @throws Exception error from the operation
+     */
+    public R await() throws Exception {
+        return this.run();
+    }
+
+    /**
+     * Runs the code inside {@link #run()} on a new Thread asynchronously and returns immediately.
+     *
+     * @param callback receives the result or error from the operation
+     */
+    public void async(final Callback<R> callback) {
+        new Thread(new Runnable() {
+            @Override
+            public void run() {
+                try {
+                    callback.onResult(ReturningRunnable.this.run());
+                } catch (final Exception e) {
+                    if (operationDescription == null) {
+                        callback.onError(e);
+                    } else {
+                        callback.onError(new Exception(operationDescription, e));
+                    }
+                }
+            }
+        }).start();
+    }
+}