Create a Promise class to simplify dealing with async results.

base/android/java/src/org/chromium/base/Promise.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 android.os.Handler;
+import android.os.Looper;
+
+import java.util.LinkedList;
+import java.util.List;
+
+/**
+ * A Promise class to be used as a placeholder for a result that will be provided asynchronously.
+ * It must only be accessed from a single thread.
+ * @param <T> The type the Promise will be fulfilled with.
+ */
+public class Promise<T> {
+    private T mResult;
+    private boolean mFulfilled;
+    private final List<Callback<T>> mCallbacks = new LinkedList<Callback<T>>();
+    private final Thread mThread;
+
+    /**
+     * Creates an unfulfilled promise.
+     */
+    public Promise() {
+        mThread = Thread.currentThread();
+    }
+
+    /**
+     * Queues a {@link Callback} to be run when the Promise is fulfilled or runs it
+     * instantly if the Promise is already fulfilled.
+     */
+    public void then(Callback<T> callback) {
+        checkThread();
+
+        if (mFulfilled) {
+            callback.onResult(mResult);

[Bernhard Bauer, 2016/04/13 16:24:21]

I think you may also want want to run this callback asynchronously, to prevent
reentrancy issues.

[PEConn, 2016/05/25 09:52:12]

Done.

+        } else {
+            mCallbacks.add(callback);
+        }
+    }
+
+    /**
+     * Fulfills the Promise with the result and passes it to any {@link Callback}s
+     * previously queued with {@link Promise#then(Callback)}.
+     */
+    public void fulfill(final T result) {
+        checkThread();
+        assert !mFulfilled;
+
+        mFulfilled = true;
+        mResult = result;
+        for (final Callback<T> callback : mCallbacks) {
+            // Post the callbacks to the Thread looper so we don't get a long chain of callbacks
+            // holding up the thread.
+            new Handler(Looper.myLooper()).post(new Runnable() {
+                @Override
+                public void run() {
+                    callback.onResult(mResult);
+                }
+            });
+        }
+        mCallbacks.clear();
+    }
+
+    /**
+     * Convenience method to return a Promise fulfilled with the given result.
+     */
+    public static <T> Promise<T> fulfilled(T result) {
+        Promise<T> promise = new Promise<T>();
+        promise.fulfill(result);
+        return promise;
+    }
+
+    private void checkThread() {
+        assert mThread == Thread.currentThread() : "Promise must only be used on a single Thread.";
+    }
+}