Android的ListView使用

ListView

ListView 简单用法

• width和height是match_parent
• 数据可以从数据库,网上下载.也可以是数组
• 数组中的数据是 无法直接传递给ListView的,要借助适配器,其中ArrayAdapter最好用
• ArrayAdapter可以用泛型指定数据类型,在构造函数中吧要适配的数据传入
• ArrayAdapter有多个构造函数重载,要选择最适合的一种.因为传入数据是字符串,所以ArrayAdapter
• 构造函数传入(上下文,ListView子项布局id,适配数据)
• 最后调用ListView的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完成 了

  public class MainActivity extends AppCompatActivity {
  
  private String[] data = {"Apple","Banana","Orange","Watermelon","Pear","Grape",
                              "Pineapple","Strawberry","Cherry","Mango","Apple",
                               "Banana","Orange","Watermelon","Pear","Grape",
                              "Pineapple","Strawberry","Cherry","Mango"};
  
  @Override
  protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
      ArrayAdapter adapter = new ArrayAdapter(
              MainActivity.this,android.R.layout.simple_list_item_1,data);
      ListView listView = (ListView) findViewById(R.id.list_view);
      listView.setAdapter(adapter);
     }
  }
  

定制ListView的界面

1. 定义一个实体类,作为ListView适配器的适配类型,新建Fruit

   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. 自定义布局 LinearLayout 要 TextVIew垂直布局居中

3. 创建适配器

   1. 继承自ArrayAdapter 泛型Fruit 新建类FruitAdapter

   2. Fruit重写一组父类的构造函数,用于将 上下文,ListView子项布局的id和数据传递进来

   3. 重写getView()方法,这个方法每个子项被滚动到屏幕内的时候会被调用.

   4. 在getView()方法中,首先通过getItem()方法得到当前项的Fruit实例,然后使用LayoutInflater 来为这个子项加载传入的布局

   5. inflate()接收三个参数 ()

   6. 调用View的findViewById() 获取ImageView和TextView的实例,并分别调用他们的setImageResource()和setText()方法来设置显示的图片和文字,最后布局返回

      public class FruitAdapter extends ArrayAdapter {
             private int resourceId;
      
      public FruitAdapter(Context context, int textViewResourceId, List objects) {
             super(context, textViewResourceId, objects);
             resourceId = textViewResourceId;
      }
      
      @NonNull
      @Override
      public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
             Fruit fruit = (Fruit) getItem(position); // 获取当前项的Fruit实例
             View view = LayoutInflater.from(getContext()).inflate(resourceId, parent, false);
             ImageView fruitImage = (ImageView) view.findViewById(R.id.fruit_image);
             TextView fruitName = (TextView) view.findViewById(R.id.fruit_name);
             fruitImage.setImageResource(fruit.getImageId());
             fruitName.setText(fruit.getName());
             return view;
      
      }
      }  
      
      

   7. initFruit()方法 初始化所有水果数据

   8. onCreat() 方法中创建FruitAdapter对象,并将FruitAdapter作为适配器传递给ListVIew,这样定制ListView界面的任务就完成了

      public class MainActivity extends AppCompatActivity {
      
      private List fruitList = new ArrayList<>();
      
       @Override
       protected void onCreate(Bundle savedInstanceState) {
           super.onCreate(savedInstanceState);
           setContentView(R.layout.activity_main);
           initFruits(); // 初始化水果数据
           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() {
           for (int i = 0; i < 2; i++) {
               Fruit apple = new Fruit("Apple", R.drawable.apple_pic);
               fruitList.add(apple);
               Fruit banana = new Fruit("banana", R.drawable.banana_pic);
               Fruit orange = new Fruit("orange", R.drawable.orange_pic);
               fruitList.add(orange);
               Fruit watermelon = new Fruit("watermelon", R.drawable.watermelon_pic);
               fruitList.add(watermelon);
               Fruit pear = new Fruit("pear", R.drawable.pear_pic);
               fruitList.add(pear);
               Fruit grape = new Fruit("grape", R.drawable.grape_pic);
               fruitList.add(grape);
               Fruit pineapple = new Fruit("pineapple", R.drawable.pineapple_pic);
               fruitList.add(pineapple);
               Fruit strawberry = new Fruit("strawberry", R.drawable.strawberry_pic);
               fruitList.add(strawberry);
               Fruit cherry = new Fruit("cherry", R.drawable.cherry_pic);
               fruitList.add(cherry);
               Fruit mango = new Fruit("mango", R.drawable.mango_pic);
               fruitList.add(mango);
           }
       }
      }
      

提升ListView运行效率 修改FruitAdapter

在getView() 方法中还有 convertView参数,用于将之前加载好的布局进行缓存,以后可重用

• 对getView()方法进行判断,如果convertView为null,则使用LayoutInflater 加载布局,不为null则直接对convertView进行重用

  public class FruitAdapter extends ArrayAdapter {
    ...
    @Override
    public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
        Fruit fruit = (Fruit) getItem(position); // 获取当前项的Fruit实例
        View view;
        if(convertView == null){
            view = LayoutInflater.from(getContext()).inflate(resourceId, parent, false);
        }else{
            view = convertView;
        }
        ImageView fruitImage = (ImageView) view.findViewById(R.id.fruit_image);
        TextView fruitName = (TextView) view.findViewById(R.id.fruit_name);
        fruitImage.setImageResource(fruit.getImageId());
        fruitName.setText(fruit.getName());
        return view;
    }
  }
  

继续优化 借助ViewHolder来对部分性能进行优化 修改FruitAdapter P120

1. 新建一个内部类ViewHolder,用于对控件的实例进行缓存
2. 当converView为null时,创建一个ViewHolder对象,并将控件的实例都存放在ViewHolder里
3. 然后调用View的setTag() 方法,将ViewHolder对象存储在View中.
4. 当convertView不为null时,则调用View的getTag()方法,吧ViewHolder重新取出.
5. 这样所有空间的实例都缓存在了ViewHolder里,就没有必要每次都通过findViewById() 方法来获取控件实例

   public class FruitAdapter extends ArrayAdapter {
     ...
     @Override
     public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
         Fruit fruit = (Fruit) getItem(position); // 获取当前项的Fruit实例
         View view;
         ViewHoler viewHolder;
         if(convertView == null){
             view = LayoutInflater.from(getContext()).inflate(resourceId, parent, false);
             viewHolder = new ViewHoler();
             viewHolder.fruitImage = (ImageView)   view.findViewById(R.id.fruit_image);
             viewHolder.fruitName = (TextView) view.findViewById(R.id.fruit_name);
             view.setTag(viewHolder); // 将ViewHolder存储在View中
         }else{
             view = convertView;
             viewHolder = (ViewHoler) view.getTag(); // 重新获取ViewHolder
         }
         viewHolder.fruitImage.setImageResource(fruit.getImageId());
         viewHolder.fruitName.setText(fruit.getName());
         return view;
     }
     class ViewHoler{
         ImageView fruitImage;
         TextView fruitName;
       }
   }
   

为ListView添加点击事件

• 在MainActivity 添加 点击事件 Toast

1. 使用setOnItemClickListener() 方法为ListView 注册一个监听器
2. 点击任何一个子项是,回调onItemClick() 方法
3. 这个方法可以通过position 参数判断用户点击的是哪个子项,然后获取相应的水果
4. 通过Toast将水果的名字显示出来

   public class MainActivity extends AppCompatActivity {
   
     private List fruitList = new ArrayList<>();
   
     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);
         initFruits(); // 初始化水果数据
         FruitAdapter adapter = new FruitAdapter(MainActivity.this,R.layout.fruit_item,fruitList);
         ListView 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(MainActivity.this,fruit.getName(),Toast.LENGTH_SHORT).show();
           }
       });
     }
   
     ...
   }
   
   
   
   
   

这是我学习Android的笔记,参考书<<第一行代码-第二版>>,感谢郭霖大神
如有不足之处,可以一起讨论
附上郭霖的blog http://blog.csdn.net/guolin_blog