ListView

1.ListView允许用户通过手指上下滑动的方式将屏幕外的数据滚动到屏幕内，同时屏幕上原有的数据会滚动出屏幕

2.在布局中加入ListView控件

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <ListView
        android:id="@+id/list_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent">

    </ListView>

</LinearLayout>

3.适配

ListView是用来展示大量数据的，这些数据可以从网上下载，也可以从数据库中读取。这里我们用data数组来测试。

数组中的数据无法直接传递给ListView，需要借助适配器，这里使用ArrayAdapter，他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。

最后调用ListViewdsetAdapter（）方法，将构造好的适配器对象传递过去，建立ListView和数据之间的关系。

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);
    }
}

4.定制ListView的界面

定制一个实体类作为ListView适配器的适配类型

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);
    }
}

5.为ListViewd子项指定一个自定义布局

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" 
    android:layout_width="wrap_content"
    android:layout_height="wrap_content">
<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"/>
</LinearLayout>

6.创建自定义适配器，并将泛型指定为Fruit类

public class FruitAdapter extends ArrayAdapter<Fruit>{
    private int resourceId;
    public FruitAdapter(Context context, int textViewResourceId, List<Fruit> objects){
        super (context,textViewResourceId,objects);
        resourceId = textViewResourceId;
    } //重写了父类的构造函数，用于将上下文、ListView子项布局的id和数据都穿过来

    @NonNull
    @Override
    public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) { //该方法使每个子项在滚动到屏幕内时被调用
        Fruit fruit = getItem(position); //获取当前项的Fruit实例
        View view = LayoutInflater.from(getContext()).inflate(resourceId,parent,false); //为子项加载我们传入的布局
        ImageView fruitImage = (ImageView) view.findViewById( R.id.fruit_image);
        TextView fruitName = (TextView) view.findViewById(R.id.fruit_name);
        fruitImage.setImageResource(fruit.getImageID()); //显示图片
        fruitName.setText(fruit.getName()); //显示文字
        return view;
    }
}

7.定制界面

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++){//for循环将所有数据填两变，一遍不足以充满整个屏幕
            Fruit apple = new Fruit("Apple",R.drawable.apple);//在构造函数中将水果的名字和对应的图片id传入
            fruitList.add(apple); //把创建好的对象添加到水果列表中
            Fruit banana = new Fruit("Banana",R.drawable.banana);
            fruitList.add(banana);
            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);
        }
    }
}

8.提升ListView的运行效率

（1）在getView（）方法中进行了判断，如果convertView为null，则使用LayoutInflater去加载布局，如果不为null则直接对convertView进行重用

public class FruitAdapter extends ArrayAdapter<Fruit>{zeprivate int resourceId;
    public FruitAdapter(Context context, int textViewResourceId, List<Fruit> objects){
        super (context,textViewResourceId,objects);
        resourceId = textViewResourceId;
    } //重写了父类的构造函数，用于将上下文、ListView子项布局的id和数据都穿过来

    @NonNull
    @Override
    public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) { //该方法使每个子项在滚动到屏幕内时被调用
        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()); //显示图片
        fruitName.setText(fruit.getName()); //显示文字
        return view;
    }
}

（2）借助viewHolder来进行优化

public class FruitAdapter extends ArrayAdapter<Fruit>{
    private int resourceId;
    public FruitAdapter(Context context, int textViewResourceId, List<Fruit> objects){
        super (context,textViewResourceId,objects);
        resourceId = textViewResourceId;
    } //重写了父类的构造函数，用于将上下文、ListView子项布局的id和数据都穿过来

    @NonNull
    @Override
    public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) { //该方法使每个子项在滚动到屏幕内时被调用
        Fruit fruit = getItem(position); //获取当前项的Fruit实例
        View view;
        ViewHolder viewHolder;
        if (convertView == null){
            view = LayoutInflater.from(getContext()).inflate(resourceId,parent,false);
            viewHolder = new ViewHolder();
            viewHolder.fruitImage = (ImageView) view.findViewById(R.id.fruit_image);
            viewHolder.fruitName = (TextView) view.findViewById(R.id.fruit_name);
            view.setTag(viewHolder);//将ViewHolder储存在View中
        }else {
            view = convertView;
            viewHolder = (ViewHolder) view.getTag();//重新获取ViewHolder
        }
        viewHolder.fruitImage.setImageResource(fruit.getImageId());
        viewHolder.fruitName.setText(fruit.getName());
        return view;
       
    }
    class ViewHolder{ //内部类，用于对控件的实例进行缓存
        ImageView fruitImage;
        TextView fruitName;
    }
}

9.ListView的点击事件

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();
            }
        });
    }