摘要：是的計(jì)算結(jié)果對(duì)象，通過(guò)來(lái)進(jìn)行更新。在線程中調(diào)用，而后的。使用下面通過(guò)兩種不同的改變條目來(lái)介紹的使用。改變第三個(gè)位置的對(duì)象將新數(shù)據(jù)給更新條目后的效果為由圖可知，第四個(gè)位置的條目顯示變?yōu)?。示例代碼在目錄下參考文章使用高效更新詳解帶來(lái)的新工具類(lèi)

DiffUtil是recyclerview support library v7 24.2.0版本中新增的類(lèi)，根據(jù)Google官方文檔的介紹，DiffUtil的作用是比較兩個(gè)數(shù)據(jù)列表并能計(jì)算出一系列將舊數(shù)據(jù)表轉(zhuǎn)換成新數(shù)據(jù)表的操作。這個(gè)概念比較抽象，換一種方式理解，DiffUtil是一個(gè)工具類(lèi)，當(dāng)你的RecyclerView需要更新數(shù)據(jù)時(shí)，將新舊數(shù)據(jù)集傳給它，它就能快速告知adapter有哪些數(shù)據(jù)需要更新。

那么相比直接調(diào)用adapter.notifyDataSetChange()方法，使用DiffUtil有什么優(yōu)勢(shì)呢？它能在收到數(shù)據(jù)集后，提高UI更新的效率，而且你也不需要自己對(duì)新老數(shù)據(jù)集進(jìn)行比較了。

顧名思義，凡是數(shù)據(jù)集的比較DiffUtil都能做，所以用處并不止于更新RecyclerView。DiffUtil也提供了回調(diào)讓你可以進(jìn)行其他操作。本文只介紹使用DiffUtil更新RecyclerView。

在使用DiffUtil前我們先簡(jiǎn)單看看DiffUtil的特性。DiffUtil使用Eugene W. Myers的Difference算法來(lái)計(jì)算出將一個(gè)數(shù)據(jù)集轉(zhuǎn)換為另一個(gè)的最小更新量，也就是用最簡(jiǎn)單的方式將一個(gè)數(shù)據(jù)集轉(zhuǎn)換為另一個(gè)。除此之外，DiffUtil還可以識(shí)別一項(xiàng)數(shù)據(jù)在數(shù)據(jù)集中的移動(dòng)。Eugene的算法對(duì)控件進(jìn)行了優(yōu)化，在查找兩個(gè)數(shù)據(jù)集間最少加減操作時(shí)的空間復(fù)雜度為O(N)，時(shí)間復(fù)雜度為O(N+D^2)。而如果添加了對(duì)數(shù)據(jù)條目移動(dòng)的識(shí)別，復(fù)雜度就會(huì)提高到O(N^2)。所以如果數(shù)據(jù)集中數(shù)據(jù)不存在移位情況，你可以關(guān)閉移動(dòng)識(shí)別功能來(lái)提高性能。當(dāng)數(shù)據(jù)集較大時(shí)，你應(yīng)該在后臺(tái)線程計(jì)算數(shù)據(jù)集的更新。

DiffUtil.Callback：這是最核心的類(lèi),你可以將它理解成比較新老數(shù)據(jù)集時(shí)的規(guī)則。

DiffUtil：通過(guò)靜態(tài)方法DiffUtil.calculateDiff(DiffUtil.Callback)來(lái)計(jì)算數(shù)據(jù)集的更新。

DiffResult：是DiffUtil的計(jì)算結(jié)果對(duì)象，通過(guò)DiffResult.dispatchUpdatesTo(RecyclerView.Adapter)來(lái)進(jìn)行更新。

代碼模式為

DiffUtil.DiffResult diffResult = DiffUtil.calculateDiff(new DiffCallBack(mDatas, newDatas), true);
mAdapter.setDatas(newDatas);
diffResult.dispatchUpdatesTo(mAdapter);

dispatchUpdatesTo()方法它會(huì)自動(dòng)計(jì)算新老數(shù)據(jù)集的差異，并根據(jù)差異情況，自動(dòng)調(diào)用以下四個(gè)方法

adapter.notifyItemRangeInserted(position, count);
adapter.notifyItemRangeRemoved(position, count);
adapter.notifyItemMoved(fromPosition, toPosition);
adapter.notifyItemRangeChanged(position, count, payload);

    public abstract static class Callback {
        /**
         * 返回舊數(shù)據(jù)集的大小
         *
         * @return The size of the old list.
         */
        public abstract int getOldListSize();

        /**
         * 返回新數(shù)據(jù)集的大小
         *
         * @return The size of the new list.
         */
        public abstract int getNewListSize();

        /**
         * 比較兩個(gè)Item對(duì)象是否是同一個(gè)對(duì)象
         *
         * @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);

        /**
         * 比較兩個(gè)Item對(duì)象的內(nèi)容是否相同
         * 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.
         * 

         * 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.
         * 

         * 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);

        /**
         * areItemsTheSame()返回true而areContentsTheSame()返回false時(shí)調(diào)用，也就是說(shuō)兩個(gè)對(duì)象代表的數(shù)據(jù)是一條，但是內(nèi)容更新了。
         * 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.
         * 

         * 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.
         * 

         * 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;
        }
    }

自定義類(lèi)繼承DiffUtil.Callback，通過(guò)重寫(xiě)特定方法給出數(shù)據(jù)比較邏輯。

調(diào)用DiffUtil.calculateDiff(DiffUtil.Callback callback,boolean detectMove)來(lái)計(jì)算更新，得到DiffResult對(duì)象。第二個(gè)參數(shù)可省，意為是否探測(cè)數(shù)據(jù)的移動(dòng)，是否關(guān)閉需要根據(jù)數(shù)據(jù)集情況來(lái)權(quán)衡。當(dāng)數(shù)據(jù)集很大時(shí)，此操作可能耗時(shí)較長(zhǎng)，需要異步計(jì)算。

在UI線程中調(diào)用DiffResult.dispatchUpdatesTo(RecyclerView.Adapter)，而后Adapter的onBindViewHolder(RecyclerView.ViewHolder holder, int position, Listpayloads)。注意這個(gè)方法比必須覆蓋的onBindViewHolder(RecyclerView.ViewHolder holder, int position)方法多一個(gè)參數(shù)payloads，而里面存儲(chǔ)了數(shù)據(jù)的更新。

新建一個(gè)Bean為Item：

package com.michael.materialdesign.bean;

/**
 * Created by liuguoquan on 2016/10/18.
 */

public class Item {

  public int id = 0;
  public String name;

  public Item(int id, String name) {
    this.id = id;
    this.name = name;
  }

  public int getId() {
    return id;
  }

  public void setId(int id) {
    this.id = id;
  }

  public String getName() {
    return name;
  }

  public void setName(String name) {
    this.name = name;
  }
}

新建Adapter

public class DiffUtilAdapter extends RecyclerView.Adapter {

  private Context mContext;
  private List mDatas;

  public DiffUtilAdapter(Context context, List datas) {
    this.mContext = context;
    this.mDatas = datas;
  }

  public void setDatas(List mDatas) {
    this.mDatas = mDatas;
  }

  @Override public DiffItemHolder onCreateViewHolder(ViewGroup parent, int viewType) {
    View view = LayoutInflater.from(mContext).inflate(R.layout.item_diff_util, parent, false);
    return new DiffItemHolder(view);
  }

  @Override public void onBindViewHolder(DiffItemHolder holder, int position) {
    Item info = mDatas.get(position);
    holder.mInfo.setText(info.getName());
    Log.d("lgq","onBindViewHolder");
  }

    //payloads就是DiffUtil.Callback中的getChangePayload方法返回的數(shù)據(jù)集
  @Override
  public void onBindViewHolder(DiffItemHolder holder, int position, List