Android RecyclerView中的ViewHolder

摘要：在使用提供的组件以列表的格式显示数据时，使用过组件和组件。当我们自定义一个时，继承了的一个内部类可以看到，内部已经帮我们定义好了抽象类，当我们自定义时，需要定义好要继承的类。

在使用android提供的组件以列表的格式显示数据时，使用过ListView组件和RecyclerView组件。目前一般推荐使用RecyclerView，因为RecyclerView本身的缓存和效率比ListView高，且支持灵活的布局方式，所以会被大家采用。相信大家在使用ListView时，如果要显示的数据多，肯定多会想到优化Adaper的getView()方法，下面给出一个例子：

public class UsersAdapter extends ArrayAdapter {

    private static class ViewHolder {
        TextView name;
        TextView home;
    }

    public UsersAdapter(Context context, ArrayList users) {
       super(context, R.layout.item_user, users);
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
       User user = getItem(position);    
       ViewHolder viewHolder;
       if (convertView == null) {
          viewHolder = new ViewHolder();
          LayoutInflater inflater = LayoutInflater.from(getContext());
          convertView = inflater.inflate(R.layout.item_user, parent, false);
          viewHolder.name = (TextView) convertView.findViewById(R.id.tvName);
          viewHolder.home = (TextView)convertView.findViewById(R.id.tvHome);
          convertView.setTag(viewHolder);
       } else {
           viewHolder = (ViewHolder) convertView.getTag();
       }
       viewHolder.name.setText(user.name);
       viewHolder.home.setText(user.hometown);
       return convertView;
   }
}

在这边，我们在ListView需要用到的Adapter中定义了一个内部类ViewHolder，它存储了我们要加载的view的所有子view结构，如果这个view已经被加载过只是暂时被回收， 当需要再次展示的话我们就不需要重新加载整个view，也不需要通过findViewById()来寻找要加载的view的子view，可以直接找到这个view，将要展示的数据设置即可返回显示。

但是在RecyclerView中，我们并不需要做这么多，我们先看一个RecyclerView的简单使用步骤：

定义一个RecyclerView的布局文件以及要展示的item的布局文件

定义一个adapter

public class ClassAdapter extends RecyclerView.Adapter {

private List classVOs;

static class ClassViewHolder extends RecyclerView.ViewHolder {

    View classView;
    TextView className;

    public ClassViewHolder(View itemView) {
        super(itemView);
        classView = itemView;
        className = (TextView) itemView.findViewById(R.id.t_class_name);
    }
}

public ClassAdapter(List classVOs){
    this.classVOs = classVOs;
}

@Override
public ClassViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
    View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.fragement_class_item, null, false);
    final ClassViewHolder holder = new ClassViewHolder(view);
    holder.classView.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            int position = holder.getAdapterPosition();
            ClassVO classVO = classVOs.get(position);
            Toast.makeText(view.getContext(), "you click class "+classVO.getId(), Toast.LENGTH_SHORT).show();
        }
    });
    return holder;
}

@Override
public void onBindViewHolder(ClassViewHolder holder, int position) {
    ClassVO classVO = classVOs.get(position);
    holder.className.setText(classVO.getName());
}

@Override
public int getItemCount() {
    return classVOs.size();
}

}

设置RecyclerView的布局和adapter

recyclerView = (RecyclerView) classListView.findViewById(R.id.t_classList);
layoutManager = new LinearLayoutManager(this.getContext());
recyclerView.setLayoutManager(layoutManager);
if (classVOs!=null){
    adapter = new ClassAdapter(classVOs);
}
recyclerView.setAdapter(adapter);

既然我们要说的是RecyclerView中的ViewHolder，但是我们的使用步骤中并没有多带带提出ViewHolder，因为在我们上面说的使用步骤的第二步———“定义一个adapter”，就涉及到ViewHolder。
当我们自定义一个adapter时，继承了RecyclerView的一个内部类：

 /**
     * Base class for an Adapter
     *
     * 
Adapters provide a binding from an app-specific data set to views that are displayed
     * within a {@link RecyclerView}.
     */
    public static abstract class Adapter {
        private final AdapterDataObservable mObservable = new AdapterDataObservable();
        private boolean mHasStableIds = false;

        /**
         * Called when RecyclerView needs a new {@link ViewHolder} of the given type to represent
         * an item.
         * 

         * This new ViewHolder should be constructed with a new View that can represent the items
         * of the given type. You can either create a new View manually or inflate it from an XML
         * layout file.
         * 

         * The new ViewHolder will be used to display items of the adapter using
         * {@link #onBindViewHolder(ViewHolder, int, List)}. Since it will be re-used to display
         * different items in the data set, it is a good idea to cache references to sub views of
         * the View to avoid unnecessary {@link View#findViewById(int)} calls.
         *
         * @param parent The ViewGroup into which the new View will be added after it is bound to
         *               an adapter position.
         * @param viewType The view type of the new View.
         *
         * @return A new ViewHolder that holds a View of the given view type.
         * @see #getItemViewType(int)
         * @see #onBindViewHolder(ViewHolder, int)
         */
        public abstract VH onCreateViewHolder(ViewGroup parent, int viewType);

        /**
         * Called by RecyclerView to display the data at the specified position. This method should
         * update the contents of the {@link ViewHolder#itemView} to reflect the item at the given
         * position.
         * 

         * Note that unlike {@link android.widget.ListView}, RecyclerView will not call this method
         * again if the position of the item changes in the data set unless the item itself is
         * invalidated or the new position cannot be determined. For this reason, you should only
         * use the position parameter while acquiring the related data item inside
         * this method and should not keep a copy of it. If you need the position of an item later
         * on (e.g. in a click listener), use {@link ViewHolder#getAdapterPosition()} which will
         * have the updated adapter position.
         *
         * Override {@link #onBindViewHolder(ViewHolder, int, List)} instead if Adapter can
         * handle efficient partial bind.
         *
         * @param holder The ViewHolder which should be updated to represent the contents of the
         *        item at the given position in the data set.
         * @param position The position of the item within the adapter"s data set.
         */
        public abstract void onBindViewHolder(VH holder, int position);

        /**
         * Called by RecyclerView to display the data at the specified position. This method
         * should update the contents of the {@link ViewHolder#itemView} to reflect the item at
         * the given position.
         * 

         * Note that unlike {@link android.widget.ListView}, RecyclerView will not call this method
         * again if the position of the item changes in the data set unless the item itself is
         * invalidated or the new position cannot be determined. For this reason, you should only
         * use the position parameter while acquiring the related data item inside
         * this method and should not keep a copy of it. If you need the position of an item later
         * on (e.g. in a click listener), use {@link ViewHolder#getAdapterPosition()} which will
         * have the updated adapter position.
         * 

         * Partial bind vs full bind:
         * 

         * The payloads parameter is a merge list from {@link #notifyItemChanged(int, Object)} or
         * {@link #notifyItemRangeChanged(int, int, Object)}.  If the payloads list is not empty,
         * the ViewHolder is currently bound to old data and Adapter may run an efficient partial
         * update using the payload info.  If the payload is empty,  Adapter must run a full bind.
         * Adapter should not assume that the payload passed in notify methods will be received by
         * onBindViewHolder().  For example when the view is not attached to the screen, the
         * payload in notifyItemChange() will be simply dropped.
         *
         * @param holder The ViewHolder which should be updated to represent the contents of the
         *               item at the given position in the data set.
         * @param position The position of the item within the adapter"s data set.
         * @param payloads A non-null list of merged payloads. Can be empty list if requires full
         *                 update.
         */

可以看到，RecyclerView内部已经帮我们定义好了ViewHolder抽象类，当我们自定义Adapter时，需要定义好要继承ViewHolder的类(Adapter)。

ViewHolder的作用我们在上文介绍使使用ListView时已经说到，简而言之就是提高消息，下面我们看一看源码中对ViewHolder的介绍：

/**
     * A ViewHolder describes an item view and metadata about its place within the RecyclerView.
     *
     * 
{@link Adapter} implementations should subclass ViewHolder and add fields for caching
     * potentially expensive {@link View#findViewById(int)} results.
     *
     * 
While {@link LayoutParams} belong to the {@link LayoutManager},
     * {@link ViewHolder ViewHolders} belong to the adapter. Adapters should feel free to use
     * their own custom ViewHolder implementations to store data that makes binding view contents
     * easier. Implementations should assume that individual item views will hold strong references
     * to ViewHolder objects and that RecyclerView instances may hold
     * strong references to extra off-screen item views for caching purposes
     */
    public static abstract class ViewHolder {
        ......
    }

可以看出两点重要的地方：

Adapter implementations should subclass ViewHolder and add fields for caching potentially expensive {@link View#findViewById(int)} results(adapter应当拥有ViewHolder的子类，并且ViewHolder内部应当存储一些子view，避免时间代价很大的findViewById操作)

Adapters should feel free to use their own custom ViewHolder implementations to store data that makes binding view content easier

其RecyclerView内部定义的ViewHolder类包含很多复杂的属性，内部使用场景也有很多，而我们经常使用的也就是onCreateViewHolder()方法和onBindViewHolder()方法，onCreateViewHolder()方法在RecyclerView需要一个新类型。item的ViewHolder时调用来创建一个ViewHolder，而onBindViewHolder()方法则当RecyclerView需要在特定位置的item展示数据时调用。

参考文章：

http://www.jianshu.com/p/d7ec...

https://guides.codepath.com/a...