ListView
一、ListView的简单用法
1. ListView 允许用户通过手指上下滑动的方式将屏幕外的数据滚动到屏幕内,同时屏幕上原有的数据则会滚出屏幕。
2. 实现:
A. 效果图:
B. 代码:
activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.example.thinkpad.listviewtest.MainActivity">
<ListView
android:id="@+id/list_view"
android:layout_width="match_parent"
android:layout_height="match_parent">
</ListView>
</android.support.constraint.ConstraintLayout>
MainActivity.java
package com.example.thinkpad.listviewtest;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.ArrayAdapter;
import android.widget.ListView;
public class MainActivity extends AppCompatActivity {
private String[]data = {"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);
//因为此处提供的数据是String类型,所以适配器的泛型指定为String。
//在这个适配器的构造函数中传入三个参数
//第一个参数为当前上下文;第二个参数为ListView子项布局的id;第三个参数为适配的数据
ArrayAdapter<String> adapter = new ArrayAdapter<String>(
MainActivity.this,android.R.layout.simple_list_item_1,data);
//android.R.layout.simple_list_item_1 是android内置的布局文件,
//仅包含一个TextView,可用于简单地显示一段文本
ListView listView = (ListView)findViewById(R.id.list_view);
listView.setAdapter(adapter);
}
}
二、定制ListView的界面
1. 效果图:
2. 代码:
A. Fruit.java(实体类)
package com.example.thinkpad.listviewtest;
public class Fruit {
private String name; //水果名称
private int imageId; //水果对应图片的资源id
public Fruit(String name,int imageId){ //满参构造函数
this.name = name;
this.imageId = imageId;
}
public String getName(){ //获取水果名称
return name;
}
public int getImageId(){ //获取水果对应图片资源id
return imageId;
}
}
B. fruit_item.xml(自定义的子项布局)
<?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" />
<!--用于显示水果的图片-->
<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"/>
<!--用于显示水果名称,并让TextView在垂直方向上居中显示,左外边距为10dp-->
</LinearLayout>
C. FruitAdapter.java(自定义的适配器)
package com.example.thinkpad.listviewtest;
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;
import java.util.List;
public class FruitAdapter extends ArrayAdapter<Fruit> {
private int resourceId; //子项布局id
//构造函数,重写父类构造函数,用于将上下文,子项布局id以及数据传递进来。
public FruitAdapter(Context context, int textViewResourceId, List<Fruit> objects){
super(context,textViewResourceId,objects);
resourceId = textViewResourceId;
}
@Override
//重写了getView()方法,这个方法在每个子项被滚动到屏幕内的时候会被调用
public View getView(int position,View convertView,ViewGroup parent) {
Fruit fruit = getItem(position); //获取当前项的Fruit实例
//使用LayoutInflater来为这个子项加载我们传入的布局。
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);
//获取实例,并设置显示的图片和文字
fruitImage.setImageResource(fruit.getImageId());
fruitName.setText(fruit.getName());
return view;
}
}
D. activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.example.thinkpad.listviewtest.MainActivity">
<ListView
android:id="@+id/list_view"
android:layout_width="match_parent"
android:layout_height="match_parent">
</ListView>
</android.support.constraint.ConstraintLayout>
E. MainActivity.java
package com.example.thinkpad.listviewtest;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.ListView;
import java.util.ArrayList;
import java.util.List;
public class MainActivity extends AppCompatActivity {
private List<Fruit> mFruitList = 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 adapter = new FruitAdapter(MainActivity.this,R.layout.fruit_item,mFruitList);
ListView listView = (ListView)findViewById(R.id.list_view);
listView.setAdapter(adapter);
}
//初始化水果数据,传入水果名称和对应的图片资源id,循环两遍是为了充满整个屏幕,达到演示效果
private void initFruits(){
for (int i = 0 ; i < 2 ; i++){
Fruit apple = new Fruit("Apple",R.drawable.apple_pic);
mFruitList.add(apple);
Fruit banana = new Fruit("Banana",R.drawable.banana_pic);
mFruitList.add(banana);
Fruit orange = new Fruit("Orange",R.drawable.orange_pic);
mFruitList.add(orange);
Fruit watermelon = new Fruit("Watermelon",R.drawable.watermelon_pic);
mFruitList.add(watermelon);
Fruit pear = new Fruit("Pear",R.drawable.pear_pic);
mFruitList.add(pear);
Fruit grape = new Fruit("Grape",R.drawable.grape_pic);
mFruitList.add(grape);
Fruit pineapple = new Fruit("Pineapple",R.drawable.pineapple_pic);
mFruitList.add(pineapple);
Fruit strawberry = new Fruit("Strawberry",R.drawable.strawberry_pic);
mFruitList.add(strawberry);
Fruit cherry = new Fruit("Cherry",R.drawable.cherry_pic);
mFruitList.add(cherry);
Fruit mango = new Fruit("Mango",R.drawable.mango_pic);
mFruitList.add(mango);
}
}
}
三、提升ListView的效率(在二的基础上修改)
1. 修改FruitAdapter.java
package com.example.thinkpad.listviewtest;
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;
import java.util.List;
public class FruitAdapter extends ArrayAdapter<Fruit> {
private int resourceId; //子项布局id
//构造函数,重写父类构造函数,用于将上下文,子项布局id以及数据传递进来。
public FruitAdapter(Context context, int textViewResourceId, List<Fruit> objects){
super(context,textViewResourceId,objects);
resourceId = textViewResourceId;
}
@Override
//重写了getView()方法,这个方法在每个子项被滚动到屏幕内的时候会被调用
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){
//使用LayoutInflater来为这个子项加载我们传入的布局。
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{
//convertView参数用于将之前加载好的布局进行缓存,以便之后进行重用。
view = convertView;
viewHolder = (ViewHolder)view.getTag();//重新获取ViewHolder
}
//设置显示的图片和文字
viewHolder.fruitImage.setImageResource(fruit.getImageId());
viewHolder.fruitName.setText(fruit.getName());
return view;
}
class ViewHolder{ //内部类,用于对控件的实例进行缓存
ImageView fruitImage;
TextView fruitName;
}
}
2. 两点优化
A. 原先的代码中,FruitAdapter 的 getView() 方法每次都将布局重新加载一遍,使得 ListView 效率低。
→getView() 方法中的参数convertView用于将之前加载好的布局进行缓存,若无缓存则加载布局,之后convertView将不为null,可直接赋值给view。可以说布局的框架不用重复加载了,但此时的控件实例还得重复获取。
B. getView() 方法每次都会调用View的findViewById()方法来获取控件实例。
→可借助内部类ViewHolder对控件实例进行缓存,当convertView为null时,创建这个内部类的对象,并将控件实例存放在该对象中,然后调用View的方法setTag()把该对象存储到View中;当convertView不为null时,则调用View的getTag()方法,把ViewHolder对象重新取出来,并获得控件实例。
以上两点优化,可解决重复加载布局以及重复获取控件实例带来的效率低下的问题。
3. ListView 的点击事件
A. 效果图:
B. 代码:(MainActivity.java)
package com.example.thinkpad.listviewtest;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ListView;
import android.widget.Toast;
import java.util.ArrayList;
import java.util.List;
public class MainActivity extends AppCompatActivity {
private List<Fruit> mFruitList = 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 adapter = new FruitAdapter(MainActivity.this,R.layout.fruit_item,mFruitList);
ListView listView = (ListView)findViewById(R.id.list_view);
listView.setAdapter(adapter);
//注册监听器,当用户点击ListView中任意一子项时,均会回调onItemClick()方法
listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {
//改方法通过参数i判断用户点击的是哪一个子项
Fruit fruit = mFruitList.get(i);
Toast.makeText(MainActivity.this,fruit.getName(),Toast.LENGTH_SHORT).show();
}
});
}
//初始化水果数据,传入水果名称和对应的图片资源id,循环两遍是为了充满整个屏幕,达到演示效果
private void initFruits(){
for (int i = 0 ; i < 2 ; i++){
Fruit apple = new Fruit("Apple",R.drawable.apple_pic);
mFruitList.add(apple);
Fruit banana = new Fruit("Banana",R.drawable.banana_pic);
mFruitList.add(banana);
Fruit orange = new Fruit("Orange",R.drawable.orange_pic);
mFruitList.add(orange);
Fruit watermelon = new Fruit("Watermelon",R.drawable.watermelon_pic);
mFruitList.add(watermelon);
Fruit pear = new Fruit("Pear",R.drawable.pear_pic);
mFruitList.add(pear);
Fruit grape = new Fruit("Grape",R.drawable.grape_pic);
mFruitList.add(grape);
Fruit pineapple = new Fruit("Pineapple",R.drawable.pineapple_pic);
mFruitList.add(pineapple);
Fruit strawberry = new Fruit("Strawberry",R.drawable.strawberry_pic);
mFruitList.add(strawberry);
Fruit cherry = new Fruit("Cherry",R.drawable.cherry_pic);
mFruitList.add(cherry);
Fruit mango = new Fruit("Mango",R.drawable.mango_pic);
mFruitList.add(mango);
}
}
}
整理学习自郭霖大佬的《第一行代码》
目前小白一名,持续学习Android中,如有错误请批评指正!