Android的ListView使用

ListView

ListView 簡單用法

  • width和height是match_parent
  • 數(shù)據(jù)可以從數(shù)據(jù)庫,網(wǎng)上下載.也可以是數(shù)組
  • 數(shù)組中的數(shù)據(jù)是 無法直接傳遞給ListView的,要借助適配器,其中ArrayAdapter最好用
  • ArrayAdapter可以用泛型指定數(shù)據(jù)類型,在構(gòu)造函數(shù)中吧要適配的數(shù)據(jù)傳入
  • ArrayAdapter有多個(gè)構(gòu)造函數(shù)重載,要選擇最適合的一種.因?yàn)閭魅霐?shù)據(jù)是字符串,所以ArrayAdapter<String>
  • 構(gòu)造函數(shù)傳入(上下文,ListView子項(xiàng)布局id,適配數(shù)據(jù))
  • 最后調(diào)用ListView的setAdapter()方法,將構(gòu)建好的適配器對象傳遞進(jìn)去 ,這樣ListView和數(shù)據(jù)之間的關(guān)聯(lián)就建立完成 了 
    <ListView
  android:id="@+id/list_view"
  android:layout_width="match_parent"
  android:layout_height="match_parent"></ListView>

    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<String> adapter = new ArrayAdapter<String>(
            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. 定義一個(gè)實(shí)體類,作為ListView適配器的適配類型,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. 自定義布局 LinearLayout 要 TextVIew垂直布局居中

    <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" />

  3. 創(chuàng)建適配器

    1. 繼承自ArrayAdapter 泛型Fruit 新建類FruitAdapter

    2. Fruit重寫一組父類的構(gòu)造函數(shù),用于將 上下文,ListView子項(xiàng)布局的id和數(shù)據(jù)傳遞進(jìn)來

    3. 重寫getView()方法,這個(gè)方法每個(gè)子項(xiàng)被滾動到屏幕內(nèi)的時(shí)候會被調(diào)用.

    4. 在getView()方法中,首先通過getItem()方法得到當(dāng)前項(xiàng)的Fruit實(shí)例,然后使用LayoutInflater 來為這個(gè)子項(xiàng)加載傳入的布局

    5. inflate()接收三個(gè)參數(shù) ()

    6. 調(diào)用View的findViewById() 獲取ImageView和TextView的實(shí)例,并分別調(diào)用他們的setImageResource()和setText()方法來設(shè)置顯示的圖片和文字,最后布局返回

      public class FruitAdapter extends ArrayAdapter {
       private int resourceId;

public FruitAdapter(Context context, int textViewResourceId, List<Fruit> 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); // 獲取當(dāng)前項(xiàng)的Fruit實(shí)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 初始化所有水果數(shù)據(jù)

    8. onCreat() 方法中創(chuàng)建FruitAdapter對象,并將FruitAdapter作為適配器傳遞給ListVIew,這樣定制ListView界面的任務(wù)就完成了

      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);
     initFruits(); // 初始化水果數(shù)據(jù)
     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運(yùn)行效率 修改FruitAdapter

在getView() 方法中還有 convertView參數(shù),用于將之前加載好的布局進(jìn)行緩存,以后可重用

  • 對getView()方法進(jìn)行判斷,如果convertView為null,則使用LayoutInflater 加載布局,不為null則直接對convertView進(jìn)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); // 獲取當(dāng)前項(xiàng)的Fruit實(shí)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;
  }
}

繼續(xù)優(yōu)化 借助ViewHolder來對部分性能進(jìn)行優(yōu)化 修改FruitAdapter ??P120

  1. 新建一個(gè)內(nèi)部類ViewHolder,用于對控件的實(shí)例進(jìn)行緩存
  2. 當(dāng)converView為null時(shí),創(chuàng)建一個(gè)ViewHolder對象,并將控件的實(shí)例都存放在ViewHolder里
  3. 然后調(diào)用View的setTag() 方法,將ViewHolder對象存儲在View中.
  4. 當(dāng)convertView不為null時(shí),則調(diào)用View的getTag()方法,吧ViewHolder重新取出.
  5. 這樣所有空間的實(shí)例都緩存在了ViewHolder里,就沒有必要每次都通過findViewById() 方法來獲取控件實(shí)例 
    public class FruitAdapter extends ArrayAdapter {
  ...
  @Override
  public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
      Fruit fruit = (Fruit) getItem(position); // 獲取當(dāng)前項(xiàng)的Fruit實(shí)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); // 將ViewHolder存儲在View中
      }else{
          view = convertView;
          viewHolder = (ViewHoler) view.getTag(); // 重新獲取ViewHolder
      }
      viewHolder.fruitImage.setImageResource(fruit.getImageId());
      viewHolder.fruitName.setText(fruit.getName());
      return view;
  }
  class ViewHoler{
      ImageView fruitImage;
      TextView fruitName;
    }
}

為ListView添加點(diǎn)擊事件

  • 在MainActivity 添加 點(diǎn)擊事件 Toast
  1. 使用setOnItemClickListener() 方法為ListView 注冊一個(gè)監(jiān)聽器
  2. 點(diǎn)擊任何一個(gè)子項(xiàng)是,回調(diào)onItemClick() 方法
  3. 這個(gè)方法可以通過position 參數(shù)判斷用戶點(diǎn)擊的是哪個(gè)子項(xiàng),然后獲取相應(yīng)的水果
  4. 通過Toast將水果的名字顯示出來 
    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);
      initFruits(); // 初始化水果數(shù)據(jù)
      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();
        }
    });
  }

  ...
}

這是我學(xué)習(xí)Android的筆記,參考書<<第一行代碼-第二版>>,感謝郭霖大神
如有不足之處,可以一起討論
附上郭霖的blog http://blog.csdn.net/guolin_blog

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