Create a Promise class to simplify dealing with async results.

base/android/junit/src/org/chromium/base/PromiseTest.java

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package org.chromium.base;
+
+import static org.junit.Assert.assertEquals;
+
+import org.chromium.testing.local.LocalRobolectricTestRunner;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.robolectric.annotation.Config;
+
+/** Unit tests for {@link Promise}. */
+@RunWith(LocalRobolectricTestRunner.class)
+@Config(manifest = Config.NONE)
+public class PromiseTest {
+    // We need a simple mutable reference type for testing.
+    private static class Value {
+        private int mValue;
+
+        public int get() {
+            return mValue;
+        }
+
+        public void set(int value) {
+            mValue = value;
+        }
+    }
+
+    /** Tests that the callback is called on fulfillment. */
+    @Test
+    public void callback() {
+        final Value value = new Value();
+
+        Promise<Integer> promise = new Promise<Integer>();
+        promise.then(new Callback<Integer>() {
+            @Override
+            public void onResult(Integer result) {
+                value.set(result);
+            }
+        });
+
+        assertEquals(value.get(), 0);
+
+        promise.fulfill(new Integer(1));
+        assertEquals(value.get(), 1);
+    }
+
+    /** Tests that multiple callbacks are called. */
+    @Test
+    public void multipleCallbacks() {
+        final Value value = new Value();
+
+        Promise<Integer> promise = new Promise<Integer>();
+        promise.then(new Callback<Integer>() {
+            @Override
+            public void onResult(Integer result) {
+                value.set(1);
+            }
+        });
+        promise.then(new Callback<Integer>() {
+            @Override
+            public void onResult(Integer result) {
+                value.set(2);
+            }
+        });
+
+        assertEquals(value.get(), 0);
+
+        promise.fulfill(new Integer(0));
+        assertEquals(value.get(), 2);
+    }
+
+    /** Tests that a callback is called immediately when given to a fulfilled Promise. */
+    @Test
+    public void callbackOnFulfilled() {
+        final Value value = new Value();
+
+        Promise<Integer> promise = Promise.fulfilled(new Integer(0));
+        assertEquals(value.get(), 0);
+
+        promise.then(new Callback<Integer>() {
+            @Override
+            public void onResult(Integer result) {
+                value.set(1);
+            }
+        });
+
+        assertEquals(value.get(), 1);
+    }
+}