谷歌发布了Android的ViewPager2测试版，让我们看看它有什么不同。

Android ViewPager2

首先使用ViewPager2,先在build.gradle添加依赖

implementation 'androidx.viewpager2:viewpager2:1.0.0-alpha01'

Android ViewPager2的特性

从右到左的布局支持
垂直方向
RecyclerView.Adapter取代PagerAdapter
registerOnPageChangeCallback 取代 addPageChangeListener
更高效的notifyDataSetChanged

ViewPaver2与ViewPager的区别

同样在xml中使用一样。

 <androidx.viewpager2.widget.ViewPager2
       android:id="@+id/viewpager2"
       android:layout_width="match_parent"
       android:layout_height="match_parent"/>

在以前的ViewPager的addOnPageChangeListener方法，我们需要覆盖三个方法。回顾下代码如下

 viewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
            @Override
            public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
                
            }

            @Override
            public void onPageSelected(int position) {

            }

            @Override
            public void onPageScrollStateChanged(int state) {

            }
        });

在ViewPager2中，不再需要这样的重写。

 mviewPager2.registerOnPageChangeCallback(new ViewPager2.OnPageChangeCallback() {
            @Override
            public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
                super.onPageScrolled(position, positionOffset, positionOffsetPixels);
            }
        });

垂直方向属性

默认是水平方向ORIENTATION_HORIZONTAL,垂直是ORIENTATION_VERTICAL

mviewPager2.setOrientation(ViewPager2.ORIENTATION_VERTICAL);

ViewPager2与RecycleView的结合使用

接下来，我们使用RecyclerView.Adapter来结合ViewPager2一起使用。不再需要像ViewPager使用FragmentStateAdapter。

项目结构如下:

struct.png

activity_main.xml代码

<?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=".MainActivity">

   <androidx.viewpager2.widget.ViewPager2
       android:id="@+id/viewpager2"
       android:layout_width="match_parent"
       android:layout_height="match_parent"/>


</androidx.constraintlayout.widget.ConstraintLayout>

MainActivity的代码

public class MainActivity extends AppCompatActivity {
    ViewPager2 mviewPager2;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mviewPager2 = findViewById(R.id.viewpager2);
        List<String> list = new ArrayList<>();
        list.add("页面一");
        list.add("页面二");
        list.add("页面三");
        list.add("页面四");

        mviewPager2.setAdapter(new ViewPagerAdapter(this, list, mviewPager2));

    }
}

列表Item的布局recycleview_item.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <androidx.appcompat.widget.AppCompatTextView
        android:id="@+id/tvTitle"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:textColor="@android:color/white"
        android:textSize="32sp"
        tools:text="item" />
</RelativeLayout>

ViewPagerAdapter的代码如下，使用跟RecycleView的adapter完全一样

package com.chaowen.androidviewpager2;

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.RelativeLayout;
import android.widget.TextView;

import java.util.List;

import androidx.annotation.NonNull;
import androidx.recyclerview.widget.RecyclerView;
import androidx.viewpager2.widget.ViewPager2;

/**
 * @Author: wuchaowen
 * @Description:
 * @Time:
 **/
public class ViewPagerAdapter extends RecyclerView.Adapter<ViewPagerAdapter.ViewHolder> {
    private List<String> mData;
    private LayoutInflater mInflater;
    private ViewPager2 viewPager2;


    private int[] colorArray = new int[]{android.R.color.black, android.R.color.holo_blue_dark, android.R.color.holo_green_dark, android.R.color.holo_red_dark};


    public ViewPagerAdapter(Context context, List<String> data, ViewPager2 viewPager2) {
        this.mInflater = LayoutInflater.from(context);
        this.mData = data;
        this.viewPager2 = viewPager2;
    }

    @NonNull
    @Override
    public ViewPagerAdapter.ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View view = mInflater.inflate(R.layout.recycleview_item, parent, false);
        return new ViewHolder(view);
    }

    @Override
    public void onBindViewHolder(@NonNull ViewPagerAdapter.ViewHolder holder, int position) {
        String animal = mData.get(position);
        holder.myTextView.setText(animal);
        holder.relativeLayout.setBackgroundResource(colorArray[position]);

    }

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

    public class ViewHolder extends RecyclerView.ViewHolder {
        TextView myTextView;
        RelativeLayout relativeLayout;

        ViewHolder(View itemView) {
            super(itemView);
            myTextView = itemView.findViewById(R.id.tvTitle);
            relativeLayout = itemView.findViewById(R.id.container);

        }
    }
}

运行效果

viewpager.gif

踩坑1 Androidx和Android support库不能共存

Manifest merger failed : Attribute application@appComponentFactory value=(android.support.v4.app.CoreComponentFactory) from [com.android.support:support-compat:28.0.0] AndroidManifest.xml:22:18-91
    is also present at [androidx.core:core:1.1.0-alpha04] AndroidManifest.xml:22:18-86 value=(androidx.core.app.CoreComponentFactory).
    Suggestion: add 'tools:replace="android:appComponentFactory"' to <application> element at AndroidManifest.xml:6:5-22:19 to override.

解决方案

将build tools更新到3.2.0，gradle更新到4.6，依赖库统一更新到28.0.0，这一步非常重要，否则会导致第二步不能完全转换成功
选择工程右键→Refactor→Migrate to Androidx

viewpager2.png
选中所有需要重命名的目录，执行Do Refactor

viewpager2_1.png
发现已经看不到support库了，运行成功安装app。

项目代码下载

android ViewPager2的使用教程