ListView和RecyclerView
ListView
1. 先介绍ListView的简单用法
- 1.1. 首先新建一个ListViewTest项目,然后修改activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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"
android:orientation="vertical"
tools:context="com.example.listviewtest.MainActivity">
<ListView
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/list_view"/>
</LinearLayout>
在布局中引入ListView还算比较简单,先为ListView指定一个id,然后将宽度和高度属性指定为match_parent,这样ListView就能占满整个布局的空间
- 1.2. 接下来修改MainActivity中的代码
public class MainActivity extends AppCompatActivity {
private String[] data = {"apple", "banana", "orange", "watermelon", "pear",
"grope", "pineapple", "strawberry", "cherry", "mango", "apple", "banana",
"orange", "watermelon", "pear", "grope", "pineapple", "strawberry", "cherry", "mango"};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
ListView listView = (ListView) findViewById(R.id.list_view);
ArrayAdapter<String> adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, data);
listView.setAdapter(adapter);
listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
Toast.makeText(MainActivity.this,"你点击了"+data[position]+"",Toast.LENGTH_SHORT).show();
}
});
}
}
- 1.3. 创建数组存放数据:创建了一个数组来存放水果的名称。
- 1.4. 创建适配器:但是数组中的数据是无法直接传递给ListView的,需要借助适配器来完成,Android中提供了很多适配器的实现类,其中有比较好用的ArrayAdapter,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,ArrayAdapter有多个构造函数的重载,可以根据实际情况选择最合适的一种。这里由于我们提供的数据是字符串,因此将ArrayAdapter的泛型指定为String,然后在ArrayAdapter中一次传入当前上下文、ListView子项布局的id和要适配的数据。这里使用了android.R.layout.simple_list_item_1作为ListView子项布局的id,这是Android内置的布局文件,里面只有一个TextView,用于可现实简单的一段文本。这样适配器就构建好了。
- 1.5. 设置适配器:然后将构建好的适配器传进去,调用ListView的setAdapter方法,这样ListView和数据之间的关联就建立完成了
- 1.6. 设置子项的监听:最后设置的是item的点击事件的监听,通过position判断用户点击的是哪一个子项
2. 定制ListView界面
- 2.1. 准备一些图片对应上面的水果,接着定义一个实体类作为适配器适配的数据类型:
package com.example.listviewtest;
/**
* Created by 侯允林 on 2018/5/21.
*/
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;
}
}
- 2.2. 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 version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="horizontal">
<ImageView
android:id="@+id/fruit_image"
android:layout_width="100dp"
android:layout_height="100dp" />
<TextView
android:id="@+id/fruit_name"
android:layout_width="wrap_content"
android:layout_height="100dp"
android:layout_gravity="center"
android:layout_marginLeft="10dp"
android:gravity="center"/>
</LinearLayout>
- 2.3. 接着创建一个自定义的适配器,这个适配器继承自ArrayAdapter,并将泛型指定为Fruit类
package com.example.listviewtest;
import android.content.Context;
import android.support.annotation.LayoutRes;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
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;
/**
* Created by 侯允林 on 2018/5/21.
*/
public class FruitAdapter extends ArrayAdapter<Fruit> {
private int resourceId;
public FruitAdapter(@NonNull Context context, @LayoutRes int resource, @NonNull List<Fruit> objects) {
super(context, resource, objects);
this.resourceId = resource;
}
@NonNull
@Override
public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
Fruit fruit = getItem(position);//ArrayAdapter里面的泛型方法,
/*
public @Nullable T getItem(int position) {
return mObjects.get(position);
}*/
View view = LayoutInflater.from(getContext()).inflate(resourceId, parent, false);
//returns:The root View of the inflated hierarchy. If root was supplied and attachToRoot is true,
// this is root; otherwise it is the root of the inflated XML file.
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FruitAdapter中重写了父类的一组构造函数,用于将上下文、ListView的子项布局的id和数据都传递进来,然后重写了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,在这个方法中:
- 我们先通过getItem获得这个滚动到屏幕内的Fruit的实例
- 然后用LayoutInflater来为这个子项加载我们传入的布局,LayoutInflater的inflate方法接受三个参数,不多说
- 接下来调用View的findViewById()方法分别获取到ImageView和TextView的实例,并为它们设置图片和文字
- 最后将这个view返回
这样我们的适配器就完成了
- 2.4. 修改MainActivity中的代码
public class SecondActivity extends AppCompatActivity {
private List<Fruit> fruitList = new ArrayList<>();
private ListView fruit_list=null;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.second_layout);
ActionBar bar=getSupportActionBar();
if(bar!=null){
bar.hide();
}
fruit_list=(ListView)findViewById(R.id.fruit_list);
initFruit();
FruitAdapter adapter=new FruitAdapter(this,R.layout.fruit_item,fruitList);
fruit_list.setAdapter(adapter);
}
private void initFruit() {
for (int i = 0; i <= 2; i++) {
Fruit lemon = new Fruit("柠檬", R.drawable.lemon);
Fruit banana = new Fruit("香蕉", R.drawable.banana);
Fruit pear = new Fruit("梨子", R.drawable.pear);
Fruit mango = new Fruit("芒果", R.drawable.mango);
Fruit strawberry = new Fruit("草莓", R.drawable.strawberry);
Fruit xihongshi = new Fruit("西红柿", R.drawable.xihongshi);
Fruit mihoutao = new Fruit("猕猴桃", R.drawable.mihoutao);
Fruit taozi = new Fruit("桃子", R.drawable.taozi);
fruitList.add(lemon);
fruitList.add(banana);
fruitList.add(pear);
fruitList.add(mango);
fruitList.add(strawberry);
fruitList.add(xihongshi);
fruitList.add(mihoutao);
fruitList.add(taozi);
}
}
}
这样定制的ListView就完成了
- 2.5. 提升ListView的运行效率
这个listview的运行效率比较低,原因是,每滚动一个子项进入屏幕都要加载一次getView方法,当ListView快速滚动的时候就会成为性能的瓶颈,所以修改方案:
public class FruitAdapter extends ArrayAdapter<Fruit> {
private int resourceId;
public MyAdapter(@NonNull Context context, @LayoutRes int resource, @NonNull List<Fruit> objects) {
super(context, resource, objects);
this.resourceId=resource;
}
@NonNull
@Override
public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
View view=null;
if(convertView==null) {
view = LayoutInflater.from(getContext()).inflate(resourceId, parent, false);//同时使resource里面的宽高属性生效,false是不将resourceId添加到parent中
// 因为有了父布局之后就不能添加到listview中了
}else {
view=convertView;
}
ImageView fruit_image=(ImageView)view.findViewById(R.id.fruit_image);
TextView fruit_name=(TextView)view.findViewById(R.id.fruit_name);
Fruit fruit=getItem(position);
fruit_image.setImageResource(fruit.getImageId());
fruit_name.setText(fruit.getName());
return view;
}
}
可以看到在getView方法中对convertView进行了判断,如果为null则用LayoutInflater去加载不,如果不为null就直接对convertView重用,这样就大大提升了运行效率。
这块代码还是可以继续优化的,虽然已经不再去重复加载布局,但是,还是会去每次都去通过findViewById去获取控件的实例,可以借助一个ViewHolder来对这部分性能进行优化:
package com.example.listviewtest;
import android.content.Context;
import android.support.annotation.LayoutRes;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
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;
/**
* Created by 侯允林 on 2018/5/21.
*/
public class ViewHolderAdapter extends ArrayAdapter<Fruit> {
private int resourceId;
public ViewHolderAdapter(@NonNull Context context, @LayoutRes int resource, @NonNull List<Fruit> objects) {
super(context, resource, objects);
this.resourceId=resource;
}
@NonNull
@Override
public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
Fruit fruit=getItem(position);
View view;
ViewHolder viewHolder;
if(convertView==null){
view= LayoutInflater.from(getContext()).inflate(resourceId,parent,false);
viewHolder=new ViewHolder();
viewHolder.imageView=view.findViewById(R.id.fruit_image);
viewHolder.textView=view.findViewById(R.id.fruit_name);
view.setTag(viewHolder);
}else{
view =convertView;
viewHolder=(ViewHolder) view.getTag();
}
viewHolder.imageView.setImageResource(fruit.getImageId());
viewHolder.textView.setText(fruit.getName());
return view;
}
class ViewHolder{
ImageView imageView;
TextView textView;
}
}
我们定义了一个内部类,当convertView为null的时候,先用LayoutInflater去加载子项布局,然后获取子项布局中各个控件的实例保存到ViewHolder中,在调用view的setTag方法,将这个ViewHolder的实例保存到view中,接下来就可以对这个ViewHolder重用了,因为这个ViewHoder中保存了控件的实例所以就不会重复去获取控件的实例了,这样一来大大提高了运行速率
更强大的滚动控件RecyclerView
ListView虽然功能强大,但是也有其缺点,其扩展性不好,只能纵向滚动,必须用一些小技巧去提高它的运行速率,因此Android就提供了更强大的滚动控件——RecyclerView
- 1.1. 和百分比布局类似,RecyclerView属于新增的控件,为了让RecyclerView在所有Android版本上都能使用,Android团队采用了同样的方法,将RecyclerView定义在support库中,所以得在build.gradle中添加依赖库: compile 'com.android.support:recyclerview-v7:26.0.0-alpha1'然后点击Sync Now来进行同步
- 1.2. 在activity_main中:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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.recyclerviewtest.MainActivity"
android:orientation="vertical">
<android.support.v7.widget.RecyclerView
android:id="@+id/recycler_view"
android:layout_width="match_parent"
android:layout_height="match_parent" />
</LinearLayout>
注意的是RecyclerView不是内置在系统SDK中的所以使用的时候要将完整的包名写出来,并且得将高和宽设置为match_parent这样RecyclerView就能占满整个屏幕空间了。
- 1.3. 接下来为RecyclerView准备一个适配器
package com.example.recyclerviewtest;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;
import android.widget.TextView;
import java.util.List;
/**
* Created by 侯允林 on 2018/5/22.
*/
public class FruitAdapter extends RecyclerView.Adapter<FruitAdapter.ViewHolder> {
public FruitAdapter(List<Fruit> fruitList) {
this.mFruitList = fruitList;
}
static class ViewHolder extends RecyclerView.ViewHolder {
ImageView fruit_image;
TextView fruit_name;
public ViewHolder(View itemView) {
super(itemView);
fruit_image = (ImageView) itemView.findViewById(R.id.fruit_image);
fruit_name = (TextView) itemView.findViewById(R.id.fruit_name);
}
}
private List<Fruit> mFruitList;
@Override
public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.fruit_second_item, parent, false);
ViewHolder holder = new ViewHolder(view);
return holder;
//在这里创建一个ViewHolder
}
@Override
public void onBindViewHolder(ViewHolder holder, int position) {
Fruit fruit = mFruitList.get(position);
holder.fruit_image.setImageResource(fruit.getImageId());
holder.fruit_name.setText(fruit.getName());
//在这里面设置ImageView和TextView的数据来源
}
@Override
public int getItemCount() {
return mFruitList.size();
}
}
- 新建FruitAdapter类,先让这个继承自RecyclerView.Adapter,再在FruitAdapter类中建一个静态内部类ViewHolder并继承RecyclerView.ViewHolder,并重写它的构造方法,这个内部类先放在这,然后将泛型指定为这个内部类。
- 创建FruitAdapter的构造函数,这个方法用于把要展示的数据源传进来,并赋值给一个全局变量,这样我们的后续操作就在这个数据源的基础上进行
- 接着重写onCreateViewHolder、onBindViewHolder和getItemCount方法
- onCreateViewHolder方法是用于创建ViewHolder实例的,在这个方法中将fruit_item布局加载进来,然后创建一个ViewHolder实例,并将加载进来的布局传到构造函数中
- onBindViewHolder方法是用于对RecyclerView子项的数据进行赋值的,会再每个子项被滚动到屏幕内的时候执行,通过position获取对应的Fruit实例,然后再将数据设置到ViewHolder的ImageView和TextView中
- getItemCount告诉RecyclerView一共有多少子项
这样适配器就准备好了
- 1.4. 接下来就在MainActivity中使用这个RecyclerView:
package com.example.recyclerviewtest;
import android.content.Intent;
import android.support.v7.app.ActionBar;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.view.View;
import android.widget.Toast;
import java.util.ArrayList;
import java.util.List;
public class MainActivity extends AppCompatActivity {
RecyclerView recyclerView = null;
private List<Fruit> fruitList = new ArrayList<>();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
ActionBar bar = getSupportActionBar();
if (bar != null) {
bar.hide();
}
initFruit();
recyclerView = (RecyclerView) findViewById(R.id.recycler_view);
MyAdapter adapter = new MyAdapter(fruitList);
recyclerView.setAdapter(adapter);
LinearLayoutManager manager = new LinearLayoutManager(this);
recyclerView.setLayoutManager(manager);
}
public void button(View view){
Intent intent=new Intent(this,SecondActivity.class);
startActivity(intent);
}
private void initFruit() {
for (int i = 0; i <= 2; i++) {
Fruit lemon = new Fruit("柠檬", R.drawable.lemon);
Fruit banana = new Fruit("香蕉", R.drawable.banana);
Fruit pear = new Fruit("梨子", R.drawable.pear);
Fruit mango = new Fruit("芒果", R.drawable.mango);
Fruit strawberry = new Fruit("草莓", R.drawable.strawberry);
Fruit xihongshi = new Fruit("西红柿", R.drawable.xihongshi);
Fruit mihoutao = new Fruit("猕猴桃", R.drawable.mihoutao);
Fruit taozi = new Fruit("桃子", R.drawable.taozi);
fruitList.add(lemon);
fruitList.add(banana);
fruitList.add(pear);
fruitList.add(mango);
fruitList.add(strawberry);
fruitList.add(xihongshi);
fruitList.add(mihoutao);
fruitList.add(taozi);
}
}
}
- 2.1. 实现横向滚动
- 2.2. 修改fruit_item代码
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="vertical"
android:layout_marginLeft="5dp"
android:layout_marginRight="5dp">
<ImageView
android:id="@+id/fruit_image"
android:layout_width="100dp"
android:layout_height="100dp" />
<TextView
android:id="@+id/fruit_name"
android:layout_width="100dp"
android:layout_height="100dp"
android:layout_gravity="center"
android:gravity="center"/>
</LinearLayout>
- 2.3.修改MainActivity中的代码
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_second);
ActionBar bar = getSupportActionBar();
if (bar != null) {
bar.hide();
}
initFruit();
recyclerView = (RecyclerView) findViewById(R.id.recycler_view);
FruitAdapter adapter = new FruitAdapter(fruitList);
recyclerView.setAdapter(adapter);
LinearLayoutManager manager = new LinearLayoutManager(this);
manager.setOrientation(LinearLayoutManager.HORIZONTAL);
recyclerView.setLayoutManager(manager);
}
能注意到多了一个LinearLayoutManager,它的setOrientation方法来设置布局的排列方向,默认是纵向排列的,传入LinearLayoutManager。HORIZONTAL表示让布局横向排列
- 2.4. 设置点击事件
- 方法一:在onBindViewHolder里面设置水果图片和文字的监听
@Override
public void onBindViewHolder(final ViewHolder holder, int position) {
final Fruit fruit = mFruitlist.get(position);
holder.fruit_name.setText(fruit.getName());
holder.fruit_image.setImageResource(fruit.getImageId());
holder.fruit_image.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Toast.makeText(v.getContext(),"你点击了"+fruit.getName()+"的图片",Toast.LENGTH_SHORT).show();
}
});
holder.fruit_name.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Toast.makeText(v.getContext(),"你点击了"+fruit.getName()+"的名字",Toast.LENGTH_SHORT).show();
}
});
}
- 方法二:在onCreateViewHolder中
public ViewHolder onCreateViewHolder(final ViewGroup parent, int viewType) {
View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.fruit_item, parent, false);
final ViewHolder holder=new ViewHolder(view);
holder.fruit_image.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
int position=holder.getAdapterPosition(); //返回此项在适配器的位置(The adapter position of the item if it still exists in the adapter)
Fruit fruit=mFruitlist.get(position);
Toast.makeText(v.getContext(),"你点击了"+fruit.getName()+"的图片",Toast.LENGTH_SHORT).show();
}
});
holder.fruit_name.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
int position=holder.getAdapterPosition();
Fruit fruit=mFruitlist.get(position);
Toast.makeText(v.getContext(),"你点击了"+fruit.getName()+"的名字",Toast.LENGTH_SHORT).show();
}
});
return holder;
}
通过调用holder.getAdapterPosition();能够获取子项的位置
关于ListView和RecyclerView就先讲到这儿,以后用到再补充。