初探ListView和Adapter

关于Android Adapter（适配器），参考Devin Zhang’s blog。简单的说，Adapter起到的作用是使得前端的显示和后端的数据能够适配，用以下代码作为例子

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public class MainActivity extends Activity{

    private List<fruit> myFruits = new ArrayList<fruit>();//将数据定义为实例

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        initialFruits();
        setContentView(R.layout.activity_main);//与R.layout.activity_main布局绑定
        ListView lv = (ListView) findViewById(R.id.lv);
        fruitAdapter adapter = new fruitAdapter(MainActivity.this,R.layout.list_item,myFruits);//fruitAdpter的三个参数依次为，上下文context,ListView单项布局文件，集合类型的数据
        lv.setAdapter(adapter);//用adapter做适配器（就理解为ListView看得懂并且存放有数据的容器）
    }
    //初始化实例变量myFruits
    private void initialFruits(){
        fruit orange = new fruit("orange",R.drawable.pa);
        myFruits.add(orange);
        fruit grape = new fruit("grape",R.drawable.pb);
        myFruits.add(grape);
        fruit milk = new fruit("milk",R.drawable.pc);
        myFruits.add(milk);
        fruit bean = new fruit("bean",R.drawable.pd);
        myFruits.add(bean);
        fruit kid = new fruit("kid",R.drawable.pe);
        myFruits.add(kid);
        fruit banana = new fruit("banana",R.drawable.pf);   
        myFruits.add(banana);
        fruit lamp = new fruit("lamp",R.drawable.pg);
        myFruits.add(lamp);
        fruit hack = new fruit("hack",R.drawable.ph);
        myFruits.add(hack);
        fruit keyboard = new fruit("keyboard",R.drawable.pi);
        myFruits.add(keyboard);
        fruit water = new fruit("water",R.drawable.pj);
        myFruits.add(water);
        fruit mouse = new fruit("mouse",R.drawable.pk);
        myFruits.add(mouse);
        fruit ear = new fruit("ear",R.drawable.pl);
        myFruits.add(ear);
    }
}

关于fruitAdapter类

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public class fruitAdapter extends ArrayAdapter<fruit>{

    private int resourceId;
    //textViewResourceId即为传入的ListView的单项布局文件id
    public fruitAdapter(Context context,int textViewResourceId, List<fruit> objects) {
        super(context, textViewResourceId, objects);
        //adapter对象的resourceId实例变量保存ListView的单项布局文件id，以供getView()使用
        resourceId = textViewResourceId;
    }

    //在每个子项被滚动到屏幕内的时候调用getView()
    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        fruit newFruit = getItem(position);
        View view = LayoutInflater.from(getContext()).inflate(resourceId,null);
        ImageView imageView = (ImageView) view.findViewById(R.id.fruit_image);
        TextView textView = (TextView) view.findViewById(R.id.fruit_name);
        imageView.setImageResource(newFruit.getId());
        textView.setText(newFruit.getName());
        return  view;
    }
}

对于getView(),《第一行代码》中给出的解释是“在每个子项被滚动到屏幕内的时候会被调用”。这句话的另一层意思是：凡是ListView中可见的子项，均调用了getView()方法。事实上也只能如此，因为在构造函数中并未告诉ListView如何呈现样式。也因此呢，整个ListView的过程是：创建Adapter(需调用构造函数)->特定的ListView指定该Adapter->在屏幕需要显示子项时调用getView()

接着附上fruit类的代码

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public class fruit {
    private String name;
    private int pic_id;

    public fruit(String fname,int id){
        this.name = fname;
        this.pic_id = id;
    }

    public String getName(){
        return name;
    }

    public int getId(){
        return pic_id;
    }
}

最后就出现了这样的ListView
界面界面
界面界面

这样看来图片的大小得事先选好，图片越大，速度越慢

滑动的时候确实会卡顿，优化如下:
getView()的convertView用于缓存加载好的布局，不至于每次有子项滑入屏幕内都要加载一次布局

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//在每个子项被滚动到屏幕内的时候调用getView()
    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        View view;
        ViewHolder viewHolder;
        fruit myFruit = getItem(position);
        if(convertView == null){
            viewHolder = new ViewHolder();
            view = LayoutInflater.from(getContext()).inflate(resourceId,null);
            viewHolder.imageView = (ImageView) view.findViewById(R.id.fruit_image);
            viewHolder.textView = (TextView) view.findViewById(R.id.fruit_name);
            view.setTag(viewHolder);
        }
        else{
            view = convertView;
            viewHolder = (ViewHolder) view.getTag();
        }
        viewHolder.imageView.setImageResource(myFruit.getId());
        viewHolder.textView.setText(myFruit.getName());
        return view;
    }

从优化ListView过程中，看出至始至终都是使用第一次加载好的view，索性在view里面再加上存有控件实例的viewHolder

写子项->适配器->item->mainActivity(传数据)

posted @ 2016-03-20 21:59  Gabyler  阅读(310)  评论(0)  编辑  收藏  举报