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Android 基础(四)滚动组件与Adapter

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Android滚动控件

ListView的基本应用

因为要展示大量数据,这就涉及到将数据注入到view中的过程。

不过,数组中的数据是无法直接传递到view的,这需要借助适配器来实现。

Android是完全遵循MVC模式设计的框架,Activity是Controller,layout是View,因为layout五花八门,很多数据都不能直接绑定上去,所以Android引入了Adapter这个机制作为复杂数据的展示的转换载体,所以各种Adapter只不过是转换的方式和能力不一样而已。
几种常用的Adapter:

技术图片

展示简单的字符串ListView

public class ListActivity extends BaseActivity {

    private String[] data = { "Apple","Banana","Orange","WaterMelon",
            "Pear","Grape","Pineapple","Strawberry","Strawberry","Strawberry","Strawberry",
            "Strawberry","Strawberry","Strawberry","Strawberry","Strawberry","Strawberry","Strawberry"
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_list);
        ArrayAdapter<String> adapter = new ArrayAdapter<String>(
                ListActivity.this, android.R.layout.simple_list_item_1,data
        );
        ListView listView = (ListView) findViewById(R.id.list_view);
        listView.setAdapter(adapter);
    }
}

上文中的android.R.layout.simple_list_item_1用的是android自带的布局,如果要自定义一个View,需要手写一个item的布局,然后写一个适配器,需要重写构造函数和getView方法。

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); // 获取当前项的Fruit实例
        View view;
        ViewHolder viewHolder;
        if (convertView == null) {
           // 加载布局,listitem不需要添加父布局
            view = LayoutInflater.from(getContext()).inflate(resourceId, parent, false);
           // new一个viewHolder,拥有对view中各个控件的引用
            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 {
            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;

    }

}

这里优化,当convertView为空的时候才去加载布局,创建view。如果不为空则直接用缓存中的view实例convertView,就省去了findbyview这种创建view实例的过程。通过获取viewHolder得到view实例中各个控件的引用,因为数据是不能复用的,所以需要重新set相关控件的数据。

关于缓存:https://blog.csdn.net/Double2hao/article/details/49077739

调用比较简单

 	@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_list);
        initFruits();
        FruitAdapter adapter = new FruitAdapter(ListActivity.this,R.layout.fruit_item,fruitList);
        ListView listView = (ListView) findViewById(R.id.list_view);
        listView.setAdapter(adapter);
    }

    private void initFruits() {
        for (int i = 0; i < 10; ++i) {
            Fruit apple = new Fruit("Apple",R.drawable.img_1);
            fruitList.add(apple);

            Fruit banana = new Fruit("banana",R.drawable.img_2);
            fruitList.add(banana);
        }
    }

RecyclerView的基本应用

ListView是有很多缺点的,因为效率不高所以需要加很多优化。

将上文的demo改成使用RecyclerView实现:

  1. 新建适配器,让这个适配器继承自RecyclerView.Adapter,并将泛型指定为新适配器的ViewHolder。
  2. ViewHolder是在适配器中定义的一个内部类,需要继承RecyclerView.ViewHolder
  3. ViewHolder的构造函数中要传入一个View参数,通常是RecyclerView子项最外层布局,于是就可以通过findViewById的方法来获取布局中的ImageViewTextView的实例。
  4. 因为继承自RecyclerView.Adapter,所以必须重写onCreateViewHolderonBindViewHoldergetItemCount
public class RecyclerFruitAdapter extends RecyclerView.Adapter<RecyclerFruitAdapter.ViewHolder> {

    private List<Fruit> mFruitList; // 数据源,存实例对象

    public RecyclerFruitAdapter(List<Fruit> fruitList) {
        this.mFruitList = fruitList;
    }

    @NonNull
    @Override
    public ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.fruit_item,parent,false);
        final ViewHolder viewHolder = new ViewHolder(view);
        viewHolder.fruitView.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Fruit fruit = mFruitList.get(viewHolder.getAdapterPosition());
                Toast.makeText(v.getContext(), "you clicked view " + fruit.getName(), Toast.LENGTH_SHORT).show();
            }
        });
        viewHolder.fruitImage.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Fruit fruit = mFruitList.get(viewHolder.getAdapterPosition());
                Toast.makeText(v.getContext(), "you clicked image " + fruit.getName(), Toast.LENGTH_SHORT).show();
            }
        });
        return viewHolder;
    }

    @Override
    public void onBindViewHolder(@NonNull ViewHolder holder, int position) {
        Fruit fruit = mFruitList.get(position);
        holder.fruitImage.setImageResource(fruit.getImageId());
        holder.fruitName.setText(fruit.getName());
    }

    @Override
    public int getItemCount() {
        return mFruitList.size();
    }

    static class ViewHolder extends RecyclerView.ViewHolder {

        View fruitView;
        ImageView fruitImage;
        TextView fruitName;

        public ViewHolder(@NonNull View itemView) {
            super(itemView);
            fruitView = itemView;
            fruitImage = (ImageView) itemView.findViewById(R.id.fruit_image);
            fruitName = (TextView) itemView.findViewById(R.id.fruit_name);
        }

    }

}
  • onCreateViewHolder:是用来创建ViewHolder实例的,在这里将item布局加载,然后将加载好的view传入holder的构造函数中创建ViewHolder实例。
  • onBindViewHolder:是对item的数据进行赋值,会在每个子项被滚动到屏幕内的时候执行。通过position参数可以得到当前项的数据,即fruit实例。然后再对保存的ViewHolder中的fruitImagefruitName对象赋值。(省去获得实例的过程)
  • getItemCount:返回RecycleView的长度
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_recycle);
        initFruits();
        RecyclerView recyclerView = (RecyclerView) findViewById(R.id.recycler_view);
        LinearLayoutManager layoutManager = new LinearLayoutManager(this);
        recyclerView.setLayoutManager(layoutManager);
        RecyclerFruitAdapter fruitAdapter = new RecyclerFruitAdapter(fruitList);
        recyclerView.setAdapter(fruitAdapter);
    }

这里多了一个LayoutManager,用于指定RecyclerView的布局方式,这里使用LinearLayoutManager就是线性布局的意思。

简单实现的xml如下:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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=".Recycler.RecycleActivity">

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/recycler_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>

</androidx.constraintlayout.widget.ConstraintLayout>
<?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"> #一个item宽度刚好是父布局的宽度,高度则用当前刚刚好包住里面的内容

    <ImageView
        android:id="@+id/fruit_image"
        android:layout_height="80dp"
        android:layout_width="100dp"
        />

    <TextView
        android:id="@+id/fruit_name"
        android:layout_marginLeft="8dp"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp" />

</LinearLayout>

横向滚动布局

首先要修改fruit_item,将每个item的宽度设成一个固定的值,并将item里水平布局改成垂直的。

<?xml version="1.0" encoding="utf-8"?>
<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_height="50dp"
        android:layout_width="80dp"
        />

    <TextView
        android:id="@+id/fruit_name"
        android:layout_marginTop="10dp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center_horizontal" />

</LinearLayout>

在之前的调用基础上,加上LinearLayoutManagersetOrientatian方法来设置布局的排列方向。默认是纵向排列,传入LinearLayoutManager.HORIZONTAL表示让布局横向排列。

layoutManager.setOrientation(LinearLayoutManager.HORIZONTAL);

瀑布流布局

RecyclerView提供了GridLayoutManagerStaggeredGridLayoutManager两种内置的布局排列方式,前者可以实现网格布局,后者可以用于实现瀑布流布局。

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_margin="10dp"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <ImageView
        android:id="@+id/fruit_image"
        android:layout_height="100dp"
        android:layout_width="match_parent"
        />

    <TextView
        android:id="@+id/fruit_name"
        android:layout_marginTop="10dp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="left" />

</LinearLayout>

这里因为每个item的宽度可以通过列数来自动适配,不是一个固定值,所以match_parent就行

在调用时也比较简单,这里为了凸出效果,文字设定为随机长度

public class RecycleActivity extends AppCompatActivity {

    private List<Fruit> fruitList = new ArrayList<>();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_recycle);
        initFruits();
        RecyclerView recyclerView = (RecyclerView) findViewById(R.id.recycler_view);
        StaggeredGridLayoutManager layoutManager = new StaggeredGridLayoutManager(2,StaggeredGridLayoutManager.VERTICAL);
        recyclerView.setLayoutManager(layoutManager);
        RecyclerFruitAdapter fruitAdapter = new RecyclerFruitAdapter(fruitList);
        recyclerView.setAdapter(fruitAdapter);
    }

    private void initFruits() {
        for (int i = 0; i < 10; ++i) {
            Fruit apple = new Fruit(getRandomLengthName("Apple"),R.drawable.img_1);
            fruitList.add(apple);

            Fruit banana = new Fruit(getRandomLengthName("banana"),R.drawable.img_2);
            fruitList.add(banana);
        }
    }

    private String getRandomLengthName(String name) {
        Random random = new Random();
        int length = random.nextInt(20) + 1;
        StringBuilder builder = new StringBuilder();
        for (int i = 0; i < length; i++) {
            builder.append(name);
        }
        return builder.toString();
    }
}

Android 基础(四)滚动组件与Adapter

标签:cli   sdn   double   instance   dem   实现   exti   protected   固定   

原文地址:https://www.cnblogs.com/smallocean/p/14448826.html

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