利用ViewHolder优化自定义Adapter的典型写法 最近写Adapter写得多了，慢慢就熟悉了。 用ViewHolder，主要是进行一些性能优化，减少一些不必要的重复操作。（WXD同学教我的。） 具体不分析了
利用ViewHolder优化自定义Adapter的典型写法
最近写Adapter写得多了，慢慢就熟悉了。
用ViewHolder，主要是进行一些性能优化，减少一些不必要的重复操作。（WXD同学教我的。）
具体不分析了，直接上一份代码吧：
public class MarkerItemAdapter extends BaseAdapter
{
private Context mContext = null;
private List<MarkerItem> mMarkerData = null;
public MarkerItemAdapter(Context context, List<MarkerItem> markerItems)
{
mContext = context;
mMarkerData = markerItems;
}
public void setMarkerData(List<MarkerItem> markerItems)
{
mMarkerData = markerItems;
}
@Override
public int getCount()
{
int count = 0;
if (null != mMarkerData)
{
count = mMarkerData.size();
}
return count;
}
@Override
public MarkerItem getItem(int position)
{
MarkerItem item = null;
if (null != mMarkerData)
{
item = mMarkerData.get(position);
}
return item;
}
@Override
public long getItemId(int position)
{
return position;
}
@Override
public View getView(int position, View convertView, ViewGroup parent)
{
ViewHolder viewHolder = null;
if (null == convertView)
{
viewHolder = new ViewHolder();
LayoutInflater mInflater = LayoutInflater.from(mContext);
convertView = mInflater.inflate(R.layout.item_marker_item, null);
viewHolder.name = (TextView) convertView.findViewById(R.id.name);
viewHolder.description = (TextView) convertView
.findViewById(R.id.description);
viewHolder.createTime = (TextView) convertView
.findViewById(R.id.createTime);
convertView.setTag(viewHolder);
}
else
{
viewHolder = (ViewHolder) convertView.getTag();
}
// set item values to the viewHolder:
MarkerItem markerItem = getItem(position);
if (null != markerItem)
{
viewHolder.name.setText(markerItem.getName());
viewHolder.description.setText(markerItem.getDescription());
viewHolder.createTime.setText(markerItem.getCreateDate());
}
return convertView;
}
private static class ViewHolder
{
TextView name;
TextView description;
TextView createTime;
}
}
其中MarkerItem是自定义的类，其中包含name，description，createTime等字段，并且有相应的get和set方法。
ViewHolder是一个内部类，其中包含了单个项目布局中的各个控件。
单个项目的布局，即R.layout.item_marker_item如下：
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical"
android:padding="5dp">
<TextView
android:id="@+id/name"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="Name"
android:textSize="20sp"
android:textStyle="bold" />
<TextView
android:id="@+id/description"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="Description"
android:textSize="18sp" />
<TextView
android:id="@+id/createTime"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="CreateTime"
android:textSize="16sp" />
</LinearLayout>
官方的API Demos中也有这个例子：
package com.example.android.apis.view中的List14：
/*
* Copyright (C) 2008 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.example.android.apis.view;
import android.app.ListActivity;
import android.content.Context;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.TextView;
import android.widget.ImageView;
import android.graphics.BitmapFactory;
import android.graphics.Bitmap;
import com.example.android.apis.R;
/**
* Demonstrates how to write an efficient list adapter. The adapter used in this example binds
* to an ImageView and to a TextView for each row in the list.
*
* To work efficiently the adapter implemented here uses two techniques:
* - It reuses the convertView passed to getView() to avoid inflating View when it is not necessary
* - It uses the ViewHolder pattern to avoid calling findViewById() when it is not necessary
*
* The ViewHolder pattern consists in storing a data structure in the tag of the view returned by
* getView(). This data structures contains references to the views we want to bind data to, thus
* avoiding calls to findViewById() every time getView() is invoked.
*/
public class List14 extends ListActivity {
private static class EfficientAdapter extends BaseAdapter {
private LayoutInflater mInflater;
private Bitmap mIcon1;
private Bitmap mIcon2;
public EfficientAdapter(Context context) {
// Cache the LayoutInflate to avoid asking for a new one each time.
mInflater = LayoutInflater.from(context);
// Icons bound to the rows.
mIcon1 = BitmapFactory.decodeResource(context.getResources(), R.drawable.icon48x48_1);
mIcon2 = BitmapFactory.decodeResource(context.getResources(), R.drawable.icon48x48_2);
}
/**
* The number of items in the list is determined by the number of speeches
* in our array.
*
* @see android.widget.ListAdapter#getCount()
*/
public int getCount() {
return DATA.length;
}
/**
* Since the data comes from an array, just returning the index is
* sufficent to get at the data. If we were using a more complex data
* structure, we would return whatever object represents one row in the
* list.
*
* @see android.widget.ListAdapter#getItem(int)
*/
public Object getItem(int position) {
return position;
}
/**
* Use the array index as a unique id.
*
* @see android.widget.ListAdapter#getItemId(int)
*/
public long getItemId(int position) {
return position;
}
/**
* Make a view to hold each row.
*
* @see android.widget.ListAdapter#getView(int, android.view.View,
* android.view.ViewGroup)
*/
public View getView(int position, View convertView, ViewGroup parent) {
// A ViewHolder keeps references to children views to avoid unneccessary calls
// to findViewById() on each row.
ViewHolder holder;
// When convertView is not null, we can reuse it directly, there is no need
// to reinflate it. We only inflate a new View when the convertView supplied
// by ListView is null.
if (convertView == null) {
convertView = mInflater.inflate(R.layout.list_item_icon_text, null);
// Creates a ViewHolder and store references to the two children views
// we want to bind data to.
holder = new ViewHolder();
holder.text = (TextView) convertView.findViewById(R.id.text);
holder.icon = (ImageView) convertView.findViewById(R.id.icon);
convertView.setTag(holder);
} else {
// Get the ViewHolder back to get fast access to the TextView
// and the ImageView.
holder = (ViewHolder) convertView.getTag();
}
// Bind the data efficiently with the holder.
holder.text.setText(DATA[position]);
holder.icon.setImageBitmap((position & 1) == 1 ? mIcon1 : mIcon2);
return convertView;
}
static class ViewHolder {
TextView text;
ImageView icon;
}
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setListAdapter(new EfficientAdapter(this));
}
private static final String[] DATA = Cheeses.sCheeseStrings;
}
其中布局：
<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright (C) 2007 The Android Open Source Project
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
-->
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="horizontal"
android:layout_width="match_parent"
android:layout_height="match_parent">
<ImageView android:id="@+id/icon"
android:layout_width="48dip"
android:layout_height="48dip" />
<TextView android:id="@+id/text"
android:layout_gravity="center_vertical"
android:layout_width="0dip"
android:layout_weight="1.0"
android:layout_height="wrap_content" />
</LinearLayout>
更多关于Adapter优化的文章：
ViewHolder到底用什么修饰？static？final？static final？
原文中提到：
对于这篇文章，我理解。ViewHolder的构造和复用，与静不静态没关。核心是ListView（AdapterView），通过getView(int position, View convertView, ViewGroup parent) 的convertView会为开发者传入一个可以复用的对象。开发者需要利用该对象，减少应用内存的消耗。
如果从减少内存消耗的角度来开。我认为ViewHolder还是应该修饰成static比较好。这样ViewHolder中可以减少MainActivity的this指针，由于减少了一个this指针的引用，也会对MainActivity的引用计数大大减少。MainActivity的this指针继承于android的Context上下文，对于Context的回收遗漏，是Android内存管理中很大的问题。我们减少了对Context的引用，可以更容易减少Context引用计数出现问题。