最常用和最難用的Android控件ListView

更新時間：2016年10月26日 16:55:28 作者：王裕杰

這篇文章主要為大家詳細(xì)介紹了最常用和最難用的Android控件ListView的使用方法,具有一定的參考價值，感興趣的小伙伴們可以參考一下

ListView允許用戶通過手指上下滑動的方式將屏幕外的數(shù)據(jù)滾動到屏幕內(nèi),同時屏幕上原有的數(shù)據(jù)則會滾動出屏幕.

1. ListView的簡單用法
首先新建一個ListViewTest項目,然后修改activity_main.xml代碼.

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:layout_height="match_parent" 
  android:layout_width="match_parent">
 <ListView 
  android:id="@+id/list_view"
  android:layout_width="match_parent"
  android:layout_height="match_parent"></ListView>
</LinearLayout>

為ListView指定一個id,然后將寬度和高度都修改為match_parent,這樣ListView就占據(jù)了整個布局的空間.

ListView布局

接下來修改MainActivity中的代碼.

public class MainActivity extends Activity {
 private String[] data = {"Apple","Banana","Orange","Watermelon",
   "Pear","Grape","Pineapple","Strawberry","Cherry","Mango"};
 @Override
 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
  //先創(chuàng)建適配器,并且把內(nèi)容放入去.
  ArrayAdapter<String> adapter = new ArrayAdapter<String>
  (MainActivity.this, android.R.layout.simple_list_item_1,data);
  ListView listView = (ListView) findViewById(R.id.list_view);
  //調(diào)用ListView的對象把適配器傳進去.
  listView.setAdapter(adapter);
 }


}

數(shù)組中的數(shù)據(jù)是無法直接傳遞給ListView的,我們需要借助適配器來完成,其中最好用的是ArrayAdapter它可以通過泛型來指定要添加的數(shù)據(jù)類型,然后在構(gòu)造函數(shù)中把要適配的數(shù)據(jù)傳入即可.注意我們使用了android.R.layout.simple_list_item_1作為ListView的子項布局的id,以及要適配的數(shù)據(jù).

最后,我們要調(diào)用ListView的SetAdapter()方法,將構(gòu)造好的適配器對象傳遞進去,這樣ListView和數(shù)據(jù)之間的關(guān)聯(lián)就建立完成了.

ListView與數(shù)據(jù)關(guān)聯(lián)

2. 定制ListView的界面
接著定義一個實體類,作為ListView適配器的適配類型,新建類Fruit,需要準(zhǔn)備一組圖片.

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;
 }

}

Fruit類中只有兩個字段,name表示水果的名字,imageId表示水果對應(yīng)圖片的資源id.

然后需要為ListView的子項指定一個我們自定義的布局,在layout目錄下新建fruit_item.xml

<?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="match_parent" >

 <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"
  android:layout_marginLeft="10dip" />

</LinearLayout>

在這個布局里,我們定義了一個ImageView用于顯示水果的圖片,又定義了一個TextView用于顯示水果的名稱.

接下來需要創(chuàng)建一個自定義的適配器,這個適配器繼承自ArrayAdapter,并將泛型指定為Fruit類.新建類FruitAdapter,代碼如下:

public class FruitAdapter extends ArrayAdapter<Fruit> {

 private int resourceId;

 public FruitAdapter(Context context, int textViewResourceId,
   List<Fruit> objects) {
  super(context, textViewResourceId, objects);
  resourceId = textViewResourceId;
 }

 @Override
 public View getView(int position, View convertView, ViewGroup parent) {
  Fruit fruit = getItem(position);
  View view = LayoutInflater.from(getContext()).inflate(resourceId, null);
  ImageView fruitImage = (ImageView) view.findViewById(R.id.fruit_image);
  TextView fruitName = (TextView) view.findViewById(R.id.fruit_name);
  fruitImage.setImageResource(fruit.getImageId()); 
  fruitName.setText(fruit.getName());
  return view;
 }

}

FruitAdapter重寫了父類的一組構(gòu)造函數(shù),用于將上下文,ListView子項布局的id和數(shù)據(jù)都傳遞進來.另外又重寫了getView()方法,首先通過getItem()方法得到當(dāng)前項的Fruit的實例,然后使用LayoutInflater來為這個子項加載我們傳入的布局,接著調(diào)用View的fndViewById()方法分別獲取到ImageView和TextView的實例,并分別調(diào)用它們的setImageResource和setText方法來設(shè)置顯示的圖片和文字,最好將布局返回.

下面修改MainActivity中的代碼,如下所示:

public class MainActivity extends Activity {

 private List<Fruit> fruitList = new ArrayList<Fruit>();

 @Override
 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
  initFruits();
  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() {
  Fruit apple = new Fruit("Apple", R.drawable.apple_pic);
  fruitList.add(apple);
  Fruit banana = new Fruit("Banana", R.drawable.banana_pic);
  fruitList.add(banana);
  Fruit orange = new Fruit("Orange", R.drawable.orange_pic);
  fruitList.add(orange);
  Fruit watermelon = new Fruit("Watermelon", R.drawable.watermelon_pic);
  fruitList.add(watermelon);
  Fruit pear = new Fruit("Pear", R.drawable.pear_pic);
  fruitList.add(pear);
  Fruit grape = new Fruit("Grape", R.drawable.grape_pic);
  fruitList.add(grape);
  Fruit pineapple = new Fruit("Pineapple", R.drawable.pineapple_pic);
  fruitList.add(pineapple);
  Fruit strawberry = new Fruit("Strawberry", R.drawable.strawberry_pic);
  fruitList.add(strawberry);
  Fruit cherry = new Fruit("Cherry", R.drawable.cherry_pic);
  fruitList.add(cherry);
  Fruit mango = new Fruit("Mango", R.drawable.mango_pic);
  fruitList.add(mango);
 }

}

可以看到,這里添加了一個initFruits()方法,用于初始化所有水果的數(shù)據(jù),在Fruit類構(gòu)造函數(shù)將水果的名字和對應(yīng)圖片id傳入,然后把創(chuàng)建好的對象添加到水果列表中,接著我們再onCreate()方法中創(chuàng)建了FruitAdapter對象,并將FruitAdapter作為適配器傳遞給ListView.

3. 提升ListView的運行效率
因為在FruitAdapter的getView()方法中每次都將布局重新加載了一次,當(dāng)ListView快速滾動的時候就會成為性能的阻礙.

public class FruitAdapter extends ArrayAdapter<Fruit> {

 private int resourceId;

 public FruitAdapter(Context context, int textViewResourceId,
   List<Fruit> objects) {
  super(context, textViewResourceId, objects);
  resourceId = textViewResourceId;
 }

 @Override
 public View getView(int position, View convertView, ViewGroup parent) {
  Fruit fruit = getItem(position); // 獲取當(dāng)前項的Fruit實例
  View view;
  if (convertView == null) {
   view = LayoutInflater.from(getContext()).inflate(resourceId, null);
  } 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());
  fruitName.setText(fruit.getName());
  return view;
 }

}

所以,我們再getView()方法中進行了判斷,如果convertView為空,則使用LayoutInflater去加載布局,如果不為空則直接對convertView進行重用.

每次在getView()方法中還是會調(diào)用View的findViewById()方法來獲取一次控件的實例,我們還可以借助一個ViewHolder來對這部分性能進行優(yōu)化,修改FruitAdapter`中的代碼,如下所示:

public class FruitAdapter extends ArrayAdapter<Fruit> {

 private int resourceId;

 public FruitAdapter(Context context, int textViewResourceId,
   List<Fruit> objects) {
  super(context, textViewResourceId, objects);
  resourceId = textViewResourceId;
 }

 @Override
 public View getView(int position, View convertView, ViewGroup parent) {
  Fruit fruit = getItem(position);
  View view;
  ViewHolder viewHolder;
  if (convertView == null) {
   view = LayoutInflater.from(getContext()).inflate(resourceId, null);
   viewHolder = new ViewHolder();
   viewHolder.fruitImage = (ImageView) view.findViewById(R.id.fruit_image);
   viewHolder.fruitName = (TextView) view.findViewById(R.id.fruit_name);
   view.setTag(viewHolder);
  } else {
   view = convertView;
   viewHolder = (ViewHolder) view.getTag();
  }
  viewHolder.fruitImage.setImageResource(fruit.getImageId());
  viewHolder.fruitName.setText(fruit.getName());
  return view;
 }

 class ViewHolder {

  ImageView fruitImage;

  TextView fruitName;

 }

}

我們新建了一個內(nèi)部類ViewHolder,用于對控件的實例進行緩存.當(dāng)convertView為空的時候,創(chuàng)建一個ViewHolder對象,并將控件的實例都存放在ViewHolder里,然后調(diào)用View的SetTag()方法,將ViewHolder對象存儲在View中.當(dāng)convertView不為空的時候則調(diào)用View的getTag()方法,把ViewHolder重新取出.這樣所有控件的實例都緩存在ViewHolder里,就沒有必要每次都通過findViewById()方法來獲取控件實例了.

4. ListView的點擊事件

  listView.setOnItemClickListener(new 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();
   }
  });

最終效果圖：