关于RecyclerView中Viewholder和View的缓存机制的探究

这个题目放在草稿箱里面许久了，一直没有动力提笔。趁现在公司人还没有来齐，工作量还不是很大，就挤出来时间来把它完善了。

 

我们知道，RecyclerView是经典的ListView的进化与升华，它比ListView更加灵活，但也因此引入了一定的复杂性。最新的v7支持包新添加了RecyclerView。

我们知道，ListView通过使用ViewHolder来提升性能。ViewHolder通过保存item中使用到的控件的引用来减少findViewById的调用，以此使ListView滑动得更加顺畅。但这种模式即使不使用也无妨。

对于还不是很熟悉RecyclerView的程序员，可以先从这两篇文章里面了解一下它的基本用法：

Android L新控件RecyclerView简介

RecyclerView和CardView简介

这里对RecyclerView的基本使用方式，就不再赘述，只讨论RecyclerView中的ViewHolder的缓存机制。其实它的缓存机制大致原理跟ListView是相似的，只是在具体的实现细节上面有些差异。对于这些RecyclerView的缓存机制，Android L新控件RecyclerView简介这篇文章的结尾进行了总结性的概括，这里讨论的内容要远比这篇简介的内容要复杂和多。

首先，我定义了三个不同的Adapter，里面分别对应了三种情况：item类型只有一种；item类型有两种；item类型有三种。里面均重写了onCreateViewHolder方法，用于记录生成的ViewHolder或者View的数目。内容可以从Log里面查询得到。三个Adapter的细节分别是：

/**   
 * @Title: MyRVAdapter.java 
 * @Package com.example.recyclerviewtest.adapter 
 * @Description: TODO
 * @author SilentKnight || happychinapc[at]gmail[dot]com   
 * @date 2015 2015年1月22日 下午4:54:25 
 * @version V1.0.0   
 */
package com.example.recyclerviewtest.adapter;

import java.util.List;

import android.content.Context;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.view.View.OnClickListener;
import android.widget.TextView;
import android.widget.Toast;

import com.example.recyclerviewtest.R;

/**
 * @ClassName: MyRVAdapter
 * @Description: TODO
 * @author SilentKnight || happychinapc@gmail.com
 * @date 2015年1月22日 下午4:54:25
 * 
 */
public class MyRVAdapter extends RecyclerView.Adapter<MyRVAdapter.ViewHolder> {
    private static int COUNT_CACHE_VIEW = 0;
    private static final String ADAPTER_TAG = MyRVAdapter.class.getSimpleName();
    private List<String> dataSet;
    private Context context;

    public MyRVAdapter(Context context, List<String> dataSet) {
        this.context = context;
        this.dataSet = dataSet;
    }

    /*
     * (non-avadoc) <p>Title: getItemId</p> <p>Description: </p>
     * 
     * @params @param position
     * 
     * @params @return
     * 
     * @overrided @see
     * android.support.v7.widget.RecyclerView.Adapter#getItemId(int)
     */
    @Override
    public long getItemId(int position) {
        // TODO Auto-generated method stub
        return position;
    }

    /*
     * (non-avadoc) <p>Title: getItemCount</p> <p>Description: </p>
     * 
     * @params @return
     * 
     * @overrided @see
     * android.support.v7.widget.RecyclerView.Adapter#getItemCount()
     */
    @Override
    public int getItemCount() {
        // TODO Auto-generated method stub
        return dataSet.size();
    }

    /*
     * (non-avadoc) <p>Title: onBindViewHolder</p> <p>Description: </p>
     * 
     * @params @param arg0
     * 
     * @params @param arg1
     * 
     * @overrided @see
     * android.support.v7.widget.RecyclerView.Adapter#onBindViewHolder
     * (android.support.v7.widget.RecyclerView.ViewHolder, int)
     */
    @Override
    public void onBindViewHolder(ViewHolder holder, final int arg1) {
        // TODO Auto-generated method stub
        holder.tv.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                Toast.makeText(context, "You clickd TextView at index of "+arg1,
                        Toast.LENGTH_SHORT).show();
            }
        });
        holder.tv.setText(dataSet.get(arg1));
    }

    /*
     * (non-avadoc) <p>Title: onCreateViewHolder</p> <p>Description: </p>
     * 
     * @params @param arg0
     * 
     * @params @param arg1
     * 
     * @params @return
     * 
     * @overrided @see
     * android.support.v7.widget.RecyclerView.Adapter#onCreateViewHolder
     * (android.view.ViewGroup, int)
     */

    @Override
    public ViewHolder onCreateViewHolder(ViewGroup viewGroup, int arg1) {
        // TODO Auto-generated method stub
        Log.i(ADAPTER_TAG, "itemTV---" + ++COUNT_CACHE_VIEW);
        View itemLayout = LayoutInflater.from(viewGroup.getContext()).inflate(
                R.layout.recycler_view_item_layout_tv, null);
        return new ViewHolder(itemLayout);
    }

    public static class ViewHolder extends RecyclerView.ViewHolder {
        public TextView tv;

        /**
         * <p>
         * Title: MainActivity.java
         * </p>
         * <p>
         * Description:
         * </p>
         * 
         * @param @param itemView
         */
        public ViewHolder(View itemView) {
            super(itemView);
            // TODO Auto-generated constructor stub
            tv = (TextView) itemView.findViewById(R.id.rv_item_tv);
        }
    }
}

/**   
 * @Title: MyRVAdapter2.java 
 * @Package com.example.recyclerviewtest.adapter 
 * @Description: TODO
 * @author SilentKnight || happychinapc[at]gmail[dot]com   
 * @date 2015 2015年1月22日 下午4:55:31 
 * @version V1.0.0   
 */
package com.example.recyclerviewtest.adapter;

import java.util.List;

import android.content.Context;
import android.content.Intent;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.view.View.OnClickListener;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast;

import com.example.recyclerviewtest.GridViewHorizontalTest;
import com.example.recyclerviewtest.GridViewVerticalTest;
import com.example.recyclerviewtest.R;

/**
 * @ClassName: MyRVAdapter2
 * @Description: TODO
 * @author SilentKnight || happychinapc@gmail.com
 * @date 2015年1月22日 下午4:55:31
 * 
 */
public class MyRVAdapter2 extends RecyclerView.Adapter<MyRVAdapter2.ViewHolder> {
    private static int COUNT_CACHE_VIEW_1 = 0;
    private static int COUNT_CACHE_VIEW_2 = 0;
    private static final String ADAPTER_TAG = MyRVAdapter2.class
            .getSimpleName();
    private static final int TYPE_TV = 0x000;
    private static final int TYPE_IV = 0x0001;
    private List<String> dataSet;
    private Context context;

    public MyRVAdapter2(Context context, List<String> dataSet) {
        this.context = context;
        this.dataSet = dataSet;
    }

    /*
     * (non-avadoc) <p>Title: getItemId</p> <p>Description: </p>
     * 
     * @params @param position
     * 
     * @params @return
     * 
     * @overrided @see
     * android.support.v7.widget.RecyclerView.Adapter#getItemId(int)
     */
    @Override
    public long getItemId(int position) {
        // TODO Auto-generated method stub
        return position;
    }

    /*
     * (non-avadoc) <p>Title: getItemViewType</p> <p>Description: </p>
     * 
     * @params @param position
     * 
     * @params @return
     * 
     * @overrided @see
     * android.support.v7.widget.RecyclerView.Adapter#getItemViewType(int)
     */
    @Override
    public int getItemViewType(int position) {
        // TODO Auto-generated method stub
        if (position % 2 == 0) {
            return TYPE_TV;
        } else {
            return TYPE_IV;
        }
    }

    /*
     * (non-avadoc) <p>Title: getItemCount</p> <p>Description: </p>
     * 
     * @params @return
     * 
     * @overrided @see
     * android.support.v7.widget.RecyclerView.Adapter#getItemCount()
     */
    @Override
    public int getItemCount() {
        // TODO Auto-generated method stub
        return dataSet.size();
    }

    /*
     * (non-avadoc) <p>Title: onBindViewHolder</p> <p>Description: </p>
     * 
     * @params @param arg0
     * 
     * @params @param arg1
     * 
     * @overrided @see
     * android.support.v7.widget.RecyclerView.Adapter#onBindViewHolder
     * (android.support.v7.widget.RecyclerView.ViewHolder, int)
     */
    @Override
    public void onBindViewHolder(ViewHolder holder, int arg1) {
        // TODO Auto-generated method stub
        if (getItemViewType(arg1) == TYPE_TV) {
            holder.tv.setOnClickListener(new OnClickListener() {

                @Override
                public void onClick(View v) {
                    // TODO Auto-generated method stub
                    context.startActivity(new Intent(context,
                            GridViewHorizontalTest.class));
                }
            });
            holder.tv.setText(dataSet.get(arg1));
        } else {
            holder.iv.setOnClickListener(new OnClickListener() {

                @Override
                public void onClick(View v) {
                    // TODO Auto-generated method stub
                    context.startActivity(new Intent(context,
                            GridViewVerticalTest.class));
                }
            });
        }
    }

    /*
     * (non-avadoc) <p>Title: onCreateViewHolder</p> <p>Description: </p>
     * 
     * @params @param arg0
     * 
     * @params @param arg1
     * 
     * @params @return
     * 
     * @overrided @see
     * android.support.v7.widget.RecyclerView.Adapter#onCreateViewHolder
     * (android.view.ViewGroup, int)
     */

    @Override
    public ViewHolder onCreateViewHolder(ViewGroup viewGroup, int arg1) {
        // TODO Auto-generated method stub
        if (getItemViewType(arg1) == TYPE_TV) {
            Log.i(ADAPTER_TAG, "itemTV---" + ++COUNT_CACHE_VIEW_1);
            View itemLayout = LayoutInflater.from(viewGroup.getContext())
                    .inflate(R.layout.recycler_view_item_layout_tv, null);
            return new ViewHolder(itemLayout, TYPE_TV);
        } else {
            Log.i(ADAPTER_TAG, "itemIV---" + ++COUNT_CACHE_VIEW_2);
            View itemLayout = LayoutInflater.from(viewGroup.getContext())
                    .inflate(R.layout.recycler_view_item_layout_iv, null);
            return new ViewHolder(itemLayout, TYPE_IV);
        }
    }

    public static class ViewHolder extends RecyclerView.ViewHolder {
        public TextView tv;
        public ImageView iv;

        /**
         * <p>
         * Title: MainActivity.java
         * </p>
         * <p>
         * Description:
         * </p>
         * 
         * @param @param itemView
         */
        public ViewHolder(View itemView, int itemType) {
            super(itemView);
            // TODO Auto-generated constructor stub
            if (itemType == TYPE_TV) {
                tv = (TextView) itemView.findViewById(R.id.rv_item_tv);
            } else {
                iv = (ImageView) itemView.findViewById(R.id.rv_item_iv);
            }
        }
    }
}

/**   
 * @Title: MyRVAdapter3.java 
 * @Package com.example.recyclerviewtest.adapter 
 * @Description: TODO
 * @author SilentKnight || happychinapc[at]gmail[dot]com   
 * @date 2015 2015年1月22日 下午4:56:33 
 * @version V1.0.0   
 */
package com.example.recyclerviewtest.adapter;

import java.util.List;

import android.content.Context;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.view.View.OnClickListener;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast;

import com.example.recyclerviewtest.R;

/**
 * @ClassName: MyRVAdapter3
 * @Description: TODO
 * @author SilentKnight || happychinapc@gmail.com
 * @date 2015年1月22日 下午4:56:33
 * 
 */
public class MyRVAdapter3 extends RecyclerView.Adapter<MyRVAdapter3.ViewHolder> {
    private static int COUNT_CACHE_VIEW_1 = 0;
    private static int COUNT_CACHE_VIEW_2 = 0;
    private static int COUNT_CACHE_VIEW_3 = 0;
    private static final String ADAPTER_TAG = MyRVAdapter3.class
            .getSimpleName();
    private static final int TYPE_TV = 0x000;
    private static final int TYPE_IV = 0x0001;
    private static final int TYPE_TV_2 = 0x0002;
    private List<String> dataSet;
    private Context context;

    public MyRVAdapter3(Context context, List<String> dataSet) {
        this.context = context;
        this.dataSet = dataSet;
    }

    /*
     * (non-avadoc) <p>Title: getItemId</p> <p>Description: </p>
     * 
     * @params @param position
     * 
     * @params @return
     * 
     * @overrided @see
     * android.support.v7.widget.RecyclerView.Adapter#getItemId(int)
     */
    @Override
    public long getItemId(int position) {
        // TODO Auto-generated method stub
        return position;
    }

    /*
     * (non-avadoc) <p>Title: getItemViewType</p> <p>Description: </p>
     * 
     * @params @param position
     * 
     * @params @return
     * 
     * @overrided @see
     * android.support.v7.widget.RecyclerView.Adapter#getItemViewType(int)
     */
    @Override
    public int getItemViewType(int position) {
        // TODO Auto-generated method stub
        if (position % 3 == 0) {
            return TYPE_TV;
        } else if (position % 3 == 1) {
            return TYPE_IV;
        } else {
            return TYPE_TV_2;
        }
    }

    /*
     * (non-avadoc) <p>Title: getItemCount</p> <p>Description: </p>
     * 
     * @params @return
     * 
     * @overrided @see
     * android.support.v7.widget.RecyclerView.Adapter#getItemCount()
     */
    @Override
    public int getItemCount() {
        // TODO Auto-generated method stub
        return dataSet.size();
    }

    /*
     * (non-avadoc) <p>Title: onBindViewHolder</p> <p>Description: </p>
     * 
     * @params @param arg0
     * 
     * @params @param arg1
     * 
     * @overrided @see
     * android.support.v7.widget.RecyclerView.Adapter#onBindViewHolder
     * (android.support.v7.widget.RecyclerView.ViewHolder, int)
     */
    @Override
    public void onBindViewHolder(ViewHolder holder, int arg1) {
        // TODO Auto-generated method stub
        if (getItemViewType(arg1) == TYPE_TV) {
            holder.tv.setOnClickListener(new OnClickListener() {

                @Override
                public void onClick(View v) {
                    // TODO Auto-generated method stub
                    Toast.makeText(context, "You clickd TextView",
                            Toast.LENGTH_SHORT).show();
                }
            });
            holder.tv.setText(dataSet.get(arg1));
        } else if (getItemViewType(arg1) == TYPE_IV) {
            holder.iv.setOnClickListener(new OnClickListener() {

                @Override
                public void onClick(View v) {
                    // TODO Auto-generated method stub

                    Toast.makeText(context, "You clickd ImageView",
                            Toast.LENGTH_SHORT).show();
                }
            });
        } else {

        }
    }

    /*
     * (non-avadoc) <p>Title: onCreateViewHolder</p> <p>Description: </p>
     * 
     * @params @param arg0
     * 
     * @params @param arg1
     * 
     * @params @return
     * 
     * @overrided @see
     * android.support.v7.widget.RecyclerView.Adapter#onCreateViewHolder
     * (android.view.ViewGroup, int)
     */

    @Override
    public ViewHolder onCreateViewHolder(ViewGroup viewGroup, int arg1) {
        // TODO Auto-generated method stub
        if (getItemViewType(arg1) == TYPE_TV) {
            Log.i(ADAPTER_TAG, "itemTV---" + ++COUNT_CACHE_VIEW_1);
            View itemLayout = LayoutInflater.from(viewGroup.getContext())
                    .inflate(R.layout.recycler_view_item_layout_tv, null);
            return new ViewHolder(itemLayout, TYPE_TV);
        } else if (getItemViewType(arg1) == TYPE_IV) {
            Log.i(ADAPTER_TAG, "itemIV---" + ++COUNT_CACHE_VIEW_2);
            View itemLayout = LayoutInflater.from(viewGroup.getContext())
                    .inflate(R.layout.recycler_view_item_layout_iv, null);
            return new ViewHolder(itemLayout, TYPE_IV);
        } else {
            Log.i(ADAPTER_TAG, "itemTV2---" + ++COUNT_CACHE_VIEW_3);
            View itemLayout = LayoutInflater.from(viewGroup.getContext())
                    .inflate(R.layout.recycler_view_item_layout_tv_2, null);
            return new ViewHolder(itemLayout, TYPE_TV_2);
        }
    }

    public static class ViewHolder extends RecyclerView.ViewHolder {
        public TextView tv;
        public TextView tv2;
        public ImageView iv;

        /**
         * <p>
         * Title: MainActivity.java
         * </p>
         * <p>
         * Description:
         * </p>
         * 
         * @param @param itemView
         */
        public ViewHolder(View itemView, int itemType) {
            super(itemView);
            // TODO Auto-generated constructor stub
            if (itemType == TYPE_TV) {
                tv = (TextView) itemView.findViewById(R.id.rv_item_tv);
            } else if (itemType == TYPE_IV) {
                iv = (ImageView) itemView.findViewById(R.id.rv_item_iv);
            } else {
                tv2 = (TextView) itemView.findViewById(R.id.rv_item_tv);
            }
        }
    }
}

同时，这个测试工程里面只有一个Activity，它的详情如下：

package com.example.recyclerviewtest;

import java.util.List;

import com.example.recyclerviewtest.adapter.MyRVAdapter;
import com.example.recyclerviewtest.adapter.MyRVAdapter2;
import com.example.recyclerviewtest.adapter.MyRVAdapter3;
import com.example.recyclerviewtest.util.Utils;

import android.support.v7.app.ActionBarActivity;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.os.Bundle;
import android.widget.LinearLayout;

public class MainActivity extends ActionBarActivity {
    private static final String TAG = MainActivity.class.getSimpleName();

    private RecyclerView rvHorizontal;
    private RecyclerView rvVertical;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        rvHorizontal = (RecyclerView) findViewById(R.id.recyclier_view_horizontal);
        rvVertical = (RecyclerView) findViewById(R.id.recyclier_view_vertical);
        List<String> dataSet = Utils.generateDataSet();
        LinearLayoutManager layoutManager = new LinearLayoutManager(this,
                LinearLayout.HORIZONTAL, false);
        rvHorizontal.setLayoutManager(layoutManager);
        layoutManager = new LinearLayoutManager(this, LinearLayout.VERTICAL,
                false);
        rvVertical.setLayoutManager(layoutManager);
        MyRVAdapter adapter = new MyRVAdapter(this, dataSet);
        MyRVAdapter2 adapter2 = new MyRVAdapter2(this, dataSet);
        MyRVAdapter3 adapter3 = new MyRVAdapter3(this, dataSet);
        rvHorizontal.setAdapter(adapter2);
        rvVertical.setAdapter(adapter);
    }

}

对于你想测试的三种情况，你可以通达代码rvHorizontal.setAdapter(adapter2)和rvVertical.setAdapter(adapter)来进行测试。在测试的过程中，可以通过改变RecyclerView的宽度来测试显示ViewHolder的数目。比如，item的宽度为60dp，你可以分别测试下RecyclerView的宽度分别为60dp、120dp和180dp情况下log的打印情况。其中，activity的布局文件如下：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.recyclerviewtest.MainActivity" >

    <android.support.v7.widget.RecyclerView
        android:id="@+id/recyclier_view_horizontal"
        android:layout_width="240dp"
        android:layout_height="70dp"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:scrollbars="horizontal" >
    </android.support.v7.widget.RecyclerView>

    <android.support.v7.widget.RecyclerView
        android:id="@+id/recyclier_view_vertical"
        android:layout_width="match_parent"
        android:layout_height="360dp"
        android:layout_below="@id/recyclier_view_horizontal"
        android:layout_centerHorizontal="true"
        android:scrollbars="vertical" >
    </android.support.v7.widget.RecyclerView>

</RelativeLayout>

其中一个item的布局如下：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    android:orientation="vertical" >

    <TextView
        android:id="@+id/rv_item_tv"
        android:layout_width="60dp"
        android:layout_height="20dp"
        android:layout_gravity="center"
        android:background="@drawable/item_selector"
        android:clickable="true"
        android:focusable="true"
        android:focusableInTouchMode="true"
        android:gravity="center" >
    </TextView>

</LinearLayout>

总之，测试的结论是这样的：在只有一种item的情况下，缓存的ViewHolder的数目为RecyclerView在滑动过程中所能在一屏内容纳的最大item个数+2。比如，在一个屏幕中只有item A可以显示，在滑动的过程最多可以出现6个item（这个最多是指所有item的个数，包括显示完全和显示不完全的总数），那么ViewHolder的缓存个数将会是8；而有至少两种item显示的情况下，每种item的ViewHolder的缓存个数为单种item在一屏内最大显示个数+1；比如，有3种item A，B，C，在滑动的过程，item A最多的情况下一屏显示了2个，那个item A对应的ViewHolder的缓存个数就是3个。

工程源文件可以从下面的链接打开下载。

点击下载源码