ListView 和 Adapter 的基础
16lz
2021-01-25
ListView 和 Adapter 的基础
工作原理:
ListView 针对List中每个item,要求 adapter “给我一个视图” (getView)。
一个新的视图被返回并显示
如果我们有上亿个项目要显示怎么办?为每个项目创建一个新视图?NO!这不可能!
实际上Android为你缓存了视图。
Android中有个叫做Recycler的构件,下图是他的工作原理: 如果你有10亿个项目(item),其中只有可见的项目存在内存中,其他的在Recycler中。
ListView先请求一个type1视图(getView)然后请求其他可见的项目。convertView在getView中是空(null)的。
当item1滚出屏幕,并且一个新的项目从屏幕低端上来时,ListView再请求一个type1视图。convertView此时不是空值了,它的值是item1。你只需设定新的数据然后返回convertView,不必重新创建一个视图。
请看下面的示例代码,这里在getView中使用了System.out进行输出
01
public class MultipleItemsList extends ListActivity {
02
03
private MyCustomAdapter mAdapter;
04
05
@Override
06
public void onCreate(Bundle savedInstanceState) {
07
super.onCreate(savedInstanceState);
08
mAdapter = new MyCustomAdapter();
09
for (int i = 0; i < 50; i++) {
10
mAdapter.addItem("item " + i);
11
}
12
setListAdapter(mAdapter);
13
}
14
15
private class MyCustomAdapter extends BaseAdapter {
16
17
private ArrayList mData = new ArrayList();
18
private LayoutInflater mInflater;
19
20
public MyCustomAdapter() {
21
mInflater = (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE);
22
}
23
24
public void addItem(final String item) {
25
mData.add(item);
26
notifyDataSetChanged();
27
}
28
29
@Override
30
public int getCount() {
31
return mData.size();
32
}
33
34
@Override
35
public String getItem(int position) {
36
return mData.get(position);
37
}
38
39
@Override
40
public long getItemId(int position) {
41
return position;
42
}
43
44
@Override
45
public View getView(int position, View convertView, ViewGroup parent) {
46
System.out.println("getView " + position + " " + convertView);
47
ViewHolder holder = null;
48
if (convertView == null) {
49
convertView = mInflater.inflate(R.layout.item1, null);
50
holder = new ViewHolder();
51
holder.textView = (TextView)convertView.findViewById(R.id.text);
52
convertView.setTag(holder);
53
} else {
54
holder = (ViewHolder)convertView.getTag();
55
}
56
holder.textView.setText(mData.get(position));
57
return convertView;
58
}
59
60
}
61
62
public static class ViewHolder {
63
public TextView textView;
64
}
65
}
执行程序,然后在Logcat中查看日志
getView 被调用 9 次 ,convertView 对于所有的可见项目是空值(如下)
02-05 13:47:32.559: INFO/System.out(947): getView 0 null
02-05 13:47:32.570: INFO/System.out(947): getView 1 null
02-05 13:47:32.589: INFO/System.out(947): getView 2 null
02-05 13:47:32.599: INFO/System.out(947): getView 3 null
02-05 13:47:32.619: INFO/System.out(947): getView 4 null
02-05 13:47:32.629: INFO/System.out(947): getView 5 null
02-05 13:47:32.708: INFO/System.out(947): getView 6 null
02-05 13:47:32.719: INFO/System.out(947): getView 7 null
02-05 13:47:32.729: INFO/System.out(947): getView 8 null
然后稍微向下滚动List,直到item10出现:
convertView仍然是空值,因为recycler中没有视图(item1的边缘仍然可见,在顶端)
02-05 13:48:25.169: INFO/System.out(947): getView 9 null
再滚动List
convertView不是空值了!item1离开屏幕到Recycler中去了,然后item11被创建
02-05 13:48:42.879: INFO/System.out(947): getView 10 android.widget.LinearLayout@437430f8
再滚动:
02-05 14:01:31.069: INFO/System.out(947): getView 11 android.widget.LinearLayout@437447d0
02-05 14:01:31.142: INFO/System.out(947): getView 12 android.widget.LinearLayout@43744ff8
02-05 14:01:31.279: INFO/System.out(947): getView 13 android.widget.LinearLayout@43743fa8
02-05 14:01:31.350: INFO/System.out(947): getView 14 android.widget.LinearLayout@43745820
02-05 14:01:31.429: INFO/System.out(947): getView 15 android.widget.LinearLayout@43746048
02-05 14:01:31.550: INFO/System.out(947): getView 16 android.widget.LinearLayout@43746870
02-05 14:01:31.669: INFO/System.out(947): getView 17 android.widget.LinearLayout@43747098
02-05 14:01:31.839: INFO/System.out(947): getView 18 android.widget.LinearLayout@437478c0
02-05 14:03:30.900: INFO/System.out(947): getView 19 android.widget.LinearLayout@43748df0
02-05 14:03:32.069: INFO/System.out(947): getView 20 android.widget.LinearLayout@437430f8
convertView 如我们所期待的非空了,在item11离开屏幕之后,它的视图(@437430f8)作为convertView容纳item21了
不同的项目布局(item layout)
我们再举一个稍微复杂的例子,在上例的list中加入一些分隔线
你需要做这些:
重(@Override)写 getViewTypeCount() �C 返回你有多少个不同的布局
重写 getItemViewType(int) �C 由position返回view type id
根据view item的类型,在getView中创建正确的convertView
以下是代码:
001
public class MultipleItemsList extends ListActivity {
002
003
private MyCustomAdapter mAdapter;
004
005
@Override
006
public void onCreate(Bundle savedInstanceState) {
007
super.onCreate(savedInstanceState);
008
mAdapter = new MyCustomAdapter();
009
for (int i = 1; i < 50; i++) {
010
mAdapter.addItem("item " + i);
011
if (i % 4 == 0) {
012
mAdapter.addSeparatorItem("separator " + i);
013
}
014
}
015
setListAdapter(mAdapter);
016
}
017
018
private class MyCustomAdapter extends BaseAdapter {
019
020
private static final int TYPE_ITEM = 0;
021
private static final int TYPE_SEPARATOR = 1;
022
private static final int TYPE_MAX_COUNT = TYPE_SEPARATOR + 1;
023
024
private ArrayList mData = new ArrayList();
025
private LayoutInflater mInflater;
026
027
private TreeSet mSeparatorsSet = new TreeSet();
028
029
public MyCustomAdapter() {
030
mInflater = (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE);
031
}
032
033
public void addItem(final String item) {
034
mData.add(item);
035
notifyDataSetChanged();
036
}
037
038
public void addSeparatorItem(final String item) {
039
mData.add(item);
040
// save separator position
041
mSeparatorsSet.add(mData.size() - 1);
042
notifyDataSetChanged();
043
}
044
045
@Override
046
public int getItemViewType(int position) {
047
return mSeparatorsSet.contains(position) ? TYPE_SEPARATOR : TYPE_ITEM;
048
}
049
050
@Override
051
public int getViewTypeCount() {
052
return TYPE_MAX_COUNT;
053
}
054
055
@Override
056
public int getCount() {
057
return mData.size();
058
}
059
060
@Override
061
public String getItem(int position) {
062
return mData.get(position);
063
}
064
065
@Override
066
public long getItemId(int position) {
067
return position;
068
}
069
070
@Override
071
public View getView(int position, View convertView, ViewGroup parent) {
072
ViewHolder holder = null;
073
int type = getItemViewType(position);
074
System.out.println("getView " + position + " " + convertView + " type = " + type);
075
if (convertView == null) {
076
holder = new ViewHolder();
077
switch (type) {
078
case TYPE_ITEM:
079
convertView = mInflater.inflate(R.layout.item1, null);
080
holder.textView = (TextView)convertView.findViewById(R.id.text);
081
break;
082
case TYPE_SEPARATOR:
083
convertView = mInflater.inflate(R.layout.item2, null);
084
holder.textView = (TextView)convertView.findViewById(R.id.textSeparator);
085
break;
086
}
087
convertView.setTag(holder);
088
} else {
089
holder = (ViewHolder)convertView.getTag();
090
}
091
holder.textView.setText(mData.get(position));
092
return convertView;
093
}
094
095
}
096
097
public static class ViewHolder {
098
public TextView textView;
099
}
100
}
运行程序,你会看到每4个item一个分割线 看看日志,无异常,所有的convertView都是空的
02-05 15:19:03.080: INFO/System.out(1035): getView 0 null type = 0
02-05 15:19:03.112: INFO/System.out(1035): getView 1 null type = 0
02-05 15:19:03.130: INFO/System.out(1035): getView 2 null type = 0
02-05 15:19:03.141: INFO/System.out(1035): getView 3 null type = 0
02-05 15:19:03.160: INFO/System.out(1035): getView 4 null type = 1
02-05 15:19:03.170: INFO/System.out(1035): getView 5 null type = 0
02-05 15:19:03.180: INFO/System.out(1035): getView 6 null type = 0
02-05 15:19:03.190: INFO/System.out(1035): getView 7 null type = 0
02-05 15:19:03.210: INFO/System.out(1035): getView 8 null type = 0
02-05 15:19:03.210: INFO/System.out(1035): getView 9 null type = 1
滚动list:
02-05 15:19:54.160: INFO/System.out(1035): getView 10 null type = 0
02-05 15:19:57.440: INFO/System.out(1035): getView 11 android.widget.LinearLayout@43744528 type = 0
02-05 15:20:01.310: INFO/System.out(1035): getView 12 android.widget.LinearLayout@43744eb0 type = 0
02-05 15:20:01.880: INFO/System.out(1035): getView 13 android.widget.LinearLayout@437456d8 type = 0
02-05 15:20:02.869: INFO/System.out(1035): getView 14 null type = 1
02-05 15:20:06.489: INFO/System.out(1035): getView 15 android.widget.LinearLayout@43745f00 type = 0
02-05 15:20:07.749: INFO/System.out(1035): getView 16 android.widget.LinearLayout@43747170 type = 0
02-05 15:20:10.250: INFO/System.out(1035): getView 17 android.widget.LinearLayout@43747998 type = 0
02-05 15:20:11.661: INFO/System.out(1035): getView 18 android.widget.LinearLayout@437481c0 type = 0
02-05 15:20:13.180: INFO/System.out(1035): getView 19 android.widget.LinearLayout@437468a0 type = 1
02-05 15:20:16.900: INFO/System.out(1035): getView 20 android.widget.LinearLayout@437489e8 type = 0
02-05 15:20:25.690: INFO/System.out(1035): getView 21 android.widget.LinearLayout@4374a8d8 type = 0
convertView对于分割线是空的,直到第一个分割线可见,当其离开屏幕,视图去到Recycler并且convertView开始起作用。
工作原理:
ListView 针对List中每个item,要求 adapter “给我一个视图” (getView)。
一个新的视图被返回并显示
如果我们有上亿个项目要显示怎么办?为每个项目创建一个新视图?NO!这不可能!
实际上Android为你缓存了视图。
Android中有个叫做Recycler的构件,下图是他的工作原理: 如果你有10亿个项目(item),其中只有可见的项目存在内存中,其他的在Recycler中。
ListView先请求一个type1视图(getView)然后请求其他可见的项目。convertView在getView中是空(null)的。
当item1滚出屏幕,并且一个新的项目从屏幕低端上来时,ListView再请求一个type1视图。convertView此时不是空值了,它的值是item1。你只需设定新的数据然后返回convertView,不必重新创建一个视图。
请看下面的示例代码,这里在getView中使用了System.out进行输出
01
public class MultipleItemsList extends ListActivity {
02
03
private MyCustomAdapter mAdapter;
04
05
@Override
06
public void onCreate(Bundle savedInstanceState) {
07
super.onCreate(savedInstanceState);
08
mAdapter = new MyCustomAdapter();
09
for (int i = 0; i < 50; i++) {
10
mAdapter.addItem("item " + i);
11
}
12
setListAdapter(mAdapter);
13
}
14
15
private class MyCustomAdapter extends BaseAdapter {
16
17
private ArrayList mData = new ArrayList();
18
private LayoutInflater mInflater;
19
20
public MyCustomAdapter() {
21
mInflater = (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE);
22
}
23
24
public void addItem(final String item) {
25
mData.add(item);
26
notifyDataSetChanged();
27
}
28
29
@Override
30
public int getCount() {
31
return mData.size();
32
}
33
34
@Override
35
public String getItem(int position) {
36
return mData.get(position);
37
}
38
39
@Override
40
public long getItemId(int position) {
41
return position;
42
}
43
44
@Override
45
public View getView(int position, View convertView, ViewGroup parent) {
46
System.out.println("getView " + position + " " + convertView);
47
ViewHolder holder = null;
48
if (convertView == null) {
49
convertView = mInflater.inflate(R.layout.item1, null);
50
holder = new ViewHolder();
51
holder.textView = (TextView)convertView.findViewById(R.id.text);
52
convertView.setTag(holder);
53
} else {
54
holder = (ViewHolder)convertView.getTag();
55
}
56
holder.textView.setText(mData.get(position));
57
return convertView;
58
}
59
60
}
61
62
public static class ViewHolder {
63
public TextView textView;
64
}
65
}
执行程序,然后在Logcat中查看日志
getView 被调用 9 次 ,convertView 对于所有的可见项目是空值(如下)
02-05 13:47:32.559: INFO/System.out(947): getView 0 null
02-05 13:47:32.570: INFO/System.out(947): getView 1 null
02-05 13:47:32.589: INFO/System.out(947): getView 2 null
02-05 13:47:32.599: INFO/System.out(947): getView 3 null
02-05 13:47:32.619: INFO/System.out(947): getView 4 null
02-05 13:47:32.629: INFO/System.out(947): getView 5 null
02-05 13:47:32.708: INFO/System.out(947): getView 6 null
02-05 13:47:32.719: INFO/System.out(947): getView 7 null
02-05 13:47:32.729: INFO/System.out(947): getView 8 null
然后稍微向下滚动List,直到item10出现:
convertView仍然是空值,因为recycler中没有视图(item1的边缘仍然可见,在顶端)
02-05 13:48:25.169: INFO/System.out(947): getView 9 null
再滚动List
convertView不是空值了!item1离开屏幕到Recycler中去了,然后item11被创建
02-05 13:48:42.879: INFO/System.out(947): getView 10 android.widget.LinearLayout@437430f8
再滚动:
02-05 14:01:31.069: INFO/System.out(947): getView 11 android.widget.LinearLayout@437447d0
02-05 14:01:31.142: INFO/System.out(947): getView 12 android.widget.LinearLayout@43744ff8
02-05 14:01:31.279: INFO/System.out(947): getView 13 android.widget.LinearLayout@43743fa8
02-05 14:01:31.350: INFO/System.out(947): getView 14 android.widget.LinearLayout@43745820
02-05 14:01:31.429: INFO/System.out(947): getView 15 android.widget.LinearLayout@43746048
02-05 14:01:31.550: INFO/System.out(947): getView 16 android.widget.LinearLayout@43746870
02-05 14:01:31.669: INFO/System.out(947): getView 17 android.widget.LinearLayout@43747098
02-05 14:01:31.839: INFO/System.out(947): getView 18 android.widget.LinearLayout@437478c0
02-05 14:03:30.900: INFO/System.out(947): getView 19 android.widget.LinearLayout@43748df0
02-05 14:03:32.069: INFO/System.out(947): getView 20 android.widget.LinearLayout@437430f8
convertView 如我们所期待的非空了,在item11离开屏幕之后,它的视图(@437430f8)作为convertView容纳item21了
不同的项目布局(item layout)
我们再举一个稍微复杂的例子,在上例的list中加入一些分隔线
你需要做这些:
重(@Override)写 getViewTypeCount() �C 返回你有多少个不同的布局
重写 getItemViewType(int) �C 由position返回view type id
根据view item的类型,在getView中创建正确的convertView
以下是代码:
001
public class MultipleItemsList extends ListActivity {
002
003
private MyCustomAdapter mAdapter;
004
005
@Override
006
public void onCreate(Bundle savedInstanceState) {
007
super.onCreate(savedInstanceState);
008
mAdapter = new MyCustomAdapter();
009
for (int i = 1; i < 50; i++) {
010
mAdapter.addItem("item " + i);
011
if (i % 4 == 0) {
012
mAdapter.addSeparatorItem("separator " + i);
013
}
014
}
015
setListAdapter(mAdapter);
016
}
017
018
private class MyCustomAdapter extends BaseAdapter {
019
020
private static final int TYPE_ITEM = 0;
021
private static final int TYPE_SEPARATOR = 1;
022
private static final int TYPE_MAX_COUNT = TYPE_SEPARATOR + 1;
023
024
private ArrayList mData = new ArrayList();
025
private LayoutInflater mInflater;
026
027
private TreeSet mSeparatorsSet = new TreeSet();
028
029
public MyCustomAdapter() {
030
mInflater = (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE);
031
}
032
033
public void addItem(final String item) {
034
mData.add(item);
035
notifyDataSetChanged();
036
}
037
038
public void addSeparatorItem(final String item) {
039
mData.add(item);
040
// save separator position
041
mSeparatorsSet.add(mData.size() - 1);
042
notifyDataSetChanged();
043
}
044
045
@Override
046
public int getItemViewType(int position) {
047
return mSeparatorsSet.contains(position) ? TYPE_SEPARATOR : TYPE_ITEM;
048
}
049
050
@Override
051
public int getViewTypeCount() {
052
return TYPE_MAX_COUNT;
053
}
054
055
@Override
056
public int getCount() {
057
return mData.size();
058
}
059
060
@Override
061
public String getItem(int position) {
062
return mData.get(position);
063
}
064
065
@Override
066
public long getItemId(int position) {
067
return position;
068
}
069
070
@Override
071
public View getView(int position, View convertView, ViewGroup parent) {
072
ViewHolder holder = null;
073
int type = getItemViewType(position);
074
System.out.println("getView " + position + " " + convertView + " type = " + type);
075
if (convertView == null) {
076
holder = new ViewHolder();
077
switch (type) {
078
case TYPE_ITEM:
079
convertView = mInflater.inflate(R.layout.item1, null);
080
holder.textView = (TextView)convertView.findViewById(R.id.text);
081
break;
082
case TYPE_SEPARATOR:
083
convertView = mInflater.inflate(R.layout.item2, null);
084
holder.textView = (TextView)convertView.findViewById(R.id.textSeparator);
085
break;
086
}
087
convertView.setTag(holder);
088
} else {
089
holder = (ViewHolder)convertView.getTag();
090
}
091
holder.textView.setText(mData.get(position));
092
return convertView;
093
}
094
095
}
096
097
public static class ViewHolder {
098
public TextView textView;
099
}
100
}
运行程序,你会看到每4个item一个分割线 看看日志,无异常,所有的convertView都是空的
02-05 15:19:03.080: INFO/System.out(1035): getView 0 null type = 0
02-05 15:19:03.112: INFO/System.out(1035): getView 1 null type = 0
02-05 15:19:03.130: INFO/System.out(1035): getView 2 null type = 0
02-05 15:19:03.141: INFO/System.out(1035): getView 3 null type = 0
02-05 15:19:03.160: INFO/System.out(1035): getView 4 null type = 1
02-05 15:19:03.170: INFO/System.out(1035): getView 5 null type = 0
02-05 15:19:03.180: INFO/System.out(1035): getView 6 null type = 0
02-05 15:19:03.190: INFO/System.out(1035): getView 7 null type = 0
02-05 15:19:03.210: INFO/System.out(1035): getView 8 null type = 0
02-05 15:19:03.210: INFO/System.out(1035): getView 9 null type = 1
滚动list:
02-05 15:19:54.160: INFO/System.out(1035): getView 10 null type = 0
02-05 15:19:57.440: INFO/System.out(1035): getView 11 android.widget.LinearLayout@43744528 type = 0
02-05 15:20:01.310: INFO/System.out(1035): getView 12 android.widget.LinearLayout@43744eb0 type = 0
02-05 15:20:01.880: INFO/System.out(1035): getView 13 android.widget.LinearLayout@437456d8 type = 0
02-05 15:20:02.869: INFO/System.out(1035): getView 14 null type = 1
02-05 15:20:06.489: INFO/System.out(1035): getView 15 android.widget.LinearLayout@43745f00 type = 0
02-05 15:20:07.749: INFO/System.out(1035): getView 16 android.widget.LinearLayout@43747170 type = 0
02-05 15:20:10.250: INFO/System.out(1035): getView 17 android.widget.LinearLayout@43747998 type = 0
02-05 15:20:11.661: INFO/System.out(1035): getView 18 android.widget.LinearLayout@437481c0 type = 0
02-05 15:20:13.180: INFO/System.out(1035): getView 19 android.widget.LinearLayout@437468a0 type = 1
02-05 15:20:16.900: INFO/System.out(1035): getView 20 android.widget.LinearLayout@437489e8 type = 0
02-05 15:20:25.690: INFO/System.out(1035): getView 21 android.widget.LinearLayout@4374a8d8 type = 0
convertView对于分割线是空的,直到第一个分割线可见,当其离开屏幕,视图去到Recycler并且convertView开始起作用。
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