Android: Add a java version of ObserverList.

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

+// Copyright 2013 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 java.lang.Iterable;
+import java.util.ArrayList;
+import java.util.Iterator;
+import java.util.List;
+import java.util.NoSuchElementException;
+
+import javax.annotation.concurrent.NotThreadSafe;
+
+/**
+ * A container for a list of observers.
+ * <p/>
+ * This container can be modified during iteration without invalidating the iterator.
+ * So, it safely handles the case of an observer removing itself or other observers from the list
+ * while observers are being notified.
+ * <p/>
+ * The implementation (and the interface) is heavily influenced by the C++ ObserverList.
+ * Notable differences:
+ *   - The iterator implements NOTIFY_EXISTING_ONLY.
+ *   - The FOR_EACH_OBSERVER closure is left to the clients to implement in terms of iterator().
+ * <p/>
+ * This class is not threadsafe. Observers MUST be added, removed and will be notified on the same
+ * thread this is created.
+ */
+@NotThreadSafe
+public class ObserverList<E> implements Iterable<E> {
+    public final List<E> mObservers = new ArrayList<E>();
+    private int mIterationDepth = 0;
+
+    public ObserverList() {}
+
+    /**
+     * Add an observer to the list.
+     * <p/>
+     * An observer should not be added to the same list more than once. If an iteration is already
+     * in progress, this observer will be not be visible during that iteration.
+     */
+    public void addObserver(E obs) {
+        // Avoid adding null elements to the list as they may be removed on a compaction.
+        if (obs == null || mObservers.contains(obs)) {
+            assert false;
+            return;
+        }
+
+        // Structurally modifying the underlying list here. This means we
+        // cannot use the underlying list's iterator to iterate over the list.
+        mObservers.add(obs);
+    }
+
+    /**
+     * Remove an observer from the list if it is in the list.
+     */
+    public void removeObserver(E obs) {
+        int index = mObservers.indexOf(obs);
+
+        if (index == -1)
+            return;
+
+        if (mIterationDepth == 0) {
+            // No one is iterating over the list.
+            mObservers.remove(obs);
+        } else {
+            mObservers.set(index, null);
+        }
+    }
+
+    public boolean hasObserver(E obs) {
+        return mObservers.contains(obs);
+    }
+
+    public void clear() {
+        if (mIterationDepth == 0) {
+            mObservers.clear();
+            return;
+        }
+
+        int size = mObservers.size();
+        for (int i = 0; i < size; i++)
+            mObservers.set(i, null);
+    }
+
+    @Override
+    public Iterator<E> iterator() {
+        return new ObserverListIterator();
+    }
+
+    /**
+     * Compact the underlying list be removing null elements.
+     * <p/>
+     * Should only be called when mIterationDepth is zero.
+     */
+    private void compact() {
+        assert mIterationDepth == 0;
+        // Safe to use the underlying list's iterator, as we know that no-one else
+        // is iterating over the list.
+        Iterator<E> it = mObservers.iterator();
+        while (it.hasNext()) {
+            E el = it.next();
+            if (el == null)
+                it.remove();
+        }
+    }
+
+    private void incrementIterationDepth() {
+        mIterationDepth++;
+    }
+
+    private void decrementIterationDepthAndCompactIfNeeded() {
+        mIterationDepth--;
+        assert mIterationDepth >= 0;
+        if (mIterationDepth == 0)
+            compact();
+    }
+
+    private int getSize() {
+        return mObservers.size();
+    }
+
+    private E getObserverAt(int index) {
+        return mObservers.get(index);
+    }
+
+    private class ObserverListIterator implements Iterator<E> {
+        private final int mListEndMarker;
+        private int mIndex = 0;
+        private boolean mIsExhausted = false;
+
+        private ObserverListIterator() {
+            ObserverList.this.incrementIterationDepth();
+            mListEndMarker = ObserverList.this.getSize();
+        }
+
+        @Override
+        public boolean hasNext() {
+            int lookupIndex = mIndex;
+            while (lookupIndex < mListEndMarker &&
+                    ObserverList.this.getObserverAt(lookupIndex) == null)
+                lookupIndex++;
+            if (lookupIndex < mListEndMarker)
+                return true;
+
+            // We have reached the end of the list, allow for compaction.
+            compactListIfNeeded();
+            return false;
+        }
+
+        @Override
+        public E next() {
+            // Advance if the current element is null.
+            while (mIndex < mListEndMarker && ObserverList.this.getObserverAt(mIndex) == null)
+                mIndex++;
+            if (mIndex < mListEndMarker)
+                return ObserverList.this.getObserverAt(mIndex++);
+
+            // We have reached the end of the list, allow for compaction.
+            compactListIfNeeded();
+            throw new NoSuchElementException();
+        }
+
+        @Override
+        public void remove() {
+            throw new UnsupportedOperationException();
+        }
+
+        private void compactListIfNeeded() {
+            if (!mIsExhausted) {
+                mIsExhausted = true;
+                ObserverList.this.decrementIterationDepthAndCompactIfNeeded();
+            }
+        }
+    }
+}