【Android UI】ListView使用ViewHolder模式提高效率

ViewHolder模式是提高ListView效率的一个很重要的方法。ViewHolder模式充分利用了ListView的视图缓存机制，避免了每次在调用getView()的时候都去通过findViewById()实例化控件。根据测试，使用ViewHolder将提高50%以上的效率。使用方式是在Adapter中定义一个内部类ViewHolder，并将布局中的控件作为成员变量。

public final class ViewHolder {
    public ImageView img;
    public TextView title;
}

接下来，只要在getView()方法中通过视图缓存机制来重用以缓存即可，下面给出具体的自定义的NotifyAdapter的代码，里面使用了ViewHolder机制。

package com.yzx.mylistview;

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.ImageView;
import android.widget.TextView;

import java.util.List;

/**
 * Created by yzx on 2016/5/2.
 */
public class NotifyAdapter extends BaseAdapter {
    private List<String> mData;
    private LayoutInflater mInflater;

    public NotifyAdapter(Context context,List<String> data){
        this.mData = data;
        mInflater = LayoutInflater.from(context);
    }

    @Override
    public int getCount() {
        return mData.size();
    }

    @Override
    public Object 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;
        //判断是否缓存
        if(convertView == null){
            holder = new ViewHolder();
            // 通过LayoutInflater实例化布局
            convertView = mInflater.inflate(R.layout.notify_item,null);
            holder.img = (ImageView) convertView.findViewById(R.id.imageView);
            holder.title = (TextView) convertView.findViewById(R.id.textView);
            convertView.setTag(holder);
        }else{
            // 通过tag找到缓存的布局
            holder = (ViewHolder) convertView.getTag();
        }
        // 设置布局中控件要显示的视图
        holder.img.setBackgroundResource(R.drawable.ic_launcher);
        holder.title.setText(mData.get(position));
        return convertView;
    }

    public final class ViewHolder {
        public ImageView img;
        public TextView title;
    }
}

给出NotifyAdapter的使用示例：

package com.yzx.mylistview;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.ListView;

import java.util.ArrayList;
import java.util.List;

/**
 * Created by yzx on 2016/5/2.
 */
public class NotifyTest extends Activity {

    private List<String> mData;
    private ListView mListView;
    private NotifyAdapter mAdapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.notify);
        mData = new ArrayList<String>();
        for(int i=0; i<20; i++){
            mData.add(""+i);
        }
        mListView = (ListView) findViewById(R.id.listview);
        mAdapter = new NotifyAdapter(this,mData);
        mListView.setAdapter(mAdapter);
    }

    public void btnAdd(View view){
        mData.add("new");
        //动态更新ListView
        mAdapter.notifyDataSetChanged();
        //设置ListView需要显示在第几项
        mListView.setSelection(mData.size()-1);
    }
}

运行结果：