Android DiffUtil

　　Android 的recyclerview-v7:24.2.0 发布后多了个DiffUtil工具类，这个工具类能够大大解放了Android开发者的一个苦恼：RecyclerView局部刷新和重新刷新时实际只改变了部分数据。

　　DiffUtil能够计算两个列表之间的差值，并计算出旧列表变换到新列表的过程（DiffResult），DiffResult可以直接应用到RecyclerView的Adapter中，DiffResult会使用Adapter的notifyItemRangeChanged等方法来更新RecyclerView。

　　DiffUtil使用了Eugene W.Myers's difference algorithm 去计算从旧集合到新集合的最小更新序列，Myers 算法并没有处理集合中的子项偏移情况，所以如果选择了要计算偏移情况的话，需要进行第二次的运算来检查子项偏移，所以会产生更多的计算时间成本。当集合的数据量较大时，Myers算法的处理时间会较长，所以不要在集合数据量大时在主线程进行Diff运算，应该放在后台线程中进行运算，将运算结果（DiffResult）在主线程进行更新RecyclerView。需要值得注意的是，子项偏移的运算会比较明显得增加Diff时间，所以在集合默认是排序情况下，关闭该功能。

　　DiffUtil的Demo可以在网络上其他地方找到，这里不再赘述，只简单提下大致使用方法。

1、calculateDiff是最核心的方法，Callback并不是我们常见的回调，更准确点说应该是一个策略

DiffUtil.DiffResult diffResult = DiffUtil.calculateDiff(DiffUtil.Callback, boolean);

2、DiffUtil.Callback可实现的方法，DiffUtil会对每个position调用对应的方法，我们只需要在Callback中告诉DiffUtil，在哪个位置中，如何判别两个列表在这个位置是否是同一个子项，如果是，有没有局部变化，如果有局部变化，都变了哪里？DiffUtil只需要知道这些，就可以计算出DiffResult

/**
     * A Callback class used by DiffUtil while calculating the diff between two lists.
     */
    public abstract static class Callback {
        /**
         * Returns the size of the old list.
         *
         * @return The size of the old list.
         */
        public abstract int getOldListSize();

        /**
         * Returns the size of the new list.
         *
         * @return The size of the new list.
         */
        public abstract int getNewListSize();

        /**
         * Called by the DiffUtil to decide whether two object represent the same Item.
         * <p>
         * For example, if your items have unique ids, this method should check their id equality.
         *
         * @param oldItemPosition The position of the item in the old list
         * @param newItemPosition The position of the item in the new list
         * @return True if the two items represent the same object or false if they are different.
         */
        public abstract boolean areItemsTheSame(int oldItemPosition, int newItemPosition);

        /**
         * Called by the DiffUtil when it wants to check whether two items have the same data.
         * DiffUtil uses this information to detect if the contents of an item has changed.
         * <p>
         * DiffUtil uses this method to check equality instead of {@link Object#equals(Object)}
         * so that you can change its behavior depending on your UI.
         * For example, if you are using DiffUtil with a
         * {@link android.support.v7.widget.RecyclerView.Adapter RecyclerView.Adapter}, you should
         * return whether the items' visual representations are the same.
         * <p>
         * This method is called only if {@link #areItemsTheSame(int, int)} returns
         * {@code true} for these items.
         *
         * @param oldItemPosition The position of the item in the old list
         * @param newItemPosition The position of the item in the new list which replaces the
         *                        oldItem
         * @return True if the contents of the items are the same or false if they are different.
         */
        public abstract boolean areContentsTheSame(int oldItemPosition, int newItemPosition);

        /**
         * When {@link #areItemsTheSame(int, int)} returns {@code true} for two items and
         * {@link #areContentsTheSame(int, int)} returns false for them, DiffUtil
         * calls this method to get a payload about the change.
         * <p>
         * For example, if you are using DiffUtil with {@link RecyclerView}, you can return the
         * particular field that changed in the item and your
         * {@link android.support.v7.widget.RecyclerView.ItemAnimator ItemAnimator} can use that
         * information to run the correct animation.
         * <p>
         * Default implementation returns {@code null}.
         *
         * @param oldItemPosition The position of the item in the old list
         * @param newItemPosition The position of the item in the new list
         *
         * @return A payload object that represents the change between the two items.
         */
        @Nullable
        public Object getChangePayload(int oldItemPosition, int newItemPosition) {
            return null;
        }
    }

　3、DiffResult应用到RecyclerView的Adapter.

 DiffResult.dispatchUpdatesTo(RecyclerView.Adapter);

　4、关于Adapter以前的一个知识点，payloads，可能有些朋友还不知道Adapter还有一个onBindViewHolder重载方法，比常见的onBindViewHolder多一个参数：List<Object> payloads。当DiffResult中有某一项出现两个列表实际上在该位置是同一个子项，但是发生了局部变化，则调用该重载方法时带上payloads，payloads中的对象可以在Callback的getChangePayload给定。payloads不会为null，所以检查payloads是否为空，为空，直接调用默认的onBindViewHolder即可。

public void onBindViewHolder(StudentViewHolder holder, int position, List<Object> payloads) {}