Android——ListView

1、ListView简单用法

（1）首先在main_activity.xml中加入ListView控件

（2）修改活动MainActivity代码

设置适配器，这里我选用的是ArrayAdapter适配器，它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。这里要注意一下arrayadapter()方法，ArrayAdapter(Context context ,int textViewResourceId ,List<T> objects)

context ----> The current context ，上下文,就是当前的Activity

textViewResourceId ---->the resource ID for a layout file contain a TextView to use when instantiating views，Android sdk中自己内置的一个布局，它里面只有一个TextView,这个参数是表明我们数组中每一条数据的布局是这个View,就是将每一条数据都显示在这个View上面，也就是当装载在这个构造函数中的layout时，其layout的ID 必须是一个TextView，简言之，第2个参数应该是ListView中每个选择项的样式，可以使用系统自带的 android.R.layout.xxx,也可以是自定义的，仅包含TextView。

List<T> objects ---> the objects to represent in the ListView，就是我们要显示的数据。listView会根据这三个参数，遍历AdapterData里面的每一条数据，读出一条，显示到第二个参数对应的布局中，这样就形成了我们看到的ListView.

当在加载布局item_layout时，要注意其xml文件要以TextView为根节点，不然就会抛“ArrayAdapter requires the resource ID to be a TextView”

2、定制ListView界面

（1）定义实体类，作为适配器的适配类型

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）为ListView的子项自定义布局

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">
    <ImageView
        android:layout_width="150dp"
        android:layout_height="150dp"
        android:id="@+id/fruit_image"/>
    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/fruit_name"
        android:textSize="20sp"
        android:textColor="@color/black"
        android:layout_gravity="center_vertical" />
</LinearLayout>

（3）自定义适配器，继承ArrayAdapter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;
    }
    public View getView(int position, View convertView, ViewGroup parent){
        //获取实例
        Fruit fruit=getItem(position);
        View view= LayoutInflater.from(getContext()).inflate(resourceId,parent,false);
        ImageView image=view.findViewById(R.id.fruit_image);
        TextView name=view.findViewById(R.id.fruit_name);
        image.setImageResource(fruit.getImageId());
        name.setText(fruit.getName());
        return view;
    }
}

FruitAdapter重写了父类的构造方法，将上下文、ListView子布局的id和数据传递进来，并重写了getView()方法，这个方法在每个子布局被滚动到屏幕内的时候会被调用，首先通过getItem()方法获得当前Fruit的实例，然后使用LayoutInflater加载布局。

（4）完善MainActivity代码

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
        initView();
        FruitAdapter adapter=new FruitAdapter(MainActivity.this,R.layout.item_layout,fruitList);
        ListView listView=findViewById(R.id.list);
        listView.setAdapter(adapter);
    }
    private void initView(){
        for(int i=0;i<3;i++){
            Fruit apple=new Fruit("Apple",R.drawable.apple);
            fruitList.add(apple);
            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);
        }
    }
}

程序结果如下：

3、ListView的提升

通过convertView将之前加载好的布局进行缓存，对适配器进行修改，如下:

public class FruitAdapter extends ArrayAdapter<Fruit> {
    private int resourceId;
    public FruitAdapter(Context context, int textViewResourceId, List<Fruit> objects){
        super(context,textViewResourceId,objects);
        resourceId=textViewResourceId;
    }
    public View getView(int position, View convertView, ViewGroup parent){
        //获取实例
        Fruit fruit=getItem(position);
        View view;
        if(convertView==null){
            view=LayoutInflater.from(getContext()).inflate(resourceId,parent,false);
        }else {
            view=convertView;
        }
        ImageView image=view.findViewById(R.id.fruit_image);
        TextView name=view.findViewById(R.id.fruit_name);
        image.setImageResource(fruit.getImageId());
        name.setText(fruit.getName());
        return view;
    }
}

借助ViewHolder对控件的实例进行优化

public class FruitAdapter extends ArrayAdapter<Fruit> {
    private int resourceId;
    public FruitAdapter(Context context, int textViewResourceId, List<Fruit> objects){
        super(context,textViewResourceId,objects);
        resourceId=textViewResourceId;
    }
    class ViewHolder{
        ImageView imageView;
        TextView tvname;
    }
    public View getView(int position, View convertView, ViewGroup parent){
        //获取实例
        Fruit fruit=getItem(position);
        View view;
        ViewHolder viewHolder;
        if(convertView==null){
            view=LayoutInflater.from(getContext()).inflate(resourceId,parent,false);
            viewHolder=new ViewHolder();
            viewHolder.imageView=view.findViewById(R.id.fruit_image);
            viewHolder.tvname=view.findViewById(R.id.fruit_name);
            //将viewHolder储存在view中
            view.setTag(viewHolder);
        }else {
            view=convertView;
            viewHolder=(ViewHolder) view.getTag();
        }
        viewHolder.imageView.setImageResource(fruit.getImageId());
        viewHolder.tvname.setText(fruit.getName());
        return view;
    }
}

新增内部类ViewHolder，对于控件实例进行缓存，当convertView为空时，创建ViewHolder对象，并将控件的实例存放在ViewHolder中，然后调用setTag()方法将View Holder重新取出

4、ListView的点击事件

在活动MainActivity中设置点击事件

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);
        //初始化
        initView();
        FruitAdapter adapter=new FruitAdapter(MainActivity.this,R.layout.item_layout,fruitList);
        ListView listView=findViewById(R.id.list);
        listView.setAdapter(adapter);
        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> adapterView, View view, int position, long id) {
                Fruit fruit=fruitList.get(position);
                Toast.makeText(MainActivity.this, fruit.getName(), Toast.LENGTH_SHORT).show();
            }
        });
    }
    private void initView(){
        for(int i=0;i<3;i++){
            Fruit apple=new Fruit("Apple",R.drawable.apple);
            fruitList.add(apple);
            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);
        }
    }
}

1、ListView简单用法

2、定制ListView界面

3、ListView的提升

4、ListView的点击事件

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