Initial checkin of AOSP Messaging app.

b/23110861

Change-Id: I11db999bd10656801e618f78ab2b2ef74136fff1

src/com/android/messaging/util/FallbackStrategies.java

@@ -0,0 +1,91 @@
+/*
+ * Copyright (C) 2015 The Android Open Source Project
+ *
+ * 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.android.messaging.util;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Provides a generic and loose-coupled framework to execute one primary and multiple fallback
+ * strategies for solving a given task.<p>
+ * Basically, what would have been a nasty try-catch that's hard to separate and maintain:
+ * <pre><code>
+ * try {
+ *     // doSomething() that may fail.
+ * } catch (Exception ex) {
+ *     try {
+ *         // fallback1() that may fail.
+ *     } catch (Exception ex2) {
+ *         try {
+ *             // fallback2() that may fail.
+ *         } catch (Exception ex3) {
+ *             // ...
+ *         }
+ *     }
+ * }
+ * </code></pre>
+ * Now becomes:<br>
+ * <pre><code>
+ * FallbackStrategies
+ *      .startWith(something)
+ *      .thenTry(fallback1)
+ *      .thenTry(fallback2)
+ *      .execute();
+ * </code></pre>
+ */
+public class FallbackStrategies<Input, Output> {
+    public interface Strategy<Input, Output> {
+        Output execute(Input params) throws Exception;
+    }
+
+    private final List<Strategy<Input, Output>> mChainedStrategies;
+
+    private FallbackStrategies(final Strategy<Input, Output> primaryStrategy) {
+        mChainedStrategies = new ArrayList<Strategy<Input, Output>>();
+        mChainedStrategies.add(primaryStrategy);
+    }
+
+    public static <Input, Output> FallbackStrategies<Input, Output> startWith(
+            final Strategy<Input, Output> primaryStrategy) {
+        return new FallbackStrategies<Input, Output>(primaryStrategy);
+    }
+
+    public FallbackStrategies<Input, Output> thenTry(final Strategy<Input, Output> strategy) {
+        Assert.isFalse(mChainedStrategies.isEmpty());
+        mChainedStrategies.add(strategy);
+        return this;
+    }
+
+    public Output execute(final Input params) {
+        final int count = mChainedStrategies.size();
+        for (int i = 0; i < count; i++) {
+            final Strategy<Input, Output> strategy = mChainedStrategies.get(i);
+            try {
+                // If succeeds, this will directly return.
+                return strategy.execute(params);
+            } catch (Exception ex) {
+                LogUtil.e(LogUtil.BUGLE_TAG, "Exceptions occured when executing strategy " +
+                        strategy + (i < count - 1 ?
+                                ", attempting fallback " + mChainedStrategies.get(i + 1) :
+                                ", and running out of fallbacks."), ex);
+                // This will fall through and continue with the next strategy (if any).
+            }
+        }
+        // Running out of strategies, return null.
+        // TODO: Should this accept user-defined fallback value other than null?
+        return null;
+    }
+}