I was surprised that getViewTypeCount() is so rarely overrided (codesearch).
If you are an expert in this – this post is not for you:-)
ListView and Adapter Basics
How it works:
-
ListView asks adapter “give me a view” (getView) for each item of the list
-
A new View is returned and displayed
Next question – what if we have one billion items? Create new view for each item? The answer is NO:-) Android caches views for you.
There’s a component in Android called “Recycler”. I drawed a picture based on Romain Guypresentation
at Google IO ’09.
-
If you have 1 billion items – there are only visible items in the memory + view in recycler.
-
ListView asks for a view type1 first time (getView) x visible items. convertView is null in getView – you create new view of type1 and return it.
-
ListView asks for a view type1 when one item1 is outside of the window and new item the same type is comming from the bottom. convertView is not null = item1. You should just set new data and return convertView back. No need to create view again.
Let’s write a simple code and put System.out to the getView:
public class MultipleItemsList extends ListActivity {
private MyCustomAdapter mAdapter;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mAdapter = new MyCustomAdapter();
for (int i = 0; i < 50; i++) {
mAdapter.addItem("item " + i);
}
setListAdapter(mAdapter);
}
private class MyCustomAdapter extends BaseAdapter {
private ArrayList mData = new ArrayList();
private LayoutInflater mInflater;
public MyCustomAdapter() {
mInflater = (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE);
}
public void addItem(final String item) {
mData.add(item);
notifyDataSetChanged();
}
@Override
public int getCount() {
return mData.size();
}
@Override
public String getItem(int position) {
return mData.get(position);
}
@Override
public long getItemId(int position) {
return position;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
System.out.println("getView " + position + " " + convertView);
ViewHolder holder = null;
if (convertView == null) {
convertView = mInflater.inflate(R.layout.item1, null);
holder = new ViewHolder();
holder.textView = (TextView)convertView.findViewById(R.id.text);
convertView.setTag(holder);
} else {
holder = (ViewHolder)convertView.getTag();
}
holder.textView.setText(mData.get(position));
return convertView;
}
}
public static class ViewHolder {
public TextView textView;
}
}
Run the program and see what happens:
getView was called 9 times. convertView is null for all visible items
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
Then scroll the list slightly down (until item 10 appears):
convertView is still null because there is still no view in the recycler (border of item1 is still visible at the top:))
02-05 13:48:25.169: INFO/System.out(947): getView 9 null
Let’s scroll list a little more:
Bingo! convertView is not null: item1 goes off the screen directly to the Recycler and item11 is created based on item1.
02-05 13:48:42.879: INFO/System.out(947): getView 10 android.widget.LinearLayout@437430f8
scroll more just to check what hapens:
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 is not null as we expected. After item11 goes off the screen, it view (@437430f8) comes as convertView for item 21. simple.
Different list items’ layouts
Let’s move to the “more complicated” example. How about to add separator somewhere to the list.
You should do the following:
-
Override getViewTypeCount() – return how many different view layouts you have
-
Override getItemViewType(int) – return correct view type id by position
-
Create correct convertView (depending on view item type) in getView
Simple, isn’t it? Code snippet:
public class MultipleItemsList extends ListActivity {
private MyCustomAdapter mAdapter;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mAdapter = new MyCustomAdapter();
for (int i = 1; i < 50; i++) {
mAdapter.addItem("item " + i);
if (i % 4 == 0) {
mAdapter.addSeparatorItem("separator " + i);
}
}
setListAdapter(mAdapter);
}
private class MyCustomAdapter extends BaseAdapter {
private static final int TYPE_ITEM = 0;
private static final int TYPE_SEPARATOR = 1;
private static final int TYPE_MAX_COUNT = TYPE_SEPARATOR + 1;
private ArrayList mData = new ArrayList();
private LayoutInflater mInflater;
private TreeSet mSeparatorsSet = new TreeSet();
public MyCustomAdapter() {
mInflater = (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE);
}
public void addItem(final String item) {
mData.add(item);
notifyDataSetChanged();
}
public void addSeparatorItem(final String item) {
mData.add(item);
// save separator position
mSeparatorsSet.add(mData.size() - 1);
notifyDataSetChanged();
}
@Override
public int getItemViewType(int position) {
return mSeparatorsSet.contains(position) ? TYPE_SEPARATOR : TYPE_ITEM;
}
@Override
public int getViewTypeCount() {
return TYPE_MAX_COUNT;
}
@Override
public int getCount() {
return mData.size();
}
@Override
public String getItem(int position) {
return mData.get(position);
}
@Override
public long getItemId(int position) {
return position;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
ViewHolder holder = null;
int type = getItemViewType(position);
System.out.println("getView " + position + " " + convertView + " type = " + type);
if (convertView == null) {
holder = new ViewHolder();
switch (type) {
case TYPE_ITEM:
convertView = mInflater.inflate(R.layout.item1, null);
holder.textView = (TextView)convertView.findViewById(R.id.text);
break;
case TYPE_SEPARATOR:
convertView = mInflater.inflate(R.layout.item2, null);
holder.textView = (TextView)convertView.findViewById(R.id.textSeparator);
break;
}
convertView.setTag(holder);
} else {
holder = (ViewHolder)convertView.getTag();
}
holder.textView.setText(mData.get(position));
return convertView;
}
}
public static class ViewHolder {
public TextView textView;
}
}
Let’s run what we wrote. Yo will see separators after each 4-th item in the list.
In the log – nothing exceptional – all convertView is null for visible items both types.
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
Scroll list and see what happens:
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 is null for separator view type until first separator is visible. When it goes off the screen – view also comes to the Recycler and convertView comes to play.
本文转自http://android.amberfog.com/?p=296
分享到:
相关推荐
ListView中getView重用好多次,有头像的布局
android listview getview多调用俩周期
it's a normal Android listview adapter.The adapter extends BaseAdapter...
基于Eclipse开发平台,Android中使用ListView和三种适配器(ArrayAdaper,SimpleAdapter,BaseAdapter)实现数据的填充。
Adapter的一种写法 package com.xxkjx.jiusanqi; import java.util.List; import android.content.Context; import android.view.LayoutInflater; import android.view.View; import android.view.ViewGroup; ...
Android listview与adapter操作和用法
listview适配器,包含 ListViewAdapter和ViewHolder以及简单使用demo代码。该适配器能快速简单绑定数据和操作显示数据。可以对其扩充修改
这是我一下午的总结,listview常用的方法,和三种基本适配器以及实际开发中通过泛型的适配器以及万能适配器,如果大家感兴趣,这里有文章的地址:http://write.blog.csdn.net/postlist
ListViewAdapter和ViewHolder代码,全面的安卓listview适配器源码。使得使用listview绑定数据显示非常方面
Android ListView adapter不同布局,当需要用到ListView叠加嵌套时候可以考虑
安卓listview自定义adapter包括demo和一些其他自定义绑定的代码
Android之自定义Adapter的ListView
ListView Adapter优化 实例,需要的朋友可以参考一下
ListView 通过adapter传递对象
主要介绍了Android ListView与getView调用卡顿问题解决办法的相关资料,这里提供实例及解决办法帮助大家解决这种问题,需要的朋友可以参考下
listview中通用adapter,方法封装,调用方便简单
1)了解 listview 和 adapter 之间怎么进行数据匹配
Android自定义adapter的listview代码
新闻列表实例NewsListView出自创新实验项目之宁夏大学校园通>项目一处。
ListView与BaseAdapter的配套使用 理解android 中adapter适配的作用 以及ListView的优化方式