| 1 | // Copyright 2013 The Chromium Authors. All rights reserved. |
| 2 | // Use of this source code is governed by a BSD-style license that can be |
| 3 | // found in the LICENSE file. |
| 4 | |
| 5 | package org.chromium.base; |
| 6 | |
| 7 | import java.lang.Iterable; |
| 8 | import java.util.ArrayList; |
| 9 | import java.util.Iterator; |
| 10 | import java.util.List; |
| 11 | import java.util.NoSuchElementException; |
| 12 | |
| 13 | import javax.annotation.concurrent.NotThreadSafe; |
| 14 | |
| 15 | /** |
| 16 | * A container for a list of observers. |
| 17 | * <p/> |
| 18 | * This container can be modified during iteration without invalidating the iterator. |
| 19 | * So, it safely handles the case of an observer removing itself or other observers from the list |
| 20 | * while observers are being notified. |
| 21 | * <p/> |
| 22 | * The implementation (and the interface) is heavily influenced by the C++ ObserverList. |
| 23 | * Notable differences: |
| 24 | * - The iterator implements NOTIFY_EXISTING_ONLY. |
| 25 | * - The FOR_EACH_OBSERVER closure is left to the clients to implement in terms of iterator(). |
| 26 | * <p/> |
| 27 | * This class is not threadsafe. Observers MUST be added, removed and will be notified on the same |
| 28 | * thread this is created. |
| 29 | */ |
| 30 | @NotThreadSafe |
| 31 | public class ObserverList<E> implements Iterable<E> { |
| 32 | public final List<E> mObservers = new ArrayList<E>(); |
| 33 | private int mIterationDepth = 0; |
| 34 | |
| 35 | public ObserverList() {} |
| 36 | |
| 37 | /** |
| 38 | * Add an observer to the list. |
| 39 | * <p/> |
| 40 | * An observer should not be added to the same list more than once. If an iteration is already |
| 41 | * in progress, this observer will be not be visible during that iteration. |
| 42 | */ |
| 43 | public void addObserver(E obs) { |
| 44 | // Avoid adding null elements to the list as they may be removed on a compaction. |
| 45 | if (obs == null || mObservers.contains(obs)) { |
| 46 | assert false; |
| 47 | return; |
| 48 | } |
| 49 | |
| 50 | // Structurally modifying the underlying list here. This means we |
| 51 | // cannot use the underlying list's iterator to iterate over the list. |
| 52 | mObservers.add(obs); |
| 53 | } |
| 54 | |
| 55 | /** |
| 56 | * Remove an observer from the list if it is in the list. |
| 57 | */ |
| 58 | public void removeObserver(E obs) { |
| 59 | int index = mObservers.indexOf(obs); |
| 60 | |
| 61 | if (index == -1) |
| 62 | return; |
| 63 | |
| 64 | if (mIterationDepth == 0) { |
| 65 | // No one is iterating over the list. |
| 66 | mObservers.remove(obs); |
| 67 | } else { |
| 68 | mObservers.set(index, null); |
| 69 | } |
| 70 | } |
| 71 | |
| 72 | public boolean hasObserver(E obs) { |
| 73 | return mObservers.contains(obs); |
| 74 | } |
| 75 | |
| 76 | public void clear() { |
| 77 | if (mIterationDepth == 0) { |
| 78 | mObservers.clear(); |
| 79 | return; |
| 80 | } |
| 81 | |
| 82 | int size = mObservers.size(); |
| 83 | for (int i = 0; i < size; i++) |
| 84 | mObservers.set(i, null); |
| 85 | } |
| 86 | |
| 87 | @Override |
| 88 | public Iterator<E> iterator() { |
| 89 | return new ObserverListIterator(); |
| 90 | } |
| 91 | |
| 92 | /** |
| 93 | * Compact the underlying list be removing null elements. |
| 94 | * <p/> |
| 95 | * Should only be called when mIterationDepth is zero. |
| 96 | */ |
| 97 | private void compact() { |
| 98 | assert mIterationDepth == 0; |
| 99 | // Safe to use the underlying list's iterator, as we know that no-one else |
| 100 | // is iterating over the list. |
| 101 | Iterator<E> it = mObservers.iterator(); |
| 102 | while (it.hasNext()) { |
| 103 | E el = it.next(); |
| 104 | if (el == null) |
| 105 | it.remove(); |
| 106 | } |
| 107 | } |
| 108 | |
| 109 | private void incrementIterationDepth() { |
| 110 | mIterationDepth++; |
| 111 | } |
| 112 | |
| 113 | private void decrementIterationDepthAndCompactIfNeeded() { |
| 114 | mIterationDepth--; |
| 115 | assert mIterationDepth >= 0; |
| 116 | if (mIterationDepth == 0) |
| 117 | compact(); |
| 118 | } |
| 119 | |
| 120 | private int getSize() { |
| 121 | return mObservers.size(); |
| 122 | } |
| 123 | |
| 124 | private E getObserverAt(int index) { |
| 125 | return mObservers.get(index); |
| 126 | } |
| 127 | |
| 128 | private class ObserverListIterator implements Iterator<E> { |
| 129 | private final int mListEndMarker; |
| 130 | private int mIndex = 0; |
| 131 | private boolean mIsExhausted = false; |
| 132 | |
| 133 | private ObserverListIterator() { |
| 134 | ObserverList.this.incrementIterationDepth(); |
| 135 | mListEndMarker = ObserverList.this.getSize(); |
| 136 | } |
| 137 | |
| 138 | @Override |
| 139 | public boolean hasNext() { |
| 140 | int lookupIndex = mIndex; |
| 141 | while (lookupIndex < mListEndMarker && |
| 142 | ObserverList.this.getObserverAt(lookupIndex) == null) |
| 143 | lookupIndex++; |
| 144 | if (lookupIndex < mListEndMarker) |
| 145 | return true; |
| 146 | |
| 147 | // We have reached the end of the list, allow for compaction. |
| 148 | compactListIfNeeded(); |
| 149 | return false; |
| 150 | } |
| 151 | |
| 152 | @Override |
| 153 | public E next() { |
| 154 | // Advance if the current element is null. |
| 155 | while (mIndex < mListEndMarker && ObserverList.this.getObserverAt(mIndex) == null) |
| 156 | mIndex++; |
| 157 | if (mIndex < mListEndMarker) |
| 158 | return ObserverList.this.getObserverAt(mIndex++); |
| 159 | |
| 160 | // We have reached the end of the list, allow for compaction. |
| 161 | compactListIfNeeded(); |
| 162 | throw new NoSuchElementException(); |
| 163 | } |
| 164 | |
| 165 | @Override |
| 166 | public void remove() { |
| 167 | throw new UnsupportedOperationException(); |
| 168 | } |
| 169 | |
| 170 | private void compactListIfNeeded() { |
| 171 | if (!mIsExhausted) { |
| 172 | mIsExhausted = true; |
| 173 | ObserverList.this.decrementIterationDepthAndCompactIfNeeded(); |
| 174 | } |
| 175 | } |
| 176 | } |
| 177 | } |