ListView详解

ListView是Android非常重要的控件之一，它有三个要素，分别是listview控件，适配器类，数据源。
ListView优化：

适配器adapter中重写getView方法时，利用convertView是否为空进行view的重用。若convertView为空，则用过inflate()方法加载布局，如果不为空，则直接对convertView进行重用，这样listview在滚动时可以表现更好的性能。
无论inflate()加载布局还是重用convertView都会调用findviewById()方法，避免重复调用，利用ViewHolder类。
listview的点击事件和属性优化: setOnItemClickListener, setOnItemLongClickListener

下面通过一个具体的例子来演示上面的几点优化：
activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
>
<ListView
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/listView"
    android:divider="@color/colorPrimary"
    android:dividerHeight="40dp"
    android:scrollbars="none"/>
    </RelativeLayout>

item_country.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:orientation="horizontal">

  <ImageView
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:id="@+id/iv_flag"/>
  <TextView
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:id="@+id/iv_name"
      android:layout_gravity="center"
      android:layout_marginLeft="10dp"
      android:layout_marginStart="10dp"/>
</LinearLayout>

Country.java

package com.nxyuntui.testproject;

public class Country {
  private String name;
  private int imageId;

  public String getName(){
      return name;
  }

  public int getImageId(){
      return imageId;
  }

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

CountryAdapter.java

public class CountryAdapter extends ArrayAdapter<Country> {

private int resourceId;
public CountryAdapter(Context context, int resource, List<Country> objects) {
    super(context, resource ,objects);
    resourceId = resource;
}

@NonNull
@Override
public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
    Country country = getItem(position);   //获取当前位置条目的数据实例
    View view;
    ViewHolder viewHolder = null;
    if (convertView == null){       
        viewHolder = new ViewHolder();
        //将xml转换成view对象
        view = View.inflate(getContext(),resourceId,null);
        viewHolder.imageView = view.findViewById(R.id.iv_flag);
        viewHolder.textView = view.findViewById(R.id.iv_name);
        view.setTag(viewHolder);    //将viewHolder保存到view对象中
    }else{
        view = convertView;
        viewHolder = (ViewHolder) view.getTag();
    }
    //设置数据
    viewHolder.imageView.setImageResource(country.getImageId());
    viewHolder.textView.setText(country.getName());
    return view;
}

 static class ViewHolder{
    ImageView imageView;
    TextView textView;
 }
}

MainActivity.java

public class MainActivity extends AppCompatActivity   {

private List <Country> countryList = new ArrayList<>();
private ListView listView;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    listView = findViewById(R.id.listView);
    initDatas();
    final CountryAdapter adapter = new CountryAdapter(this,R.layout.item_country,countryList);
    listView.setAdapter(adapter);
    //条目点击事件
    listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {
            Toast.makeText(getApplicationContext(),countryList.get(i).getName(),Toast.LENGTH_SHORT)
                    .show();
        }
    });
    //条目长按事件
    listView.setOnItemLongClickListener(new AdapterView.OnItemLongClickListener() {
        @Override
        public boolean onItemLongClick(AdapterView<?> adapterView, View view, int i, long l) {
            countryList.remove(i);
            adapter.notifyDataSetChanged();
            return true;
        }
    });
}

 public void initDatas(){
    countryList.add(new Country("美国",R.drawable.america));
    countryList.add(new Country("比利时",R.drawable.belgium));
    countryList.add(new Country("巴西",R.drawable.brazil));
    countryList.add(new Country("意大利",R.drawable.italy));
    countryList.add(new Country("葡萄牙",R.drawable.portugal));
    countryList.add(new Country("西班牙",R.drawable.spain));
    countryList.add(new Country("克罗地亚",R.drawable.croatian));
 }
}

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