[Suggestions UI] Drop Bitmap references from articles under memory pressure.

base/android/java/src/org/chromium/base/ReferencePool.java

+// Copyright 2017 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.support.annotation.Nullable;
+
+import java.lang.ref.WeakReference;
+import java.util.WeakHashMap;
+
+/**
+ * A ReferencePool allows handing out typed references to objects ("payloads") that can be dropped
+ * in one batch ("drained"), e.g. under memory pressure. In contrast to {@link WeakReference}s,
+ * which drop their referents when they get garbage collected, a reference pool gives more precise
+ * control over when exactly it is drained.
+ *
+ * <p>Internally it uses a {@link WeakHashMap} with the reference itself as a key to allow the
+ * payloads to be garbage collected regularly when the last reference goes away before the pool is
+ * drained.
+ *
+ * <p>Note that certain kinds of reference cycles that would be garbage-collected when using weak
+ * (or even strong) references could lead to leaks with a ReferencePool: For example, if a
+ * {@link Reference} is held by its own payload (including indirectly) but is otherwise not
+ * referenced, it will not get garbage-collected, because the ReferencePool will still hold a strong
+ * reference to the payload.
+ */
+public class ReferencePool {

[rmcilroy, 2017/05/10 14:38:11]

nit - not sure about the name. How about DiscardableReferencePool (similar to
DiscardableMemory in C++ land).

[Bernhard Bauer, 2017/05/10 17:47:50]

Good idea! Done. I kept variable and member names as referencePool just to keep
things a bit shorter.

+    /**
+     * The underlying data storage. The wildcard type parameter allows using a single pool for
+     * references of any type.
+     */
+    private final WeakHashMap<Reference<?>, Object> mPool = new WeakHashMap<>();
+
+    /**
+     * A reference to an object in the pool. Will be nulled out when the pool has been drained.
+     * @param <T> The type of the object.
+     */
+    public class Reference<T> {

[rmcilroy, 2017/05/10 14:38:11]

nit - DiscardableReference to make it clear it could return null even if it was
set?

[Bernhard Bauer, 2017/05/10 17:47:50]

Done.

+        /**
+         * @return The referent, or null if the pool has been drained.
+         */
+        @SuppressWarnings("unchecked")
+        @Nullable
+        public T get() {
+            return (T) mPool.get(this);
+        }
+    }
+
+    /**
+     * @param <T> The type of the object.
+     * @param payload The payload to add to the pool.
+     * @return A new reference to the {@code payload}.
+     */
+    public <T> Reference<T> put(T payload) {
+        assert payload != null;
+        Reference<T> reference = new Reference<>();
+        mPool.put(reference, payload);
+        return reference;
+    }
+
+    /**
+     * Drains the pool, removing all references to objects in the pool and therefore allowing them
+     * to be garbage collected.
+     */
+    public void drain() {
+        mPool.clear();
+    }
+}