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<String>
• 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

  <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"};@Overrideprotected 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. 定义一个实体类,作为ListView适配器的适配类型,新建Fruit
   “`java
   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
   “`xml
   

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
java
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
“`java
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
     “`java
     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