两个控件
1. ListView
基本使用示例
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);
ArrayAdapter<String> adapter = new ArrayAdapter<String>( MainActivity.this,
android.R.layout.simple_list_item_1, data);
ListView listView = (ListView) findViewById(R.id.list_view);
listView.setAdapter(adapter);
}
}
数组中的数据是无法直接传递给ListView的,我们还需要借助适配器来完成。Android中提供了很多适配器的实现类,其中常用的就是ArrayAdapter。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。ArrayAdapter有多个构造函数的重 载,应该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都是字符串,因此将 ArrayAdapter的泛型指定为String ,然后在ArrayAdapter的构造函数中依次传入当前上下文、 ListView子项布局的id,以及要适配的数据。注意,我们使用了 android.R.layout.simple_list_item_1 作为ListView子项布局的id,这是一个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。这样适配器对象就构建好了。 最后,还需要调用ListView的setAdapter() 方法,将构建好的适配器对象传递进去,这样 ListView和数据之间的关联就建立完成了。
定制ListView的界面
- 新建实体类Fruit,作为ListView适配器的适配类型。
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;
}
}
- 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:
<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>
- 创建一个自定义的适配器(新建类FruitAdapter),这个适配器继承自ArrayAdapter,并将泛型指定为Fruit 类:
public class FruitAdapter extends ArrayAdapter<Fruit> {
private int resourceId;
//构造方法
public FruitAdapter(Context context,int textViewResourceId,List<Fruit> objects) {
super(context, textViewResourceId, objects);
resourceId = textViewResourceId;
}
//重写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
//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);
fruitImage.setImageResource(fruit.getImageId());
fruitName.setText(fruit.getName());
return view;
}
}
- 修改MainActivity中的代码
public class MainActivity extends AppCompatActivity {
private List<Fruit> fruitList = new ArrayList<>();//水果实例列表
@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, fruitList);
ListView listView = (ListView) findViewById(R.id.list_view);
listView.setAdapter(adapter);
}
private void initFruits() {
//创建水果实例,并添加到水果实例列表
for (int i = 0; i < 5; i++) {
Fruit apple = new Fruit("Apple", R.drawable.apple_pic);
fruitList.add(apple);
Fruit banana = new Fruit("Banana", R.drawable.apple_pic);
fruitList.add(banana);
Fruit orange = new Fruit("Orange", R.drawable.apple_pic);
fruitList.add(orange);
}
}
}
提升ListView的运行效率
- 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。getView() 方法中还有一个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,如下所示:
public View getView(int position, View convertView, ViewGroup parent) {
Fruit fruit = getItem(position);//获取当前项的Fruit示例
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;
}
- 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FruitAdapter 中的代 码,如下所示:
public View getView(int position, View convertView, ViewGroup parent) {
Fruit fruit = getItem(position);//获取当前项的Fruit示例
View view;
ViewHolder viewHolder;
if (convertView == null){
view = LayoutInflater.from(getContext()).inflate(resourceId,parent,false);
viewHolder = new ViewHolder();
viewHolder.fruitName = (TextView) view.findViewById(R.id.fruit_name);
viewHolder.fruitImage = (ImageView) view.findViewById(R.id.fruit_image);
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;
}
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 的时候,创建一个ViewHolder 对象,并将控件的实例都存放在ViewHolder 里,然后调 用View 的setTag() 方法,将ViewHolder 对象存储在View 中。当convertView 不 为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 实例了。
ListView的点击事件
listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> adapterView, View view, int position, long l) {
Fruit fruit = fruitList.get(position);
Toast.makeText(MainActivity.this,fruit.getName(),Toast.LENGTH_SHORT).show();
}
});
2. RecyclerView
基本用法
- 打开app/build.gradle文件,在dependencies 闭包中添加语句并同步:
implementation ‘com.android.support:recyclerview-v7:28.0.0’ - 修改activity_main.xml中的代码:
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
<androidx.recyclerview.widget.RecyclerView
android:id="@+id/recycler_view"
android:layout_width="match_parent"
android:layout_height="match_parent" />
</LinearLayout>
- 新建FruitAdapter 类(适配器),继承自 RecyclerView.Adapter ,并将泛型指定为FruitAdapter.ViewHolder。ViewHolder 是在FruitAdapter 中定义的一个内部类,代码:
public class FruitAdapter extends RecyclerView.Adapter<FruitAdapter.ViewHolder> {
private List<Fruit> mFruitList;
//先定义了一个内部类ViewHolder ,ViewHolder 要继承自RecyclerView.ViewHolder 。
//然后ViewHolder 的构造函数中要传入一个View 参数,这个参数通常就是RecyclerView子项的最外层布局,
//就可以通过findViewById()方法来获取到布局中的ImageView和 TextView的实例了。
static class ViewHolder extends RecyclerView.ViewHolder {
ImageView fruitImage;
TextView fruitName;
public ViewHolder(View view) {
super(view);
fruitImage = (ImageView) view.findViewById(R.id.fruit_image);
fruitName = (TextView) view.findViewById(R.id.fruit_name);
}
}
//自己的构造方法
public FruitAdapter(List<Fruit> fruitList) {
mFruitList = fruitList;
}
//用于创建ViewHolder 实例的,将fruit_item 布局加载进来,
// 然后创建一个ViewHolder 实例,并把加载出来的布局传入到构造函数当中,
// 最后将ViewHolder 的实例返回。
public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View view = LayoutInflater.from(parent.getContext()) .inflate(R.layout.fruit_item, parent, false);
ViewHolder holder = new ViewHolder(view);
return holder;
}
//用于对 RecyclerView子项的数据进行赋值的,会在每个子项被滚动到
// 屏幕内的时候执行,这里我们通过 position 参数得到当前项的Fruit 实例,
// 然后再将数据设置到ViewHolder 的ImageView和 TextView当中即可。
public void onBindViewHolder(ViewHolder holder, int position) {
Fruit fruit = mFruitList.get(position);
holder.fruitImage.setImageResource(fruit.getImageId());
holder.fruitName.setText(fruit.getName());
}
//用于告诉RecyclerView一共有多少子项,返回数据源的长度。
public int getItemCount() {
return mFruitList.size();
}
}
- 修改MainActivity中的代码:
public class MainActivity extends AppCompatActivity {
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
RecyclerView recyclerView = (RecyclerView) findViewById(R.id.recycler_view);
LinearLayoutManager layoutManager = new LinearLayoutManager(this);
recyclerView.setLayoutManager(layoutManager);
FruitAdapter adapter = new FruitAdapter(fruitList);
recyclerView.setAdapter(adapter);
}
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);
}
}
}
横向滚动
- 把fruit_item 里的元素改成垂直排列,即修改fruit_item.xml中的代码:
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="100dp"
android:layout_height="wrap_content" >
<ImageView
android:id="@+id/fruit_image"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal" />
<TextView
android:id="@+id/fruit_name"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal"
android:layout_marginTop="10dp" />
</LinearLayout>
- 修改MainActivity中的代码:
······
LinearLayoutManager layoutManager = new LinearLayoutManager(this);
添加:layoutManager.setOrientation(LinearLayoutManager.HORIZONTAL);
recyclerView.setLayoutManager(layoutManager);
······
瀑布流布局 & 网格布局
以瀑布流布局为例
- 修改一下fruit_item.xml中的代码:
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_margin="5dp" >
<ImageView
android:id="@+id/fruit_image"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal"/>
<TextView
android:id="@+id/fruit_name"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="left"
android:layout_marginTop="10dp" />
</LinearLayout>
- 修改fruit_item.xml中的代码:
public class MainActivity extends AppCompatActivity {
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
RecyclerView recyclerView = (RecyclerView) findViewById(R.id.recycler_view);
StaggeredGridLayoutManager layoutManager = new StaggeredGridLayoutManager(3, StaggeredGridLayoutManager.VERTICAL);//主要语句,3列瀑布流布局
recyclerView.setLayoutManager(layoutManager);
FruitAdapter adapter = new FruitAdapter(fruitList);
recyclerView.setAdapter(adapter);
}
private void initFruits() {
for (int i = 0; i < 50; i++) {
Fruit apple = new Fruit("Apple", R.drawable.apple_pic);
fruitList.add(apple);
Fruit banana = new Fruit("Banana", R.drawable.apple_pic);
fruitList.add(banana);
Fruit orange = new Fruit("Orange", R.drawable.apple_pic);
fruitList.add(orange);
Fruit watermelon = new Fruit("Watermelon", R.drawable.apple_pic);
}
}
}
RecyclerView的点击事件
- 修改FruitAdapter 中的代码:
public class FruitAdapter extends RecyclerView.Adapter<FruitAdapter.ViewHolder> {
private List<Fruit> mFruitList;
static class ViewHolder extends RecyclerView.ViewHolder {
View fruitView;//新添加代码
ImageView fruitImage;
TextView fruitName;
public ViewHolder(View view) {
super(view);
fruitView = view;//新添加代码
fruitImage = (ImageView) view.findViewById(R.id.fruit_image);
fruitName = (TextView) view.findViewById(R.id.fruit_name);
}
}
public FruitAdapter(List<Fruit> fruitList) {
mFruitList = fruitList;
}
//用于创建ViewHolder 实例的,将fruit_item 布局加载进来,
// 然后创建一个ViewHolder 实例,并把加载出来的布局传入到构造函数当中,
// 最后将ViewHolder 的实例返回。
public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View view = LayoutInflater.from(parent.getContext()) .inflate(R.layout.fruit_item, parent, false);
final ViewHolder holder = new ViewHolder(view);
//新添加代码
holder.fruitView.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
int position = holder.getAdapterPosition();
Fruit fruit = mFruitList.get(position);
Toast.makeText(v.getContext(), "you clicked view " + fruit.getName(), Toast.LENGTH_SHORT).show();
}
});
//新添加代码
holder.fruitImage.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
int position = holder.getAdapterPosition();
Fruit fruit = mFruitList.get(position);
Toast.makeText(v.getContext(),"you clicked image "+fruit.getName(),Toast.LENGTH_SHORT).show();
}
});
return holder;
}
//用于对 RecyclerView子项的数据进行赋值的,会在每个子项被滚动到
// 屏幕内的时候执行,这里我们通过 position 参数得到当前项的Fruit 实例,
// 然后再将数据设置到ViewHolder 的ImageView和 TextView当中即可。
public void onBindViewHolder(ViewHolder holder, int position) {
Fruit fruit = mFruitList.get(position);
holder.fruitImage.setImageResource(fruit.getImageId());
holder.fruitName.setText(fruit.getName());
}
//用于告诉RecyclerView一共有多少子项,返回数据源的长度。
public int getItemCount() {
return mFruitList.size();
}
}