Android ListView 代码1
ListView效果
ListView允许用户通过手机上下滑动的方式将屏幕外的数据滚动到屏幕内，同时屏幕上的数据则会滚动出屏幕。
一、ListView的简单用法
activity_main.xml
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" 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>
MainActivity
public class MainActivity extends Activity {
private String[] datas={"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>adpter=new ArrayAdapter<String>(MainActivity.this,android.R.layout.simple_list_item_1,datas);//当前上下文、ListView子项布局的Id，要适配的数据
//其中android.R.layout.simple_list_item_1是Android内置的布局文件，里面只有一个TextView可用于简单地显示一段文本。
ListView listview=(ListView)findViewById(R.id.list_view);
listview.setAdapter(fruitAdapter);
}
}
ListView用来展示大量数据，我们先将数据准备好，比如这里的datas数组。不过数据无法直接传递给ListView,需要借助适配器来完成。ArrayAdapter就是Android提供的一个适配器的实现类，它可以通过泛型来指定要适配的数据类型，然后在构造函数中把要适配的数据传入。
二、定制ListView的界面
目标
实现水果名称旁边都有一个图样
步骤
1.定义一个实体类作为ListView适配器的适配对象。
Ftuit类
public class Fruit {
private String name;
private int imageId;
public Fruit(String name,int imageId){
this.name=name;
this.imageId=imageId;
}
public int getImageId() {
return imageId;
}
public String getName() {
return name;
}
}
2.为ListView的子项指定我们的自定义布局
fruit_item.xml
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
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>
3.创建自定义适配器，这个适配器继承自ArrayAdapter，并将泛型指定为Fruit类
FruitAdapter类
public class FruitAdapter extends ArrayAdapter<Fruit> {
private int resourceId;
//重写父类构造方法，用于将上下文、ListView子项布局的id和数据传递进来
public FruitAdapter(Context context,int textViewResourceId,List<Fruit> objects){
super(context,textViewResourceId,objects);
resourceId=textViewResourceId;
}
//这个方法在每个子项被滚动到屏幕内时会被调用
@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);//使用LayoutInflater来为这个子项加载我们传入的布局
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;
}
}
LayoutInflater的inflate()方法接收三个参数，第三个指定成false表示我们在父布局中声明的layout属性生效，但不会为这个View添加父布局，因为View一旦有了父布局之后，它就不能再添加到ListView中了。
4.调用View的findViewById()获取ImageView和TextView的实例，设置图片和文字
MainActivity
public class MainActivity extends Activity {
private List<Fruit>fruitList=new ArrayList<>();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
initFruits();//初始化水果数据
FruitAdapter fruitAdapter=new FruitAdapter(MainActivity.this,R.layout.fruit_item,fruitList);
ListView listview=(ListView)findViewById(R.id.list_view);
listview.setAdapter(fruitAdapter);
}
private void initFruits(){
for(int i=0;i<2;i++){
Fruit apple=new Fruit("Apple",R.drawable.apple);
fruitList.add(apple);
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);
}
}
}
运行时，先创建FruitAdapter适配器，将上下文和子项ID(R.layout.fruit_item）和数据传入，当每个子项被滑动到屏幕内时，调用getView()方法，通过LayoutInflater获得View，再通过view的findViewById()将水果的图片和名称传入。
接下来获得activity_main.xml中的ListView，将创建好的适配器传入。
三、ListView的性能提升
ListView效率很低，在FruitAdapter的getView()方法中，每次都将布局重新加载一遍，当ListView快速滚动时，这就会成为性能的瓶颈
仔细观察会发现，getView()中有一个convertView参数，这个参数用于缓存之前加载好的布局，以便之后的重用。
修改FruitAdapter中的代码1:使用convertView加载缓存布局
public class FruitAdapter extends ArrayAdapter<Fruit> {
...
@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;
}
}
修改FruitAdapter中的代码2:使用ViewHolder类保存控件(通过view的setTag()方法，将ViewHolder对象存储在View中)
public class FruitAdapter extends ArrayAdapter<Fruit> {
...
@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);
View view;
ViewHolder viewHolder;
if(convertView==null){
view=LayoutInflater.from(getContext()).inflate(resourceId,parent,false);
viewHolder=new ViewHolder();
viewHolder.fruitImage=view.findViewById(R.id.fruit_image);
viewHolder.fruitname=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;
}
}
四、ListView的点击事件
修改MainActivity中的代码
public class MainActivity extends Activity {
private List<Fruit>fruitList=new ArrayList<>();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
ActionBar actionBar=getActionBar();
if(actionBar!=null){
actionBar.hide();
}
initFruits();//初始化水果数据
FruitAdapter fruitAdapter=new FruitAdapter(MainActivity.this,R.layout.fruit_item,fruitList);
ListView listview=(ListView)findViewById(R.id.list_view);
listview.setAdapter(fruitAdapter);
listview.setOnItemClickListener(new AdapterView.OnItemClickListener(){
@Override
public void onItemClick(AdapterView<?> parent, View view, int positon, long id) {
Fruit fruit=fruitList.get(positon);
Toast.makeText(MainActivity.this, fruit.getName(), Toast.LENGTH_SHORT).show();
}
});
}
private void initFruits(){
for(int i=0;i<2;i++){
Fruit apple=new Fruit("Apple",R.drawable.apple);
fruitList.add(apple);
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);
}
}
}
我们使用setOnItemClickListenser()方法为ListView注册监听器，当点击ListView任何一个子项时，就会回调onItemClick()方法。position参数可以判断用户点击的是哪一个子项，对应着数组的下标，然后就能获取到相应的数据。
