避免list的并发修改异常的几种方式

避免list的并发修改异常的几种方式

1、使用list的snapshot，遍历它的副本

使用如下：com.bumptech.glide.manager.ActivityFragmentLifecycle#onStart()

for (LifecycleListener lifecycleListener : Util.getSnapshot(lifecycleListeners)) {
  lifecycleListener.onStart();
}

具体实现如下： com.bumptech.glide.util.Util:

/**
 * Returns a copy of the given list that is safe to iterate over and perform actions that may
 * modify the original list.
 *
 * <p>See #303, #375, #322, #2262.
 */
@NonNull
@SuppressWarnings("UseBulkOperation")
public static <T> List<T> getSnapshot(@NonNull Collection<T> other) {
  // toArray creates a new ArrayList internally and does not guarantee that the values it contains
  // are non-null. Collections.addAll in ArrayList uses toArray internally and therefore also
  // doesn't guarantee that entries are non-null. WeakHashMap's iterator does avoid returning null
  // and is therefore safe to use. See #322, #2262.
  List<T> result = new ArrayList<>(other.size());
  for (T item : other) {
    if (item != null) {
      result.add(item);
    }
  }
  return result;
}

2、使用CopyOnWriteArrayList

具体可以参考给ViewTreeObserver添加OnGlobalLayoutListener：

// Non-recursive listeners use CopyOnWriteArray
// Any listener invoked from ViewRootImpl.performTraversals() should not be recursive
@UnsupportedAppUsage
private CopyOnWriteArray<OnGlobalLayoutListener> mOnGlobalLayoutListeners;


/**
 * Register a callback to be invoked when the global layout state or the visibility of views
 * within the view tree changes
 *
 * @param listener The callback to add
 *
 * @throws IllegalStateException If {@link #isAlive()} returns false
 */
public void addOnGlobalLayoutListener(OnGlobalLayoutListener listener) {
    checkIsAlive();


    if (mOnGlobalLayoutListeners == null) {
        mOnGlobalLayoutListeners = new CopyOnWriteArray<OnGlobalLayoutListener>();
    }


    mOnGlobalLayoutListeners.add(listener);
}

/**
 * Remove a previously installed global layout callback
 *
 * @param victim The callback to remove
 *
 * @throws IllegalStateException If {@link #isAlive()} returns false
 * 
 * @see #addOnGlobalLayoutListener(OnGlobalLayoutListener)
 */
public void removeOnGlobalLayoutListener(OnGlobalLayoutListener victim) {
    checkIsAlive();
    if (mOnGlobalLayoutListeners == null) {
        return;
    }
    mOnGlobalLayoutListeners.remove(victim);
}

/**
 * Notifies registered listeners that a global layout happened. This can be called
 * manually if you are forcing a layout on a View or a hierarchy of Views that are
 * not attached to a Window or in the GONE state.
 */
public final void dispatchOnGlobalLayout() {
    // NOTE: because of the use of CopyOnWriteArrayList, we *must* use an iterator to
    // perform the dispatching. The iterator is a safe guard against listeners that
    // could mutate the list by calling the various add/remove methods. This prevents
    // the array from being modified while we iterate it.
    final CopyOnWriteArray<OnGlobalLayoutListener> listeners = mOnGlobalLayoutListeners;
    if (listeners != null && listeners.size() > 0) {
        CopyOnWriteArray.Access<OnGlobalLayoutListener> access = listeners.start();
        try {
            int count = access.size();
            for (int i = 0; i < count; i++) {
                access.get(i).onGlobalLayout();
            }
        } finally {
            listeners.end();
        }
    }
}

/**addOnGlobalLayoutListener
 * Register a callback to be invoked when the global layout state or the visibility of views
 * within the view tree changes
 *
 * @param listener The callback to add
 *
 * @throws IllegalStateException If {@link #isAlive()} returns false
 */
public void addOnGlobalLayoutListener(OnGlobalLayoutListener listener) {
    checkIsAlive();


    if (mOnGlobalLayoutListeners == null) {
        mOnGlobalLayoutListeners = new CopyOnWriteArray<OnGlobalLayoutListener>();
    }


    mOnGlobalLayoutListeners.add(listener);
}

/**
 * Notifies registered listeners that a global layout happened. This can be called
 * manually if you are forcing a layout on a View or a hierarchy of Views that are
 * not attached to a Window or in the GONE state.
 */
public final void dispatchOnGlobalLayout() {
    // NOTE: because of the use of CopyOnWriteArrayList, we *must* use an iterator to
    // perform the dispatching. The iterator is a safe guard against listeners that
    // could mutate the list by calling the various add/remove methods. This prevents
    // the array from being modified while we iterate it.
    final CopyOnWriteArray<OnGlobalLayoutListener> listeners = mOnGlobalLayoutListeners;
    if (listeners != null && listeners.size() > 0) {
        CopyOnWriteArray.Access<OnGlobalLayoutListener> access = listeners.start();
        try {
            int count = access.size();
            for (int i = 0; i < count; i++) {
                access.get(i).onGlobalLayout();
            }
        } finally {
            listeners.end();
        }
    }
}