ListView：ListView是一個能夠?qū)?shù)據(jù)集合以動態(tài)滾動的方式展示到用戶界面上的View梦染。

1. ListView的適配器模式

適配器模式.png

2. ListView的垃圾回收機制RecycleBin

RecycleBin機制.png

3. ListView的優(yōu)化

(1) convertView復(fù)用和viewHolder

//繪制ListView的每個Item
        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            ViewHolder viewHolder;
            if (convertView == null){
                viewHolder = new ViewHolder();
                convertView = LayoutInflater.from(ListViewTestActivity.this).inflate(R.layout.item_listview,null);
                viewHolder.title = convertView.findViewById(R.id.title);
                viewHolder.content= convertView.findViewById(R.id.content);
                convertView.setTag(viewHolder);
            }else {
                viewHolder = (ViewHolder)convertView.getTag();
            }
            viewHolder.title.setText("標題");
            viewHolder.info.setText("內(nèi)容");
            return convertView;
        }

        class ViewHolder{
            private TextView title;
            private TextView content;
        }

convertView是ListView優(yōu)化最重要的一點：convertView其實是緩存的作用。利用緩存convertView來做判斷怠惶，當(dāng)緩存中convertView為空時才創(chuàng)建相應(yīng)的View涨缚，存在則調(diào)用已有的View，從而提升了性能策治。
自定義viewHolder：View是一個二叉樹的結(jié)構(gòu)脓魏，每次遍歷都很耗時，利用viewHolder可以減少每次findViewById()的調(diào)用次數(shù)通惫，從而在時間上進行優(yōu)化茂翔。
(2) 三級緩存和監(jiān)聽滑動事件
三級緩存：圖片加載時需要用到緩存機制來進行。
監(jiān)聽滑動事件：盡量不要在getView()中做耗時操作履腋，保證ListView滑動的流暢性珊燎，避免滑動時卡頓。如果一定要在ListView滑動時加載，可以設(shè)置ListView的監(jiān)聽事件俐末，當(dāng)滑動停止時再去加載整個圖片。
(3) 其他

• ListView的Item布局中盡量避免半透明的元素：半透明的繪制比不透明更耗時
• 可以開啟硬件加速

4. ListView使用

(1) ListViewTestActivity.java

package comi.example.liy.mytestdemo;

import android.content.Context;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v7.app.AppCompatActivity;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.ListView;
import android.widget.TextView;

import java.util.ArrayList;

/**
 * Created by liy on 2019-11-08 15:38
 */
public class ListViewTestActivity extends AppCompatActivity {

    private ListView listView;
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_listview);
        listView = findViewById(R.id.listview);
        listView.setAdapter(new MyAdapter(this));
    }

    class MyAdapter extends BaseAdapter{
        private Context context;
        private ArrayList<NewsInfo> newsInfos = new ArrayList<>();

        public MyAdapter(Context context) {
            this.context = context;
            newsInfos = new NewsInfo().getNewsInfos();
        }

        //計算數(shù)據(jù)資源的長度:有n條數(shù)據(jù)就會調(diào)用n次getView()方法來繪制Item奄侠，當(dāng)數(shù)據(jù)長度很多時卓箫，ListView的recycleBin機制(內(nèi)存回收機制)保證滑動時不會發(fā)生OOM(內(nèi)存泄漏)
        @Override
        public int getCount() {
            if (newsInfos == null){
                return 0;
            }
            return newsInfos.size();
        }

        @Override
        public Object getItem(int position) {
            return newsInfos.get(position);
        }

        @Override
        public long getItemId(int position) {
            return position;
        }

        //繪制ListView的每個Item
        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            ViewHolder viewHolder;
            if (convertView == null){
                convertView = LayoutInflater.from(ListViewTestActivity.this).inflate(R.layout.item_listview,null);
                viewHolder = new ViewHolder();
                viewHolder.title = convertView.findViewById(R.id.title);
                viewHolder.content = convertView.findViewById(R.id.content);
                convertView.setTag(viewHolder);
            }else {
                viewHolder = (ViewHolder)convertView.getTag();
            }
            viewHolder.title.setText(newsInfos.get(position).getTitle());
            viewHolder.content.setText(newsInfos.get(position).getContent());
            return convertView;
        }

        class ViewHolder{
            private TextView title;
            private TextView content;
        }
    }

}

(2) NewsInfo.java

package comi.example.liy.mytestdemo;

import java.util.ArrayList;

/**
 * Created by liy on 2019-11-08 15:54
 */
public class NewsInfo {
    private String title;
    private String content;

    public NewsInfo() {

    }

    public NewsInfo(String title, String content) {
        this.title = title;
        this.content= content;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getContent() {
        return content;
    }

    public void setContent(String content) {
        this.content = content;
    }

    //創(chuàng)建測試數(shù)據(jù)
    public ArrayList<NewsInfo> getNewsInfos(){
        ArrayList<NewsInfo> newsInfos = new ArrayList<>();
        newsInfos.add(new NewsInfo("標題1","信息內(nèi)容1"));
        newsInfos.add(new NewsInfo("標題2","信息內(nèi)容2"));
        newsInfos.add(new NewsInfo("標題3","信息內(nèi)容3"));
        newsInfos.add(new NewsInfo("標題4","信息內(nèi)容4"));
        newsInfos.add(new NewsInfo("標題5","信息內(nèi)容5"));
        newsInfos.add(new NewsInfo("標題6","信息內(nèi)容6"));
        newsInfos.add(new NewsInfo("標題7","信息內(nèi)容7"));
        newsInfos.add(new NewsInfo("標題8","信息內(nèi)容8"));
        newsInfos.add(new NewsInfo("標題9","信息內(nèi)容9"));
        newsInfos.add(new NewsInfo("標題10","信息內(nèi)容10"));
        return newsInfos;
    }

}

(3) activity_listview.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <ListView
        android:id="@+id/listview"
        android:layout_width="match_parent"
        android:layout_height="match_parent">
    </ListView>
</LinearLayout>

(4) item_listview.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">
    <android.support.v7.widget.LinearLayoutCompat
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical">
        <TextView
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:id="@+id/title"
            android:textSize="20sp"/>
        <TextView
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:id="@+id/content"
            android:textSize="16sp"/>
    </android.support.v7.widget.LinearLayoutCompat>
</LinearLayout>

(5) 運行效果

運行效果.png

拓展：實現(xiàn)ListView的橫向滑動可置頂或刪除列表項。