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：
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：
1 /**
2 * @Title: MyRVAdapter.java
3 * @Package com.example.recyclerviewtest.adapter
4 * @Description: TODO
5 * @author SilentKnight || happychinapc[at]gmail[dot]com
6 * @date 2015 2015年1月22日 下午4:54:25
7 * @version V1.0.0
8 */
9 package com.example.recyclerviewtest.adapter;
10
11 import java.util.List;
12
13 import android.content.Context;
14 import android.support.v7.widget.RecyclerView;
15 import android.util.Log;
16 import android.view.LayoutInflater;
17 import android.view.View;
18 import android.view.ViewGroup;
19 import android.view.View.OnClickListener;
20 import android.widget.TextView;
21 import android.widget.Toast;
22
23 import com.example.recyclerviewtest.R;
24
25 /**
26 * @ClassName: MyRVAdapter
27 * @Description: TODO
28 * @author SilentKnight || [email protected]
29 * @date 2015年1月22日 下午4:54:25
30 *
31 */
32 public class MyRVAdapter extends RecyclerView.Adapter<MyRVAdapter.ViewHolder> {
33 private static int COUNT_CACHE_VIEW = 0;
34 private static final String ADAPTER_TAG = MyRVAdapter.class.getSimpleName();
35 private List<String> dataSet;
36 private Context context;
37
38 public MyRVAdapter(Context context, List<String> dataSet) {
39 this.context = context;
40 this.dataSet = dataSet;
41 }
42
43 /*
44 * (non-avadoc) <p>Title: getItemId</p> <p>Description: </p>
45 *
46 * @params @param position
47 *
48 * @params @return
49 *
50 * @overrided @see
51 * android.support.v7.widget.RecyclerView.Adapter#getItemId(int)
52 */
53 @Override
54 public long getItemId(int position) {
55 // TODO Auto-generated method stub
56 return position;
57 }
58
59 /*
60 * (non-avadoc) <p>Title: getItemCount</p> <p>Description: </p>
61 *
62 * @params @return
63 *
64 * @overrided @see
65 * android.support.v7.widget.RecyclerView.Adapter#getItemCount()
66 */
67 @Override
68 public int getItemCount() {
69 // TODO Auto-generated method stub
70 return dataSet.size();
71 }
72
73 /*
74 * (non-avadoc) <p>Title: onBindViewHolder</p> <p>Description: </p>
75 *
76 * @params @param arg0
77 *
78 * @params @param arg1
79 *
80 * @overrided @see
81 * android.support.v7.widget.RecyclerView.Adapter#onBindViewHolder
82 * (android.support.v7.widget.RecyclerView.ViewHolder, int)
83 */
84 @Override
85 public void onBindViewHolder(ViewHolder holder, final int arg1) {
86 // TODO Auto-generated method stub
87 holder.tv.setOnClickListener(new OnClickListener() {
88
89 @Override
90 public void onClick(View v) {
91 // TODO Auto-generated method stub
92 Toast.makeText(context, "You clickd TextView at index of "+arg1,
93 Toast.LENGTH_SHORT).show();
94 }
95 });
96 holder.tv.setText(dataSet.get(arg1));
97 }
98
99 /*
100 * (non-avadoc) <p>Title: onCreateViewHolder</p> <p>Description: </p>
101 *
102 * @params @param arg0
103 *
104 * @params @param arg1
105 *
106 * @params @return
107 *
108 * @overrided @see
109 * android.support.v7.widget.RecyclerView.Adapter#onCreateViewHolder
110 * (android.view.ViewGroup, int)
111 */
112
113 @Override
114 public ViewHolder onCreateViewHolder(ViewGroup viewGroup, int arg1) {
115 // TODO Auto-generated method stub
116 Log.i(ADAPTER_TAG, "itemTV---" + ++COUNT_CACHE_VIEW);
117 View itemLayout = LayoutInflater.from(viewGroup.getContext()).inflate(
118 R.layout.recycler_view_item_layout_tv, null);
119 return new ViewHolder(itemLayout);
120 }
121
122 public static class ViewHolder extends RecyclerView.ViewHolder {
123 public TextView tv;
124
125 /**
126 * <p>
127 * Title: MainActivity.java
128 * </p>
129 * <p>
130 * Description:
131 * </p>
132 *
133 * @param @param itemView
134 */
135 public ViewHolder(View itemView) {
136 super(itemView);
137 // TODO Auto-generated constructor stub
138 tv = (TextView) itemView.findViewById(R.id.rv_item_tv);
139 }
140 }
141 }
MyRVAdapter
1 /**
2 * @Title: MyRVAdapter2.java
3 * @Package com.example.recyclerviewtest.adapter
4 * @Description: TODO
5 * @author SilentKnight || happychinapc[at]gmail[dot]com
6 * @date 2015 2015年1月22日 下午4:55:31
7 * @version V1.0.0
8 */
9 package com.example.recyclerviewtest.adapter;
10
11 import java.util.List;
12
13 import android.content.Context;
14 import android.content.Intent;
15 import android.support.v7.widget.RecyclerView;
16 import android.util.Log;
17 import android.view.LayoutInflater;
18 import android.view.View;
19 import android.view.ViewGroup;
20 import android.view.View.OnClickListener;
21 import android.widget.ImageView;
22 import android.widget.TextView;
23 import android.widget.Toast;
24
25 import com.example.recyclerviewtest.GridViewHorizontalTest;
26 import com.example.recyclerviewtest.GridViewVerticalTest;
27 import com.example.recyclerviewtest.R;
28
29 /**
30 * @ClassName: MyRVAdapter2
31 * @Description: TODO
32 * @author SilentKnight || [email protected]
33 * @date 2015年1月22日 下午4:55:31
34 *
35 */
36 public class MyRVAdapter2 extends RecyclerView.Adapter<MyRVAdapter2.ViewHolder> {
37 private static int COUNT_CACHE_VIEW_1 = 0;
38 private static int COUNT_CACHE_VIEW_2 = 0;
39 private static final String ADAPTER_TAG = MyRVAdapter2.class
40 .getSimpleName();
41 private static final int TYPE_TV = 0x000;
42 private static final int TYPE_IV = 0x0001;
43 private List<String> dataSet;
44 private Context context;
45
46 public MyRVAdapter2(Context context, List<String> dataSet) {
47 this.context = context;
48 this.dataSet = dataSet;
49 }
50
51 /*
52 * (non-avadoc) <p>Title: getItemId</p> <p>Description: </p>
53 *
54 * @params @param position
55 *
56 * @params @return
57 *
58 * @overrided @see
59 * android.support.v7.widget.RecyclerView.Adapter#getItemId(int)
60 */
61 @Override
62 public long getItemId(int position) {
63 // TODO Auto-generated method stub
64 return position;
65 }
66
67 /*
68 * (non-avadoc) <p>Title: getItemViewType</p> <p>Description: </p>
69 *
70 * @params @param position
71 *
72 * @params @return
73 *
74 * @overrided @see
75 * android.support.v7.widget.RecyclerView.Adapter#getItemViewType(int)
76 */
77 @Override
78 public int getItemViewType(int position) {
79 // TODO Auto-generated method stub
80 if (position % 2 == 0) {
81 return TYPE_TV;
82 } else {
83 return TYPE_IV;
84 }
85 }
86
87 /*
88 * (non-avadoc) <p>Title: getItemCount</p> <p>Description: </p>
89 *
90 * @params @return
91 *
92 * @overrided @see
93 * android.support.v7.widget.RecyclerView.Adapter#getItemCount()
94 */
95 @Override
96 public int getItemCount() {
97 // TODO Auto-generated method stub
98 return dataSet.size();
99 }
100
101 /*
102 * (non-avadoc) <p>Title: onBindViewHolder</p> <p>Description: </p>
103 *
104 * @params @param arg0
105 *
106 * @params @param arg1
107 *
108 * @overrided @see
109 * android.support.v7.widget.RecyclerView.Adapter#onBindViewHolder
110 * (android.support.v7.widget.RecyclerView.ViewHolder, int)
111 */
112 @Override
113 public void onBindViewHolder(ViewHolder holder, int arg1) {
114 // TODO Auto-generated method stub
115 if (getItemViewType(arg1) == TYPE_TV) {
116 holder.tv.setOnClickListener(new OnClickListener() {
117
118 @Override
119 public void onClick(View v) {
120 // TODO Auto-generated method stub
121 context.startActivity(new Intent(context,
122 GridViewHorizontalTest.class));
123 }
124 });
125 holder.tv.setText(dataSet.get(arg1));
126 } else {
127 holder.iv.setOnClickListener(new OnClickListener() {
128
129 @Override
130 public void onClick(View v) {
131 // TODO Auto-generated method stub
132 context.startActivity(new Intent(context,
133 GridViewVerticalTest.class));
134 }
135 });
136 }
137 }
138
139 /*
140 * (non-avadoc) <p>Title: onCreateViewHolder</p> <p>Description: </p>
141 *
142 * @params @param arg0
143 *
144 * @params @param arg1
145 *
146 * @params @return
147 *
148 * @overrided @see
149 * android.support.v7.widget.RecyclerView.Adapter#onCreateViewHolder
150 * (android.view.ViewGroup, int)
151 */
152
153 @Override
154 public ViewHolder onCreateViewHolder(ViewGroup viewGroup, int arg1) {
155 // TODO Auto-generated method stub
156 if (getItemViewType(arg1) == TYPE_TV) {
157 Log.i(ADAPTER_TAG, "itemTV---" + ++COUNT_CACHE_VIEW_1);
158 View itemLayout = LayoutInflater.from(viewGroup.getContext())
159 .inflate(R.layout.recycler_view_item_layout_tv, null);
160 return new ViewHolder(itemLayout, TYPE_TV);
161 } else {
162 Log.i(ADAPTER_TAG, "itemIV---" + ++COUNT_CACHE_VIEW_2);
163 View itemLayout = LayoutInflater.from(viewGroup.getContext())
164 .inflate(R.layout.recycler_view_item_layout_iv, null);
165 return new ViewHolder(itemLayout, TYPE_IV);
166 }
167 }
168
169 public static class ViewHolder extends RecyclerView.ViewHolder {
170 public TextView tv;
171 public ImageView iv;
172
173 /**
174 * <p>
175 * Title: MainActivity.java
176 * </p>
177 * <p>
178 * Description:
179 * </p>
180 *
181 * @param @param itemView
182 */
183 public ViewHolder(View itemView, int itemType) {
184 super(itemView);
185 // TODO Auto-generated constructor stub
186 if (itemType == TYPE_TV) {
187 tv = (TextView) itemView.findViewById(R.id.rv_item_tv);
188 } else {
189 iv = (ImageView) itemView.findViewById(R.id.rv_item_iv);
190 }
191 }
192 }
193 }
MyRVAdapter2
1 /**
2 * @Title: MyRVAdapter3.java
3 * @Package com.example.recyclerviewtest.adapter
4 * @Description: TODO
5 * @author SilentKnight || happychinapc[at]gmail[dot]com
6 * @date 2015 2015年1月22日 下午4:56:33
7 * @version V1.0.0
8 */
9 package com.example.recyclerviewtest.adapter;
10
11 import java.util.List;
12
13 import android.content.Context;
14 import android.support.v7.widget.RecyclerView;
15 import android.util.Log;
16 import android.view.LayoutInflater;
17 import android.view.View;
18 import android.view.ViewGroup;
19 import android.view.View.OnClickListener;
20 import android.widget.ImageView;
21 import android.widget.TextView;
22 import android.widget.Toast;
23
24 import com.example.recyclerviewtest.R;
25
26 /**
27 * @ClassName: MyRVAdapter3
28 * @Description: TODO
29 * @author SilentKnight || [email protected]
30 * @date 2015年1月22日 下午4:56:33
31 *
32 */
33 public class MyRVAdapter3 extends RecyclerView.Adapter<MyRVAdapter3.ViewHolder> {
34 private static int COUNT_CACHE_VIEW_1 = 0;
35 private static int COUNT_CACHE_VIEW_2 = 0;
36 private static int COUNT_CACHE_VIEW_3 = 0;
37 private static final String ADAPTER_TAG = MyRVAdapter3.class
38 .getSimpleName();
39 private static final int TYPE_TV = 0x000;
40 private static final int TYPE_IV = 0x0001;
41 private static final int TYPE_TV_2 = 0x0002;
42 private List<String> dataSet;
43 private Context context;
44
45 public MyRVAdapter3(Context context, List<String> dataSet) {
46 this.context = context;
47 this.dataSet = dataSet;
48 }
49
50 /*
51 * (non-avadoc) <p>Title: getItemId</p> <p>Description: </p>
52 *
53 * @params @param position
54 *
55 * @params @return
56 *
57 * @overrided @see
58 * android.support.v7.widget.RecyclerView.Adapter#getItemId(int)
59 */
60 @Override
61 public long getItemId(int position) {
62 // TODO Auto-generated method stub
63 return position;
64 }
65
66 /*
67 * (non-avadoc) <p>Title: getItemViewType</p> <p>Description: </p>
68 *
69 * @params @param position
70 *
71 * @params @return
72 *
73 * @overrided @see
74 * android.support.v7.widget.RecyclerView.Adapter#getItemViewType(int)
75 */
76 @Override
77 public int getItemViewType(int position) {
78 // TODO Auto-generated method stub
79 if (position % 3 == 0) {
80 return TYPE_TV;
81 } else if (position % 3 == 1) {
82 return TYPE_IV;
83 } else {
84 return TYPE_TV_2;
85 }
86 }
87
88 /*
89 * (non-avadoc) <p>Title: getItemCount</p> <p>Description: </p>
90 *
91 * @params @return
92 *
93 * @overrided @see
94 * android.support.v7.widget.RecyclerView.Adapter#getItemCount()
95 */
96 @Override
97 public int getItemCount() {
98 // TODO Auto-generated method stub
99 return dataSet.size();
100 }
101
102 /*
103 * (non-avadoc) <p>Title: onBindViewHolder</p> <p>Description: </p>
104 *
105 * @params @param arg0
106 *
107 * @params @param arg1
108 *
109 * @overrided @see
110 * android.support.v7.widget.RecyclerView.Adapter#onBindViewHolder
111 * (android.support.v7.widget.RecyclerView.ViewHolder, int)
112 */
113 @Override
114 public void onBindViewHolder(ViewHolder holder, int arg1) {
115 // TODO Auto-generated method stub
116 if (getItemViewType(arg1) == TYPE_TV) {
117 holder.tv.setOnClickListener(new OnClickListener() {
118
119 @Override
120 public void onClick(View v) {
121 // TODO Auto-generated method stub
122 Toast.makeText(context, "You clickd TextView",
123 Toast.LENGTH_SHORT).show();
124 }
125 });
126 holder.tv.setText(dataSet.get(arg1));
127 } else if (getItemViewType(arg1) == TYPE_IV) {
128 holder.iv.setOnClickListener(new OnClickListener() {
129
130 @Override
131 public void onClick(View v) {
132 // TODO Auto-generated method stub
133
134 Toast.makeText(context, "You clickd ImageView",
135 Toast.LENGTH_SHORT).show();
136 }
137 });
138 } else {
139
140 }
141 }
142
143 /*
144 * (non-avadoc) <p>Title: onCreateViewHolder</p> <p>Description: </p>
145 *
146 * @params @param arg0
147 *
148 * @params @param arg1
149 *
150 * @params @return
151 *
152 * @overrided @see
153 * android.support.v7.widget.RecyclerView.Adapter#onCreateViewHolder
154 * (android.view.ViewGroup, int)
155 */
156
157 @Override
158 public ViewHolder onCreateViewHolder(ViewGroup viewGroup, int arg1) {
159 // TODO Auto-generated method stub
160 if (getItemViewType(arg1) == TYPE_TV) {
161 Log.i(ADAPTER_TAG, "itemTV---" + ++COUNT_CACHE_VIEW_1);
162 View itemLayout = LayoutInflater.from(viewGroup.getContext())
163 .inflate(R.layout.recycler_view_item_layout_tv, null);
164 return new ViewHolder(itemLayout, TYPE_TV);
165 } else if (getItemViewType(arg1) == TYPE_IV) {
166 Log.i(ADAPTER_TAG, "itemIV---" + ++COUNT_CACHE_VIEW_2);
167 View itemLayout = LayoutInflater.from(viewGroup.getContext())
168 .inflate(R.layout.recycler_view_item_layout_iv, null);
169 return new ViewHolder(itemLayout, TYPE_IV);
170 } else {
171 Log.i(ADAPTER_TAG, "itemTV2---" + ++COUNT_CACHE_VIEW_3);
172 View itemLayout = LayoutInflater.from(viewGroup.getContext())
173 .inflate(R.layout.recycler_view_item_layout_tv_2, null);
174 return new ViewHolder(itemLayout, TYPE_TV_2);
175 }
176 }
177
178 public static class ViewHolder extends RecyclerView.ViewHolder {
179 public TextView tv;
180 public TextView tv2;
181 public ImageView iv;
182
183 /**
184 * <p>
185 * Title: MainActivity.java
186 * </p>
187 * <p>
188 * Description:
189 * </p>
190 *
191 * @param @param itemView
192 */
193 public ViewHolder(View itemView, int itemType) {
194 super(itemView);
195 // TODO Auto-generated constructor stub
196 if (itemType == TYPE_TV) {
197 tv = (TextView) itemView.findViewById(R.id.rv_item_tv);
198 } else if (itemType == TYPE_IV) {
199 iv = (ImageView) itemView.findViewById(R.id.rv_item_iv);
200 } else {
201 tv2 = (TextView) itemView.findViewById(R.id.rv_item_tv);
202 }
203 }
204 }
205 }
MyRVAdapter3
同时，这个测试工程里面只有一个Activity，它的详情如下：
1 package com.example.recyclerviewtest;
2
3 import java.util.List;
4
5 import com.example.recyclerviewtest.adapter.MyRVAdapter;
6 import com.example.recyclerviewtest.adapter.MyRVAdapter2;
7 import com.example.recyclerviewtest.adapter.MyRVAdapter3;
8 import com.example.recyclerviewtest.util.Utils;
9
10 import android.support.v7.app.ActionBarActivity;
11 import android.support.v7.widget.LinearLayoutManager;
12 import android.support.v7.widget.RecyclerView;
13 import android.os.Bundle;
14 import android.widget.LinearLayout;
15
16 public class MainActivity extends ActionBarActivity {
17 private static final String TAG = MainActivity.class.getSimpleName();
18
19 private RecyclerView rvHorizontal;
20 private RecyclerView rvVertical;
21
22 @Override
23 protected void onCreate(Bundle savedInstanceState) {
24 super.onCreate(savedInstanceState);
25 setContentView(R.layout.activity_main);
26 rvHorizontal = (RecyclerView) findViewById(R.id.recyclier_view_horizontal);
27 rvVertical = (RecyclerView) findViewById(R.id.recyclier_view_vertical);
28 List<String> dataSet = Utils.generateDataSet();
29 LinearLayoutManager layoutManager = new LinearLayoutManager(this,
30 LinearLayout.HORIZONTAL, false);
31 rvHorizontal.setLayoutManager(layoutManager);
32 layoutManager = new LinearLayoutManager(this, LinearLayout.VERTICAL,
33 false);
34 rvVertical.setLayoutManager(layoutManager);
35 MyRVAdapter adapter = new MyRVAdapter(this, dataSet);
36 MyRVAdapter2 adapter2 = new MyRVAdapter2(this, dataSet);
37 MyRVAdapter3 adapter3 = new MyRVAdapter3(this, dataSet);
38 rvHorizontal.setAdapter(adapter2);
39 rvVertical.setAdapter(adapter);
40 }
41
42 }
MainActivity
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：
1 <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 2 xmlns:tools="http://schemas.android.com/tools" 3 android:layout_width="match_parent" 4 android:layout_height="match_parent" 5 android:paddingBottom="@dimen/activity_vertical_margin" 6 android:paddingLeft="@dimen/activity_horizontal_margin" 7 android:paddingRight="@dimen/activity_horizontal_margin" 8 android:paddingTop="@dimen/activity_vertical_margin" 9 tools:context="com.example.recyclerviewtest.MainActivity" > 10 11 <android.support.v7.widget.RecyclerView 12 android:id="@+id/recyclier_view_horizontal" 13 android:layout_width="240dp" 14 android:layout_height="70dp" 15 android:layout_alignParentTop="true" 16 android:layout_centerHorizontal="true" 17 android:scrollbars="horizontal" > 18 </android.support.v7.widget.RecyclerView> 19 20 <android.support.v7.widget.RecyclerView 21 android:id="@+id/recyclier_view_vertical" 22 android:layout_width="match_parent" 23 android:layout_height="360dp" 24 android:layout_below="@id/recyclier_view_horizontal" 25 android:layout_centerHorizontal="true" 26 android:scrollbars="vertical" > 27 </android.support.v7.widget.RecyclerView> 28 29 </RelativeLayout>
activity_main.xml
其中一个item的布局如下：
1 <?xml version="1.0" encoding="utf-8"?> 2 <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 3 android:layout_width="match_parent" 4 android:layout_height="match_parent" 5 android:gravity="center" 6 android:orientation="vertical" > 7 8 <TextView 9 android:id="@+id/rv_item_tv" 10 android:layout_width="60dp" 11 android:layout_height="20dp" 12 android:layout_gravity="center" 13 android:background="@drawable/item_selector" 14 android:clickable="true" 15 android:focusable="true" 16 android:focusableInTouchMode="true" 17 android:gravity="center" > 18 </TextView> 19 20 </LinearLayout>
recycler_view_item_layout_tv.xml
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。
时间： 2024-08-24 12:31:08