RecyclerView核心要点

RecyclerView是什么？

a flexible view for providing
a limited window into
a large data set

ListView的局限

• 只有纵向列表一种布局
• 没有支持动画的 API
• 接口设计和系统不一致
  • setOnItemClickListener()
  • setOnItemLongClickListener()
  • setSelection()
• 没有强制实现 ViewHolder
• 性能不如 RecyclerView

RecyclerView的优势

• 默认支持Linear，Grid，Staggered Grid三种布局
• 友好的ItemAnimator动画API
• 强制实现ViewHolder
• 解耦的架构设计
• 相比ListView更好的性能

LayoutManager支持的布局

RecyclerView的重要组件

RecyclerView Demo

ViewHolder究竟是什么？

• View holder 和 item view 是什么关系？一对一？一对多？多对多？
• View holder 解决的是什么问题？
• View holder 的 ListView item view 的复用有什么关系

没有实现view hodler的getView()

public class SimpleListViewAdapter extends BaseAdapter{
    @Override
    public View getView(int position, View convertView, ViewGroup parent){
        if(convertView == null){
            convertView = LayoutInflater.from(parent.getContext())
                .inflate(R.layout.list_view, parent, false);
        }
        ImageView avatar = convertView.findViewById(R.id.user_avatar);
        TextView name = convertView.findViewById(R.id.user_name);
        TextView title = convertView.findViewById(R.id.user_title);
        
        User user = getItem(position);
        Glide.with(parent.getContext()).load(user.avatarUrl()).into(avatar);
        name.setText(user.name);
        title.setText(user.title);
        return convertView;
    }
}

没有复用item view

实现了view holder的getView()

public class SimpleListViewAdapter extends BaseAdapter{
    @Override
    public View getView(int position, View convertView, ViewGroup parent){
        UserViewHolder holder;
        if(convertView == null){
            convertView = LayoutInflater.from(parent.getContext())
                .inflate(R.layout.list_view, parent, false);
            holder = new UserViewHolder(convertView);
            holder.avatar = convertView.findViewById(R.id.user_avatar);
            holder.name = convertView.findViewById(R.id.user_name);
            holder.title = convertView.findViewById(R.id.user_title);
            convertView.setTag(holder);
        } else {
            holder = (UserViewHolder)convertView.getTag();
        }
       
        User user = getItem(position);
        Glide.with(parent.getContext()).load(user.avatarUrl()).into(holder.avatar);
        holder.name.setText(user.name);
        holder.title.setText(user.title);
        return convertView;
    }
}

Item view和view holder一一对应

View holder最佳实践

static class UserViewHolder extends RecyclerView.ViewHolder{
    ImageView avatar;
    TextView name;
    UserVIewHolder(@Nonull View itemView){
        super(itemView);
        avatar = itemView.findViewById(R.id.avatar);
        name = itemView.findViewById(R.id.name);
    }
    
    void bindTo(User user){
        //bind data to UI
    }
}

//void onBindViewHolder(UserViewHolder holder, int postion){
//    holder.bindTo(userList.get(position));
//}

RecyclerView缓存机制

ListView缓存图示一

ListView缓存图示二

RecyclerView缓存图示一

RecyclerView缓存图示二

ViewCacheExtension Example

• 广告卡片
  • 每一页一共有4个广告
  • 这些广告短期内不会发生变化
• 每次滑入一个广告卡片，一般情况下都需要重新绑定
• Cache 只关心 position，不关心 view type
• RecycledViewPool 只关心 view type，都需要重新绑定
• 在 ViewCacheExtension 里保持4个广告Card缓存

注意：列表中 item/广告的 impression 统计

• ListView 通过 getView() 统计
• RecyclerView 通过 onBindViewHolder() 统计？可能错误！
• 通过 onViewAttachedToWindow() 统计

RecyclerView性能优化策略

在onBindViewHolder里设置点击监听？

onBindViewHodler里设置点击监听器会导致重复创建对象

public class SimpleAdapter extends RecyclerView.Adapter{
    @Override
    public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType){
        
    }
    @Override
    public void onBindViewHolder(ViewHolder holder, int position){
        holder.itemView.setOnClickListener(new View.OnClickListener(){
            @Override
            public void onClick(View v){
                //...
            }
        });
    }
}

改成在onCreateViewHolder里设置点击监听！

View-ViewHolder-View.OnClickListener 三者一一对应

public class SimpleAdapter extends RecyclerView.Adapter{
    @Override
    public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType){
        final SimpleViewHolder holder = new SimpleViewHolder();
        holder.itemView.setOnClickListener(new View.OnClickListener(){
            @Override
            public void onClick(View v){
                //...
            }
        });
    }
    @Override
    public void onBindViewHolder(ViewHolder holder, int position){
        
    }
}

使用LinearLayoutManager.setInitialPrefetchItemCount()

• 用户滑动到横向滑动的 item RecyclerView 的时候，由于需要创建更复杂的 RecyclerView 以及多个子 view，可能会导致页面卡顿
• 由于 RenderThread 的存在，RecyclerView 会进行 prefetch
• LinearLayoutManager.setInitialPrefetchItemCount(横向列表初次显示时可见的item个数)
  • 只有 LinearLayoutManager 有这个API
  • 只有嵌套在内部的 RecyclerView 才会生效

RecyclerView.setHasFixedSize()

//伪代码
void onContentsChanged(){
    if (mHasFixedSize){
        layoutChildren();
    } else {
        requestLayout();
    }
}

如果 Adapter 的数据变化不会导致 RecyclerView 的大小变化 –》RecyclerView.setHasFixedSize(true)

多个 RecyclerView 共用 RecycledViewPool

共用 RecycledViewPool 代码

RecyclerView.RecycledViewPool recycledViewPool = new RecyclerView.RecycledViewPool();
recyclerView1.setRecyclerViewPool(recycledViewPool);
recyclerView2.setRecyclerViewPool(recycledViewPool);
recyclerView3.setRecyclerViewPool(recycledViewPool);

DiffUtil

• DiffUtil is a utility class that can calculate the difference between two lists and output a list of update operations that converts the first list into the second one.
• 局部更新方法 notifyItemXXX() 不适用于所有情况
• notifyDataSetChange() 会导致整个布局重绘，重新绑定所有 ViewHolder，而且会失去可能的动画效果
• DiffUtil 适用于整个页面需要刷新，但是有部分数据可能相同的情况

public abstract static class Callback{
    public abstract int getOldListSize();
    public abstract int getNewListSize();
    public abstract boolean areItemsTheSame(int oldItemPosition, int newItemPosition);       public abstract boolean areContentsTheSame(int oldItemPosition, int newItemPosition);
    public Object getChangePayload(int oldItemPosition, int newItemPosition){
        return null;
    }
}

DiffUtil.Callback逻辑

public class UserDiffCallback extends DiffUtil.Callback{
    private List<User> oldList;
    private List<User> newList;
    public UserDiffCallback(List<User> oldList, List<User> newList){
        this.oldList = oldList;
        this.newList = newList;
    }
    @Override
    public int getOldListSize(){
        return oldList.size();
    }
    @Override
    public int getNewListSize(){
        return newList.size();
    }
    @Override
    public boolean areItemsTheSame(int oldItemPosition, int newItemPosition){
        return oldList.get(oldItemPosition).id == newList.get(newItemPosition).id;
    }
    @Override
    public boolean areContentsTheSame(int oldItemPosition, int newItemPosition){
        User oldUser = oldList.get(oldItemPosition);
        User newUser = newList.get(newItemPosition);
        return oldUser.id == newUser.id && oldUser.name.equals(newUser.name)
            && oldUser.profession.equals(newUser.profession);
    }
}

public class UserDiffCallback extends DiffUtil.Callback{
    private List<User> oldList;
    private List<User> newList;
    @Nullable
    @Override
    public Object getChangePayload(int oldItemPosition, int newItemPosition){
        User oldUser = oldList.get(oldItemPosition);
        User newUser = newList.get(newItemPosition);
        Bundle payload = new Bundle();
        if(oldUser.id != newUser.id){
            payload.putLong(User.KEY_ID, newUser.id);
        }
        if(!oldUser.name.equals(newUser.name)){
            payload.putString(User.KEY_NAME, newUser.name);
        }
        if(!oldUser.profession.equals(newUser.profession)){
            payload.putString(User.KEY_PROF, newUser.profession);
        }
        if(payload.size() == 0) return null;
        return payload;
    }
}

public class ShowcaseRVAdapter extends RecyclerView.Adapter<ShowcaseRvAdapter.UserViewHolder>{
    private List<User> userList;
    public ShowcaseRVAdapter(){
        userList = new ArrayList<>(UserRepo.USER_LIST);
    }
    
    public void swapData(List<User> newList, boolean diff){
        if(diff){
            DiffUtil.DiffResult diffResult = DiffUtil.calculateDiff(new UserDiffCallback(userList, newList), false);
            userList = newList;
            diffResult.dispatchUpdatesTo(this);
        } else {
            userList = newList;
            notifyDataSetChanged();
        }
    }
}

public class ShowcaseRVAdapter extends RecyclerView.Adapter<ShowcaseRvAdapter.UserViewHolder>{
    @Override
    public void onBindViewHolder(@NonNull ShowcaseRVAdapter.UserViewHolder holder, int position){
        User user = userList.get(position);
        holder.name.setText(user.name);
    }

    @Override
    public void onBindViewHolder(@NonNull UserViewHolder holder, int position, @NonNull List<Object> payloads){
        if(payloads.isEmpty()){
            onBindViewHolder(holder, position);
        } else {
            Bundle payload = (Bundle) payloads.get(0);
            for(String key: payload.keySet()){
                switch(key){
                    case User.KEY_NAME:
                        holder.name.setText(payload.getString(key));
                        break;
                }
            }
        }
    }
}

在列表很大的时候异步计算diff

• 使用 Thread/Handler 将 DiffResult 发送到主线程
• 使用 RxJava 将 calculateDiff 操作放到后台线程
• 使用 Google 提供的 AsyncListDiffer(Executor)/ListAdapter

  AsyncListDiffer / ListAdapter 代码示例

• https://developer.android.com/reference/androidx/recyclerview/widget/AsyncListDiffer
• https://developer.android.com/reference/androidx/recyclerview/widget/ListAdapter

为什么ItemDecoration可以绘制分割线？

Overlay Divider 代码

public class OverlayDivider extends RecyclerView.ItemDecoration{
    @Overrid
    public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state){
        if (mOrientation == VERTICAL_LSIT) {
            drawVertical(c, parent);
        } else {
            drawHorizontal(c, parent);
        }
    }
    
    protected void drawVertical(Canvas c, RecyclerView parent){}
    protected void drawHorizontal(Canvas c, RecyclerView parent){}
    //不需要覆写 getItemOffsets()
}

ItemDecoration 还可以做什么？

• Drawing dividers between items
• Highlights
  
• Visual grouping boundaries
  

RecyclerView 更多知识

• https://github.com/h6ah4i/android-advancedrecyclerview
• https://advancedrecyclerview.h6ah4i.com/