引言
listView就是我们过去常用的列表组件,不过现在已经基本被recycleView取代了,主要还是listview的性能和可扩展性跟不上新的recycleView,越来越少的场景能够使用它,不过recycleView脱胎于Listview和GridView,并进行了大量优化,所以学习Listview仍然很有意义。
基本用法
列表效果如下
xml文件
<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>
adapter
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); // 获取当前项的Fruit实例
View view = LayoutInflater.from(getContext()).inflate(resourceId, parent, false);
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;
}
}
activity
public class MainActivity extends AppCompatActivity {
//创建数据集合
private List<Fruit> fruitList = new ArrayList<Fruit>();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
initFruits();
//创建listview适配器对象
FruitAdapter adapter = new FruitAdapter(MainActivity.this,R.layout.fruit_item,fruitList);
ListView listView = (ListView) findViewById(R.id.list_view);
//将适配器赋予listview
listView.setAdapter(adapter);
}
//创建数据集合内容
private void initFruits() {
for (int i = 0; i < 2; i++) {
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);
}
}
}
常用api
在使用 ListView 时有很多细节可以优化,其中运行效率就是很重要的一点。目前我们 ListView 的运行效率是很低的,因为在 FruitAdapter 的 getView() 方法中,每次都将布局重新加载了一遍,当 ListView 快速滚动的时候,这就会成为性能的瓶颈。
在 getView() 方法中有一个 convertView 参数,这个参数用于将之前加载喊得布局进行缓存,以便之后可以进行重用,修改 FruitAdapter,如下所示:
public class FruitAdapter extends ArrayAdapter {
...
@Override
public View getView(int position, View convertView, ViewGroup parent) {
Fruit fruit = getItem(position); // 获取当前项的Fruit实例
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());
fruitName.setText(fruit.getName());
return view;
}
}
在getView()方法中进行了判断,如果 convertView 为null,则使用 LayoutInflater 去加载布局,如果不为null则直接对 convertView 进行重用。这样就大大提高了 ListView 的运行效率,在快速滚动的时候也可以表现出更好的性能。
目前虽然已经不会再重复去加载布局,但是每次在 getView() 方法中还是会调用 View 的 findViewById() 方法来获取一次控件的实例。我们可以借助一个 ViewHolder 来对这部分性能进行优化修改代码,修改代码,如下如下:
public class FruitAdapter extends ArrayAdapter {
private int resourceId;
public FruitAdapter(Context context, int textViewResourceId,
List objects) {
super(context, textViewResourceId, objects);
resourceId = textViewResourceId;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
Fruit fruit = getItem(position); // 获取当前项的Fruit实例
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); // 将ViewHolder存储在View中
} else {
view = convertView;
viewHolder = (ViewHolder) view.getTag(); // 重新获取ViewHolder
}
viewHolder.fruitImage.setImageResource(fruit.getImageId());
viewHolder.fruitName.setText(fruit.getName());
return view;
}
class ViewHolder {
ImageView fruitImage;
TextView fruitName;
}
}
我们新增了一个内部类 ViewHolder ,用于对控件的实例进行缓存。当 convertView 为null时,创建 ViewHolder 对象,并将控件的实例存放在 ViewHolder 里,然后调用 View 的 setTag() 方法,将 ViewHolder 对象存储在View 中。当 convertView 部null的时候,则调用View的 getTag() 方法,把 ViewHolder 重新取出。这样所有控件的实例缓存在了 ViewHolder 里,就不用每次都通过 findViewById() 方法来获取控件的实例了。
ListView 的点击事件
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();
}
});
小tips
注意使用listview的时候最好是把优化这块加上,避免因为数据量过多导致的app卡顿甚至oom异常
结语
listview虽然大幅度被recycleView取代,但是因为recyclerView的用法和原理是listview优化的一大进步的结果,尤其是优化适配器缓存这一块非常有用,所以学习listview仍有意义