一、首先我们先实现ListView的构建工作
1.定义一个实体类,作为ListView适配器的适配类型。新建Fruit类,代码如下:
package org.lxh.demo; 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; } }
2.然后需要为ListView的子项指定一个我们自定义的布局,在layout目录下新建fruit_item.xml,代码如下:
<?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="fill_parent" android:layout_height="fill_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="10sp" /> </LinearLayout>
3.接下来我们要创建一个适配器,这个适配器继承自ArrayAdapter,并指定泛型类型为Fruit类。新建FruitAdapter,代码如下:
package org.lxh.demo; import java.util.List; import android.content.Context; import android.view.LayoutInflater; import android.view.View; import android.view.ViewGroup; import android.widget.ArrayAdapter; import android.widget.ImageView; import android.widget.TextView; 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 fruitImageView=(ImageView)view.findViewById(R.id.fruit_image); TextView fruitTextView=(TextView)view.findViewById(R.id.fruit_name); fruitImageView.setImageResource(fruit.getImageId()); fruitTextView.setText(fruit.getName()); return view; } }
4.下面在main.xml中加入一个ListView:
<?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="fill_parent" android:layout_height="fill_parent" android:orientation="vertical" > <ListView android:id="@+id/list_view" android:layout_width="fill_parent" android:layout_height="wrap_content" /> </LinearLayout>
5.然后MainActivity:
package org.lxh.demo; import java.util.ArrayList; import java.util.List; import android.app.Activity; import android.os.Bundle; import android.widget.ListView; public class Hello extends Activity { private List<Fruit> fruitList = new ArrayList<Fruit>(); public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); // 生命周期方法 super.setContentView(R.layout.main); // 设置要使用的布局管理器 initFruits(); FruitAdapter adapter = new FruitAdapter(Hello.this, R.layout.fruit_item, fruitList); ListView listView = (ListView) findViewById(R.id.list_view); listView.setAdapter(adapter); } private void initFruits() { Fruit appleFruit = new Fruit("Apple", R.drawable.apple_pic); fruitList.add(appleFruit); Fruit bananaFruit = new Fruit("Banana", R.drawable.banana_pic); fruitList.add(bananaFruit); Fruit orangeFruit = new Fruit("Orange", R.drawable.orange_pic); fruitList.add(orangeFruit); Fruit waterFruit = new Fruit("Apple", R.drawable.watermelon_pic); fruitList.add(waterFruit); } }
运行实例如下:
二、提升ListView的运行效率
ListView这个控件很难用,因为它有很多细节可以优化,其中运行效率就是很重要的一点。目前我们的ListView的运行效率很低,因为在FruitAdapter的getView()方法中每次都将布局重新加载了一遍,当ListView快速滚动的时候就会成为性能的瓶颈。
仔细观察,getView()方法还有一个convertView参数,这个参数用于将之前加载好的布局进行缓存,以便之后可以进行重用。修改FruitAdapter中的代码:
package org.lxh.demo; import java.util.List; import android.content.Context; import android.view.LayoutInflater; import android.view.View; import android.view.ViewGroup; import android.widget.ArrayAdapter; import android.widget.ImageView; import android.widget.TextView; 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; if (convertView == null) { view = LayoutInflater.from(getContext()).inflate(resourceId, null); } else { view = convertView; } ImageView fruitImageView = (ImageView) view .findViewById(R.id.fruit_image); TextView fruitTextView = (TextView) view.findViewById(R.id.fruit_name); fruitImageView.setImageResource(fruit.getImageId()); fruitTextView.setText(fruit.getName()); return view; } }
可以看到,现在我们在getView()方法中进行了判断,如果convertView为空,则使用LayoutInflater去加载布局,如果不为空则直接对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变现出更好的性能。
不过目前我们的代码还是可以优化的,虽然我们现在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()方法去获取一次控件的实例。我们借助一个ViewHolder来对这部分性能进行优化,修改FruitAdapter中的代码:
package org.lxh.demo; import java.util.List; import android.content.Context; import android.view.LayoutInflater; import android.view.View; import android.view.ViewGroup; import android.widget.ArrayAdapter; import android.widget.ImageView; import android.widget.TextView; 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; } }
我们新增了一个内部类ViewHolder,用于对控件的实例进行缓存。当converView为空的时候创建一个ViewHoler对象,并将控件的实例都放在ViewHolder里,然后调用View的setTag()方法,将ViewHolder对象存储在View中。当当converView不为空则调用View的getTag()方法,把ViewHolder重新取出。这样所有的控件的实例都缓存在了ViewHolder里,就没有必要每次都通过findViewById()方法来获取控件的实例了。通过这两步优化之后,ListView的运行效率就已经非常不错了。