《第一行代码》笔记2-自定义ListView

创建一个新工程。要使用自定义ListView，需要自定义一个ListView item 的布局xml，还需要自定义一个Adapter。

1. 先在activity_main.xml中创建一个listview:

这里让listview全部充满整个屏幕。

2. 自定义listview item的布局文件。新建一个fruit_item.xml布局文件：

这里只用来显示一张图片和一个textview文字说明.

3.先声明一个Fruit类:

package com.example.listview;

/**
 * Created by archerlj on 2017/9/17.
 */

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;
    }
}

4. 自定义适配器，新建一个FruitAdapter类，继承自ArrayAdapter，泛型类型穿入Fruit:

package com.example.listview;

import android.content.Context;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.TextView;

import java.util.List;

/**
 * Created by archerlj on 2017/9/17.
 */

public class FruitAdapter extends ArrayAdapter {

    private int resourceId;

    public FruitAdapter(Context context, int resource, List objects) {
        super(context, resource, objects);
        this.resourceId = resource;
    }

    @NonNull
    @Override
    public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
        View view;
        ViewHolder viewHolder;
        if (convertView == null) {
            view = LayoutInflater.from(getContext()).inflate(resourceId, parent, false);
            viewHolder = new ViewHolder();

            viewHolder.fruitImage = view.findViewById(R.id.fruit_image);
            viewHolder.fruitName = view.findViewById(R.id.furit_name);

            view.setTag(viewHolder);
        } else {
            view = convertView;
            viewHolder = (ViewHolder) view.getTag();
        }

        Fruit fruit = getItem(position);
        viewHolder.fruitImage.setImageResource(fruit.getImageId());
        viewHolder.fruitName.setText(fruit.getName());

        return view;
    }

    class ViewHolder {
        ImageView fruitImage;
        TextView fruitName;
    }
}

这里添加一个属性变量resourceId用来存储初始化方法中穿入的listview item的布局文件的id。添加一个初始化方法。重写getView方法。这里为了优化listview的性能，添加了一个ViewHolder内部类，用来保存listview item上的控件，这样就不需要每次都通过findviewbyid去获取了，并通过setTag方法把viewholder赋值给每个listview item的view。这里convertView缓存了listview item的view。如果缓存为空，我们再根据resourceId去创建，否则就直接使用即可。

android中的listview 的内部优化机制同iOS中的UITableView对cell的优化机制是一样的，都使用了cell重用机制。

5. 在MainActivity中获取listview，并设置适配器:

package com.example.listview;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ListView;
import android.widget.Toast;

import java.util.ArrayList;
import java.util.List;

public class MainActivity extends AppCompatActivity {

    private List fruits = new ArrayList<>();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        
        initFruits();
        FruitAdapter adapter = new FruitAdapter(this, R.layout.fruit_item, fruits);
        ListView listView = (ListView) findViewById(R.id.list_view);
        listView.setAdapter(adapter);

        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView adapterView, View view, int position, long l) {
                Fruit fruit = fruits.get(position);
                Toast.makeText(MainActivity.this, fruit.getName(), Toast.LENGTH_SHORT).show();
            }
        });
    }
    
    private void initFruits() {

        for (int i = 0; i < 2; i++) {
            Fruit apple = new Fruit("Apple", R.mipmap.ic_launcher);
            fruits.add(apple);
            Fruit apple1 = new Fruit("Apple", R.mipmap.ic_launcher);
            fruits.add(apple1);
            Fruit apple2 = new Fruit("Apple", R.mipmap.ic_launcher);
            fruits.add(apple2);
            Fruit apple3 = new Fruit("Apple", R.mipmap.ic_launcher);
            fruits.add(apple3);
            Fruit apple4 = new Fruit("Apple", R.mipmap.ic_launcher);
            fruits.add(apple4);
            Fruit apple5 = new Fruit("Apple", R.mipmap.ic_launcher);
            fruits.add(apple5);
            Fruit apple6 = new Fruit("Apple", R.mipmap.ic_launcher);
            fruits.add(apple6);
        }
    }
}

运行：

Screenshot_2017-09-17-14-46-42-886_ListView.png

这样，一个简单的自定义ListView item 就实现了。