Android最常用和最难用的控件--ListView

最常用和最难用的控件—ListView

ListView的简单用法

//在布局中加入ListView控件，高度和宽度设置为match_parent，让其占满整个空间
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">
//ListView用于展示大量数据
    <ListView
        android:id="@+id/list_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>
</androidx.constraintlayout.widget.ConstraintLayout>
            
//修改MainActivity中的代码
public class MainActivity extends AppCompatActivity {
    //事先创建好一组数据，但数组中的数据是无法直接传递给ListView的，所以要借助适配器完成
    private String[] data = {"Apple","banner","Orange","Watermelon","pear","grape","pineapple","strawberry","cherry","mango",
            "Apple","banner","Orange","Watermelon","pear","grape","pineapple","strawberry","cherry","mango"};

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        //ArrayAdapter可以通过泛型来指定要适配的数据类型，然后在构造函数中把要适配的数据传入
        //ArrayAdapter有多个构造函数的重载，可以根据实际情况选择使用
        //由于提供的数据为字符串，所以这里的泛型指定为String，然后在ArrayAdapter的构造函数中一次传入当前上下文、ListView子项布局的id，以及要适配的数据
       //simple_list_item_1作为ListView子项布局的id，这是哟个Android内置的布局文件，里面只有一个TextView，用于简单的显示一段文本
        ArrayAdapter<String> adapter = new ArrayAdapter<String>(MainActivity.this,android.R.layout.simple_list_item_1,data);
        ListView listView = (ListView) findViewById(R.id.list_view);
        //最后调用setAdapter()方法将构建好的适配器对象传递进去，这样ListView和数据之间的关联就建立成了
        listView.setAdapter(adapter);
    }
} 

• 运行结果

定制ListView的界面

• 实例说明：准备好一组图片，分别对应好以上提供的每一种说过，让水果的名称旁边都有一个图样

//新建Friut类，作为ListView的适配类型
package com.example.listviewtest;
public class Fruit {
   //水果的名字
   private String name;
   //水果对应的图片
   private int imageId;
   //设置水果的名字和图片
   public Fruit(String name,int imageId){
       this.name = name;
       this.imageId = imageId;
   }
   public  String getName(){
       return  name;
   }
   public  int getImageId(){
       return imageId;
   }
}

//在layout目录下新建fruit_item.xml   为ListView的子项指定一个我们自定义的布局
<LinearLayout
   xmlns:android="http://schemas.android.com/apk/res/android"
   android:layout_width="match_parent"
   android:layout_height="wrap_content">
//图片
   <ImageView
       android:id="@+id/fruit_image"
       android:layout_width="100dp"
       android:layout_height="100dp"/>
//名称
   <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>
           
//创建一个自定义的适配器，这个适配器继承自ArrayAdapter，并将泛型指定为Friut类，新建类FriutAdapter
//FruitAdapter继承自ArrayAdapter，泛型为Fruit
public class FruitAdapter extends ArrayAdapter<Fruit> {
   private int resourceId;
   //FruitAdapter重写了一组父类的构造函数，将上下文、ListView子项布局的id、数据都传递进来
   public FruitAdapter(Context context, int textViewResourceId, List<Fruit> objects){
       super(context,textViewResourceId,objects);
       resourceId = textViewResourceId;

   }
   //重写了getView()方法，这个方法在每个子项被滚动到屏幕内的时候会被调用
   @Override
   public View getView(int position, View convertView,ViewGroup parent) {
       //使用getItem()方法得到Fruit实例
       Fruit fruit = getItem(position);
       //使用LayoutInflater来为这个子项加载我们传入的布局
       //LayoutInflater的inflate()方法接收3个参数，第三个参数false，表示只让我们在父布局中声明layout属性生效，但补位这个View加载父布局，因为一旦有了父布局之后，他就不能再添加到ListView中了
       View view = LayoutInflater.from(getContext()).inflate(resourceId,parent,false);
       //得到ImageView实例
       ImageView fruitImage = (ImageView) view.findViewById(R.id.fruit_image);
       //得到TextView实例
       TextView fruitNmae = (TextView) view.findViewById(R.id.fruit_name);
       //使用setImageResource()方法显示图片
       fruitImage.setImageResource(fruit.getImageId());
       //setText()方法显示名称
       fruitNmae.setText(fruit.getName());
       //将布局返回
       return view;
   }
}

//修改MainActivity
package com.example.listviewtest;
public class MainActivity extends AppCompatActivity {
   private List<Fruit> fruitList = new ArrayList<>();
   @Override
   protected void onCreate(Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       setContentView(R.layout.activity_main);
       //初始化所有的水果数据
       initFruits();
       //创建FruitAdapter对象，并将FruitAdapter作为适配器传递给ListView，
       FruitAdapter adapter = new FruitAdapter(MainActivity.this,R.layout.fruit_item,fruitList);
       ListView listView = (ListView) findViewById(R.id.list_view);
       listView.setAdapter(adapter);
   }

   private void initFruits() {
       //使用for循环将数据加载了两遍
       for (int i = 0;i<2;i++){
           //在Fruit类的构造函数中将水果的名称和对应的图片传入，然后把创建好的对象添加到水果列表中
           Fruit apple = new Fruit("Apple",R.drawable.apple);
           fruitList.add(apple);
           Fruit banner = new Fruit("Banner",R.drawable.banner);
           fruitList.add(banner);
           Fruit orange = new Fruit("Orange",R.drawable.orange);
           fruitList.add(orange);
           Fruit watermelon = new Fruit("Watermelon",R.drawable.watermelon);
           fruitList.add(watermelon);
           Fruit pear = new Fruit("Pear",R.drawable.pear);
           fruitList.add(pear);
           Fruit grape = new Fruit("Grape",R.drawable.grape);
           fruitList.add(grape);
           Fruit pineapple = new Fruit("Pineapple",R.drawable.pineapple);
           fruitList.add(pineapple);
           Fruit strawberry = new Fruit("Strawberry",R.drawable.strawberry);
           fruitList.add(strawberry);
           Fruit cherry = new Fruit("Cherry",R.drawable.cherry);
           fruitList.add(cherry);
           Fruit mango = new Fruit("Mango",R.drawable.mango);
           fruitList.add(mango);

       }
   }
}

• 运行结果
  

提升ListView的运行效率

• 以上例子虽然实现了功能，但是ListView的运行效率还是很低，因为在FruitAdapter的getView()方法中，每次都将布局重新加载了一遍，当ListView快速滚动的时候，这就会成为性能的瓶颈

• 解决布局重复加载问题

  //修改FriutAdapter
  public class FruitAdapter extends ArrayAdapter<Fruit> {
      private int resourceId;
      public FruitAdapter(Context context, int textViewResourceId, List<Fruit> objects){
          super(context,textViewResourceId,objects);
          resourceId = textViewResourceId;
  
      }
  
      @Override
      //convertView这个参数用于将之前加载好的布局进行缓存，以便之后可以进行重用
      public View getView(int position, View convertView,ViewGroup parent) {
          Fruit fruit = getItem(position);
          View view;
          //对convertView进行判断，如果convertView为null，就使用LayoutInflater加载布局
          if(convertView == null){
              view = LayoutInflater.from(getContext()).inflate(resourceId,parent,false);      
          }else{  //如果convertView不为null，就对convertView进行重用
              view = convertView;
              viewHolder = (ViewHolder) view.getTag();
          }
          ImageView fruitImage = (ImageView) view.findViewById(R.id.fruit_image);
          TextView fruitNmae = (TextView) view.findViewById(R.id.fruit_name);
           fruitImage.setImageResource(fruit.getImageId());
           fruitNmae.setText(fruit.getName());
           return view;
      }
  }
  
  

• 上述操作通过判断convertView是否为null来控制是否加载布局，解决了布局重复加载的问题。但是每次在getView()方法中还是会调用View的findViewById()方法来获取一次控件的实例，我们可以借助ViewHolder来对这部分性能进行优化

  //修改FriutAdapter
  public class FruitAdapter extends ArrayAdapter<Fruit> {
      private int resourceId;
      public FruitAdapter(Context context, int textViewResourceId, List<Fruit> objects){
          super(context,textViewResourceId,objects);
          resourceId = textViewResourceId;
  
      }
  
      @Override
      //convertView这个参数用于将之前加载好的布局进行缓存，以便之后可以进行重用
      public View getView(int position, View convertView,ViewGroup parent) {
          Fruit fruit = getItem(position);
          View view;
          ViewHolder viewHolder;
          //对convertView进行判断，如果convertView为null，就使用LayoutInflater加载布局
          if(convertView == null){
              view = LayoutInflater.from(getContext()).inflate(resourceId,parent,false);
              //convertView为null时创建ViewHolder对象
              viewHolder = new ViewHolder();
              //将控件的实例都存放在ViewHolder里
              viewHolder.fruitImage = (ImageView) view.findViewById(R.id.fruit_image);
              viewHolder.fruitNmae = (TextView) view.findViewById(R.id.fruit_name);
              //调用View的setTag()方法将ViewHolder对象存在View中
              view.setTag(viewHolder);
          }else{  //如果convertView不为null，就对convertView进行重用
              view = convertView;
              //convertView不为null时则调用View的getTag()ff ,把 ViewHolder重新取出，这样所有控件的实例就都缓存在了ViewHolder里，就不必每次都通过findViewById()方法来获取实例了
              viewHolder = (ViewHolder) view.getTag();
          }
           viewHolder.fruitImage.setImageResource(fruit.getImageId());
           viewHolder.fruitNmae.setText(fruit.getName());
           return view;
      }
      //新增内部类ViewHolder，用于对控件的实例进行缓存
      class ViewHolder{
          ImageView fruitImage;
          TextView fruitNmae;
      }
  }
  
  

ListView的点击事件

• 点击ListView的任何一个子项，弹出提示

  //修改MainActivity
  package com.example.listviewtest;
  public class MainActivity extends AppCompatActivity {
      private List<Fruit> fruitList = new ArrayList<>();
      @Override
      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_main);
          //初始化所有的水果数据
          initFruits();
          //创建FruitAdapter对象，并将FruitAdapter作为适配器传递给ListView，
          FruitAdapter adapter = new FruitAdapter(MainActivity.this,R.layout.fruit_item,fruitList);
          ListView listView = (ListView) findViewById(R.id.list_view);
          listView.setAdapter(adapter);
          
          //使用setOnItemClickListener()方法为ListView注册了一个件监听器，
          // 当点击ListView中的任何一个子项时，会回调OnItemClick()方法，这个方法通过position判断用户点击的是哪一项，然后会获得相应的数据，并弹出提示
          listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
              @Override
              public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                  Fruit fruit = fruitList.get(position);
                  Toast.makeText(MainActivity.this,fruit.getName(),Toast.LENGTH_SHORT).show();
              }
          });
      }
  
      private void initFruits() {
          //使用for循环将数据加载了两遍
          for (int i = 0;i<2;i++){
              //在Fruit类的构造函数中将水果的名称和对应的图片传入，然后把创建好的对象添加到水果列表中
              Fruit apple = new Fruit("Apple",R.drawable.apple);
              fruitList.add(apple);
              Fruit banner = new Fruit("Banner",R.drawable.banner);
              fruitList.add(banner);
              Fruit orange = new Fruit("Orange",R.drawable.orange);
              fruitList.add(orange);
              Fruit watermelon = new Fruit("Watermelon",R.drawable.watermelon);
              fruitList.add(watermelon);
              Fruit pear = new Fruit("Pear",R.drawable.pear);
              fruitList.add(pear);
              Fruit grape = new Fruit("Grape",R.drawable.grape);
              fruitList.add(grape);
              Fruit pineapple = new Fruit("Pineapple",R.drawable.pineapple);
              fruitList.add(pineapple);
              Fruit strawberry = new Fruit("Strawberry",R.drawable.strawberry);
              fruitList.add(strawberry);
              Fruit cherry = new Fruit("Cherry",R.drawable.cherry);
              fruitList.add(cherry);
              Fruit mango = new Fruit("Mango",R.drawable.mango);
              fruitList.add(mango);
  
          }
      }
  }
  

  • 效果图

本文地址：https://blog.csdn.net/weixin_43704586/article/details/112513698