背景

一般上下滑动的全屏界面，都是使用viewpager做。
好处是系统封装好了，可以直接调用相应的api就能使用
坏处是viewpager的刷新机制比较麻烦，而且往头部添加数据也不是很方便

后来有人使用RecyclerView+PagerSnapHelper将recyclerview打造成类似viewpage的效果
好处是recyclerview功能强大，能满足大部分需求
坏处是这俩个搭配会有很多小bug需要你慢慢优化

viewpager2

这篇文章主要讲的就是viewpage2
google官网上更新的viewpager2
首先看下本文实现的效果

Kapture 2019-08-07 at 16.24.57.gif

首先来看下viewpage2的基本用法

 implementation 'androidx.recyclerview:recyclerview:1.0.0'
 implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
 implementation 'com.scwang.smartrefresh:SmartRefreshLayout:1.1.0-alpha-27'
implementation 'androidx.viewpager2:viewpager2:1.0.0-beta02'

首先引入本文需要的第三方库
viewpager使用的是androidx版本
项目怎么转为androidx这篇文章就不讲了
直接上xml开始

        <androidx.viewpager2.widget.ViewPager2
            android:id="@+id/page2"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="vertical">

        </androidx.viewpager2.widget.ViewPager2>

mainactivity

        viewpage2 = findViewById(R.id.page2);
        smartRefreshLayout = findViewById(R.id.refresh);
        smartRefreshLayout.setEnableLoadMore(true);
        adapter = new Adapter();
        viewpage2.setAdapter(adapter);

        for (int i = 0; i < 3; i++) {
            list.add(i + "");
        }
        adapter.setList(list);

adapter的内容为

class Adapter extends RecyclerView.Adapter<ViewHolder> {

        List<String> list = new ArrayList<>();


        public void setList(List<String> list) {
            this.list = list;
            notifyDataSetChanged();
        }

        public void addHead(String s) {
            list.add(0, s);
            notifyDataSetChanged();
        }

        void addFoot(String s) {
            list.add(s);
            notifyDataSetChanged();
        }

        @NonNull
        @Override
        public ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
            View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.item, parent, false);
            return new ViewHolder(view);
        }

        @Override
        public void onBindViewHolder(@NonNull ViewHolder holder, int position) {
            holder.bindData(list.get(position) + "");
        }

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

    class ViewHolder extends RecyclerView.ViewHolder {
        TextView textView;

        public ViewHolder(@NonNull View itemView) {
            super(itemView);
            textView = itemView.findViewById(R.id.num);
        }

        void bindData(String s) {
            textView.setText(s);
        }

    }

可以看到viewpage2的adapter 是继承recycerview的adapter，
其实viewpage2内部也还是通过recyclerview+PagerSnapHelper实现的
打开viewpage2的源码可以看到

    LinearLayoutManager mLayoutManager;
    private int mPendingCurrentItem = NO_POSITION;
    private Parcelable mPendingAdapterState;
    private RecyclerView mRecyclerView;
    private PagerSnapHelper mPagerSnapHelper;
    private ScrollEventAdapter mScrollEventAdapter;
    private CompositeOnPageChangeCallback mPageChangeEventDispatcher;
    private FakeDrag mFakeDragger;
    private PageTransformerAdapter mPageTransformerAdapter;
    private boolean mUserInputEnabled = true;

所以viewapge2是支持横向和竖版的滑动的
接下来我们给viewpage2添加刷新头

 <com.scwang.smartrefresh.layout.SmartRefreshLayout
        android:id="@+id/refresh"
        android:layout_width="match_parent"
        android:layout_height="match_parent">

        <androidx.viewpager2.widget.ViewPager2
            android:id="@+id/page2"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="vertical">

        </androidx.viewpager2.widget.ViewPager2>

    </com.scwang.smartrefresh.layout.SmartRefreshLayout>

这里使用的是smartrefresh
github地址为https://github.com/scwang90/SmartRefreshLayout
接下里修改mainactivity的代码

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        viewpage2 = findViewById(R.id.page2);
        smartRefreshLayout = findViewById(R.id.refresh);
        smartRefreshLayout.setEnableLoadMore(true);
        adapter = new Adapter();
        viewpage2.setAdapter(adapter);

        for (int i = 0; i < 3; i++) {
            list.add(i + "");
        }
        adapter.setList(list);
        viewpage2.registerOnPageChangeCallback(new ViewPager2.OnPageChangeCallback() {
            @Override
            public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
                super.onPageScrolled(position, positionOffset, positionOffsetPixels);
            }

            @Override
            public void onPageSelected(int position) {
                super.onPageSelected(position);
                if (position == list.size() - 1) {
                    Toast.makeText(getApplicationContext(), position + "触发loadmore", Toast.LENGTH_SHORT).show();
                }
            }

            @Override
            public void onPageScrollStateChanged(int state) {
                super.onPageScrollStateChanged(state);
            }
        });
    }

下拉刷新通过smartrefresh的监听来做
smartrefresh的用法可以参考github
加载更多的方式可以通过smartrefresh的监听来做也可以通过viewpage2. registerOnPageChangeCallback在滑动到最后一个的时候触发loadmore
这样就实现了一个简单的类似抖音的主界面框架

本文代码https://github.com/BigSweet/ViewPage2Demo