ListView

時間  2019-11-30
標籤 listview 简体版
原文   原文鏈接

1.ListView容許用戶經過手指上下滑動的方式將屏幕外的數據滾動到屏幕內，同時屏幕上原有的數據會滾動出屏幕android

2.在佈局中加入ListView控件數據庫

<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.適配數組

ListView是用來展現大量數據的，這些數據能夠從網上下載，也能夠從數據庫中讀取。這裏咱們用data數組來測試。緩存

數組中的數據沒法直接傳遞給ListView，須要藉助適配器，這裏使用ArrayAdapter，他能夠經過泛型來指定要適配的數據類型，而後在構造函數中把要適配的數據傳入。app

最後調用ListViewdsetAdapter（）方法，將構造好的適配器對象傳遞過去，創建ListView和數據之間的關係。ide

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.定製ListView的界面函數

定製一個實體類做爲ListView適配器的適配類型佈局

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.爲ListViewd子項指定一個自定義佈局測試

<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.建立自定義適配器，並將泛型指定爲Fruit類優化

public class FruitAdapter extends ArrayAdapter<Fruit>{
    private int resourceId;
    public FruitAdapter(Context context, int textViewResourceId, List<Fruit> objects){
        super (context,textViewResourceId,objects);
        resourceId = textViewResourceId;
    } //重寫了父類的構造函數，用於將上下文、ListView子項佈局的id和數據都穿過來

    @NonNull
    @Override
    public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) { //該方法使每一個子項在滾動到屏幕內時被調用
        Fruit fruit = getItem(position); //獲取當前項的Fruit實例
        View view = LayoutInflater.from(getContext()).inflate(resourceId,parent,false); //爲子項加載咱們傳入的佈局
        ImageView fruitImage = (ImageView) view.findViewById( R.id.fruit_image);
        TextView fruitName = (TextView) view.findViewById(R.id.fruit_name);
        fruitImage.setImageResource(fruit.getImageID()); //顯示圖片
        fruitName.setText(fruit.getName()); //顯示文字
        return view;
    }
}

7.定製界面

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();//初始化水果數據
        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循環將全部數據填兩變，一遍不足以充滿整個屏幕
            Fruit apple = new Fruit("Apple",R.drawable.apple);//在構造函數中將水果的名字和對應的圖片id傳入
            fruitList.add(apple); //把建立好的對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.提高ListView的運行效率

（1）在getView（）方法中進行了判斷，若是convertView爲null，則使用LayoutInflater去加載佈局，若是不爲null則直接對convertView進行重用

public class FruitAdapter extends ArrayAdapter<Fruit>{zeprivate int resourceId;
    public FruitAdapter(Context context, int textViewResourceId, List<Fruit> objects){
        super (context,textViewResourceId,objects);
        resourceId = textViewResourceId;
    } //重寫了父類的構造函數，用於將上下文、ListView子項佈局的id和數據都穿過來

    @NonNull
    @Override
    public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) { //該方法使每一個子項在滾動到屏幕內時被調用
        Fruit fruit = getItem(position); //獲取當前項的Fruit實例
        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()); //顯示圖片
        fruitName.setText(fruit.getName()); //顯示文字
        return view;
    }
}

（2）藉助viewHolder來進行優化

public class FruitAdapter extends ArrayAdapter<Fruit>{
    private int resourceId;
    public FruitAdapter(Context context, int textViewResourceId, List<Fruit> objects){
        super (context,textViewResourceId,objects);
        resourceId = textViewResourceId;
    } //重寫了父類的構造函數，用於將上下文、ListView子項佈局的id和數據都穿過來

    @NonNull
    @Override
    public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) { //該方法使每一個子項在滾動到屏幕內時被調用
        Fruit fruit = getItem(position); //獲取當前項的Fruit實例
        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);//將ViewHolder儲存在View中
        }else {
            view = convertView;
            viewHolder = (ViewHolder) view.getTag();//從新獲取ViewHolder
        }
        viewHolder.fruitImage.setImageResource(fruit.getImageId());
        viewHolder.fruitName.setText(fruit.getName());
        return view;
       
    }
    class ViewHolder{ //內部類，用於對控件的實例進行緩存
        ImageView fruitImage;
        TextView fruitName;
    }
}

9.ListView的點擊事件

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();//初始化水果數據
        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();
            }
        });
    }
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