最常用和最難用的Android控件ListView

2019-12-12 04:56:05

字體：大中小

來源：轉載

供稿：網友

ListView允許用戶通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);  //先創建適配器,并且把內容放入去.  ArrayAdapter<String> adapter = new ArrayAdapter<String>  (MainActivity.this, android.R.layout.simple_list_item_1,data);  ListView listView = (ListView) findViewById(R.id.list_view);  //調用ListView的對象把適配器傳進去.  listView.setAdapter(adapter); }}

數組中的數據是無法直接傳遞給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據類型,然后在構造函數中把要適配的數據傳入即可.注意我們使用了android.R.layout.simple_list_item_1作為ListView的子項布局的id,以及要適配的數據.

最后,我們要調用ListView的SetAdapter()方法,將構造好的適配器對象傳遞進去,這樣ListView和數據之間的關聯就建立完成了.

ListView與數據關聯

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造函數,用于將上下文,ListView子項布局的id和數據都傳遞進來.另外又重寫了getView()方法,首先通過getItem()方法得到當前項的Fruit的實例,然后使用LayoutInflater來為這個子項加載我們傳入的布局,接著調用View的fndViewById()方法分別獲取到ImageView和TextView的實例,并分別調用它們的setImageResource和setText方法來設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據,在Fruit類構造函數將水果的名字和對應圖片id傳入,然后把創建好的對象添加到水果列表中,接著我們再onCreate()方法中創建了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; }}

我們新建了一個內部類ViewHolder,用于對控件的實例進行緩存.當convertView為空的時候,創建一個ViewHolder對象,并將控件的實例都存放在ViewHolder里,然后調用View的SetTag()方法,將ViewHolder對象存儲在View中.當convertView不為空的時候則調用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：