ListView性能优化实践-优快云博客

本文链接：https://blog.youkuaiyun.com/Jay_zjc/article/details/79773910

ListView基本用法及性能优化

一、ListView最简单的用法：借助适配器直接将数据传递给ListView。此处使用ArrayAdapter，并通过其构造方法将适配的数据传入。见ListViewActivity.java中注释一。

二、自定义ListView:实体类作为XXAdapter的适配类型、自定义适配器XXAdapter、为ListView的子项指定一个自定义的布局X_item.xml、LayoutInflater为子项加载传入的布局。

三、性能优化：当ListView快速滚动时，getView()方法每次将布局加载一遍的特性决定了版本一的性能低下；在合理利用了convertView将布局缓存以后，大大提高ListView的运行效率，可还是会每次调用view的findViewById()方法；版本三通过自定义一个ViewHolder来对这部分进行优化，将控件实例都放在其中，就不必每次都调用findViewById()了。

四、点击事件：setOnItemClickListener()。

Xml文件：

1.activity_listview.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.zjc.view.ListviewActivity">

    <ListView
        android:id="@+id/list_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</LinearLayout>

2.fruit_item.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <ImageView
        android:id="@+id/fruit_image"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />

    <TextView
        android:id="@+id/fruit_name"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center_vertical"
        android:layout_marginLeft="10dp"/>

</LinearLayout>

java文件

1.Fruit.java

package com.zjc.view;

/**
 * Created by ZJC on 2018-03-31.
 */

public class Fruit {
    private String name;

    private int imageId;

    public Fruit(String name, int imageId) {
        this.name = name;
        this.imageId = imageId;
    }

    public String getName() {
        return name;
    }

    public int getImageId() {
        return imageId;
    }
}

2.FruitAdapter.java

package com.zjc.view;

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;

import java.util.List;

/**
 * Created by ZJC on 2018-03-31.
 */

public class FruitAdapter extends ArrayAdapter<Fruit> {
    private int resourceId;
    public FruitAdapter(Context context, int textViewResourceId, List<Fruit> objects) {
        super(context, textViewResourceId, objects);
        resourceId = textViewResourceId;
    }

    /**
     * 版本一:最慢的加载方式
     * 这个方法在每个子项被滚动到屏幕内的时候会被调用,每次都将布局重新加载了一遍，当ListView快速滚动时，运行效率较为低下
     */
/*    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        Fruit fruit = getItem(position);    //获取当前Fruit实例
        //inflate中的第三个参数设置为false表示只让我们在父布局中声明的layout属性生效，但不会为这个View添加父布局
        //因为View一旦有了父布局后就不能再添加到ListView中了
        View view = LayoutInflater.from(getContext()).inflate(resourceId,parent,false);
        ImageView fruitImage = view.findViewById(R.id.fruit_image);
        TextView fruitName  = view.findViewById(R.id.fruit_name);
        fruitImage.setImageResource(fruit.getImageId());
        fruitName.setText(fruit.getName());
        return view;
    }*/

    /**版本二：正确的加载方式
     * 这个方法在前一个版本的基础上进行了优化，getView()方法中有convertView参数，用于将之前加载好的布局缓存
     */
    /*@Override
    public View getView(int position,  View convertView,  ViewGroup parent) {
        Fruit fruit = getItem(position);
        View view;
        //逻辑：如果convertView为null，就使用LayoutInflater加载布局，否则直接对convertView进行重用
        if (convertView == null) {
            view = LayoutInflater.from(getContext()).inflate(resourceId, parent, false);
        } else {
            view = convertView;
        }
        return view;
    }*/

    /**版本三：最快的加载方式
     * 这个方法在版本二的基础上进一步优化，借助ViewHolder存放控件实例
     */
    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        Fruit fruit = getItem(position);
        ViewHolder holder;
        View view;
        if (convertView == null) {
            view = LayoutInflater.from(getContext()).inflate(resourceId, parent, false);
            holder = new ViewHolder();
            holder.fruitImage = view.findViewById(R.id.fruit_image);
            holder.fruitName = view.findViewById(R.id.fruit_name);
            view.setTag(holder);    //将viewHolder储存在view中
        } else {
            view = convertView;
            holder = (ViewHolder) view.getTag(); //重新获取ViewHolder
        }
        holder.fruitImage.setImageResource(fruit.getImageId());
        holder.fruitName.setText(fruit.getName());
        return view;
    }
}

3.ListViewActivity.java

package com.zjc.view;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ListView;
import android.widget.Toast;

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


public class ListviewActivity extends AppCompatActivity {

    private List<Fruit> fruitList = new ArrayList<>();

    private ListView listView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_listview);
        /**注释一
         ArrayAdapter通过泛型来指定要匹配的数据类型，在构造函数中把要适配的数据传入，此处传入为字符串故泛型指定为String
         在ArrayAdapter的构造函数中依次传入当前上下文、ListView子项布局的id、和要适配的数据
        * */
        /*ArrayAdapter<String> adapter = new ArrayAdapter<String>(
                ListviewActivity.this, android.R.layout.simple_list_item_1,data);
        listView = findViewById(R.id.list_view);
        listView.setAdapter(adapter);*/

        initFruits();   //初始化水果数据
        /**
         * FruitAdapter作为适配器传递给ListView
        * */
        FruitAdapter adapter = new FruitAdapter(ListviewActivity.this,R.layout.fruit_item, fruitList);
        ListView listView = findViewById(R.id.list_view);
        listView.setAdapter(adapter);
        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                Fruit fruit = fruitList.get(position);
                Toast.makeText(ListviewActivity.this,"You clicked "+fruit.getName(),Toast.LENGTH_SHORT).show();
            }
        });
    }

    private void initFruits() {
        for (int i = 0;i < 5; i++){
            Fruit apple = new Fruit("apple", R.drawable.apple);
            fruitList.add(apple);
            Fruit banana = new Fruit("banana", R.drawable.banana);
            fruitList.add(banana);
            Fruit orange = new Fruit("orange", R.drawable.orange);
            fruitList.add(orange);
            Fruit strawberry = new Fruit("strawberry", R.drawable.strawberry);
            fruitList.add(strawberry);
            Fruit grape = new Fruit("grape", R.drawable.grape);
            fruitList.add(grape);
        }
    }
}

4.ViewHolder.java

package com.zjc.view;

import android.widget.ImageView;
import android.widget.TextView;

/**
 * Created by ZJC on 2018-03-31.
 */

class ViewHolder {
    ImageView fruitImage;

    TextView fruitName;
}