RecyclerView扩展知识: DiffUtil

1.简介

DiffUtil是support-v7:24.2.0中的新工具类，它用来比较两个数据集，寻找出旧数据集-》新数据集的最小变化量，定向刷新列表。

它最大的用处就是在RecyclerView刷新时，不再使用mAdapter.notifyDataSetChanged()全部刷新，全部刷新的缺点:

1. 不会触发RecyclerView的动画（删除、新增、位移、change动画）
2. 性能较低，毕竟是刷新了一遍整个RecyclerView , 极端情况下：新老数据集一模一样，效率是最低的

使用DiffUtil后，改为如下代码：

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

它会自动计算新老数据集的差异，并根据差异情况，自动调用以下四个方法:

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

2. 例子

Bean:

public class TestBean {
    
    private int id;
    private String name;

    public TestBean(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;
    }
}

DiffUtil.Callback: 用来比较数据集的差异

public class MyDiffUtilCallback extends DiffUtil.Callback{

    private List mOldItems;
    private List mNewItems;

    public void setItems(@NonNull final List oldItems, @NonNull final List newItems) {
        mOldItems = oldItems;
        mNewItems = newItems;
    }

    @Override
    public int getOldListSize() {
        return mOldItems == null ? 0 : mOldItems.size();
    }

    @Override
    public int getNewListSize() {
        return mNewItems == null ? 0 : mNewItems.size();
    }

    /**
     * 是否是同一个对象
     */
    @Override
    public boolean areItemsTheSame(final int oldItemPosition, final int newItemPosition) {
        if (mOldItems.get(oldItemPosition) == null || mNewItems.get(newItemPosition) == null){
            return false;
        }
        return mOldItems.get(oldItemPosition).getId() == mNewItems.get(newItemPosition).getId();
    }
    
    /**
     * 是否是相同内容
     */
    @Override
    public boolean areContentsTheSame(final int oldItemPosition, final int newItemPosition) {
        return mOldItems.get(oldItemPosition).getName().equals(mNewItems.get(newItemPosition).getName());
    }

    /**
     * areItemsTheSame()返回true而areContentsTheSame()返回false时调用,也就是说两个对象代表的数据是一条，但是内容更新了。
     */
    @Nullable
    @Override
    public Object getChangePayload(final int oldItemPosition, final int newItemPosition) {
        TestBean oldBean = mOldItems.get(oldItemPosition);
        TestBean newBean = mNewItems.get(newItemPosition);

        //这里就不用比较核心字段了,一定相等
        Bundle payload = new Bundle();
       
        if (!oldBean.getName().equals(newBean.getName())) {
            payload.putString("KEY_NAME", newBean.getName());
        }

        if (payload.size() == 0){
            //如果没有变化 就传空
            return null;
        }
        return payload;
    }
   
}

DiffUtilAdapter:

public class DiffUtilAdapter extends RecyclerView.Adapter {
   
    private List mList = new ArrayList();
    private LayoutInflater mInflater;
    private MyDiffUtilCallback mDiffCallback;
    
    public DiffUtilAdapter(Context mContext) {
        mDiffCallback = new MyDiffUtilCallback();
        mInflater = LayoutInflater.from(mContext);
    }

    public void setData(TestBean mData){
        mList.add(mData);
        notifyItemRangeInserted(getItemCount(), 1);
    }

    public void setData(List mData){
        mDiffCallback.setItems(mList, mData);
        DiffUtil.DiffResult diffResult = DiffUtil.calculateDiff(mDiffCallback);
        diffResult.dispatchUpdatesTo(this);
        mList.clear();
        mList.addAll(mData);
    }

    public void removeData(int index){
        mList.remove(index);
        notifyItemRemoved(index);
        if (index != mList.size()) {
            notifyItemRangeChanged(index, mList.size() - index);
        }
    }
    
    public void clear(){
        mList.clear();
        notifyDataSetChanged();
    }
    
    @Override
    @NonNull
    public DiffUtilAdapter.ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        return new ViewHolder(mInflater.inflate(R.layout.item_test, parent, false));
    }

    @Override
    public void onBindViewHolder(@NonNull ViewHolder holder, int position, @NonNull List