前言

忙碌的2019过去了，2020继续努力！

实现原理

首先来看下其初始化

 private void initialize(Context context, AttributeSet attrs) {
      ...省略...
        mRecyclerView = new RecyclerViewImpl(context);
        mRecyclerView.setId(ViewCompat.generateViewId());
        mRecyclerView.setDescendantFocusability(FOCUS_BEFORE_DESCENDANTS);

        mLayoutManager = new LinearLayoutManagerImpl(context);
        mRecyclerView.setLayoutManager(mLayoutManager);
        mRecyclerView.setScrollingTouchSlop(RecyclerView.TOUCH_SLOP_PAGING);
        setOrientation(context, attrs);

        mRecyclerView.setLayoutParams(
                new ViewGroup.LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT));
        mRecyclerView.addOnChildAttachStateChangeListener(enforceChildFillListener());

        // Create ScrollEventAdapter before attaching PagerSnapHelper to RecyclerView, because the
        // attach process calls PagerSnapHelperImpl.findSnapView, which uses the mScrollEventAdapter
        mScrollEventAdapter = new ScrollEventAdapter(this);
        // Create FakeDrag before attaching PagerSnapHelper, same reason as above
        mFakeDragger = new FakeDrag(this, mScrollEventAdapter, mRecyclerView);
        mPagerSnapHelper = new PagerSnapHelperImpl();
        mPagerSnapHelper.attachToRecyclerView(mRecyclerView);
        ...省略...
    }

看到RecyclerView、LinearLayoutManager、PagerSnapHelper是不是有点熟悉？详情可点击【Android 进阶】仿抖音系列之翻页上下滑切换视频（四）
其使用方式前文中已有详情描述，这里不再赘述。

基本使用

添加依赖

implementation "androidx.viewpager2:viewpager2:1.0.0"

布局中引入

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

Adapter

由于ViewPager2 基于RecyclerView实现，所以Adapter应继承于RecyclerView.Adapter

public class BaseAdapter extends RecyclerView.Adapter<BaseAdapter.MyViewHolder> {


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

    @Override
    public void onBindViewHolder(@NonNull MyViewHolder holder, int position) {
        int color = Color.parseColor(randomHexStr(6));
        holder.tvDemo.setBackgroundColor(color);
    }

    @Override
    public int getItemCount() {
        return 10;
    }


    class MyViewHolder extends RecyclerView.ViewHolder {
        private TextView tvDemo;

        public MyViewHolder(@NonNull View itemView) {
            super(itemView);

            tvDemo = itemView.findViewById(R.id.tvDemo);
        }
    }
   }

使用

        BaseAdapter baseAdapter = new BaseAdapter();
        vpBase.setAdapter(baseAdapter);

滑动方向

通过 setOrientation方法设置，可设置水平ORIENTATION_HORIZONTAL和垂直ORIENTATION_VERTICAL

   public void setOrientation(@Orientation int orientation) {
        mLayoutManager.setOrientation(orientation);
        mAccessibilityProvider.onSetOrientation();
    }

可见，其原理是设置LayoutManager的方向

滑动监听

通过设置registerOnPageChangeCallback方法

    public void registerOnPageChangeCallback(@NonNull OnPageChangeCallback callback) {
        mExternalPageChangeCallbacks.addOnPageChangeCallback(callback);
    }

而mExternalPageChangeCallbacks 在初始化时，被添加到mPageChangeEventDispatcher

mPageChangeEventDispatcher.addOnPageChangeCallback(mExternalPageChangeCallbacks);

mPageChangeEventDispatcher 又被设置给mScrollEventAdapter

mScrollEventAdapter.setOnPageChangeCallback(mPageChangeEventDispatcher);

mScrollEventAdapter则继承于RecyclerView.OnScrollListener

ScrollEventAdapter extends RecyclerView.OnScrollListener

简而言之，基于RecyclerView.OnScrollListener的滑动事件

禁止滑动

通过setUserInputEnabled方法

预加载

通过setOffscreenPageLimit方法

    /**
     * <p>Set the number of pages that should be retained to either side of the currently visible
     * page(s). Pages beyond this limit will be recreated from the adapter when needed. Set this to
     * {@link #OFFSCREEN_PAGE_LIMIT_DEFAULT} to use RecyclerView's caching strategy. The given value
     * must either be larger than 0, or {@code #OFFSCREEN_PAGE_LIMIT_DEFAULT}.</p>
     *
     * <p>Pages within {@code limit} pages away from the current page are created and added to the
     * view hierarchy, even though they are not visible on the screen. Pages outside this limit will
     * be removed from the view hierarchy, but the {@code ViewHolder}s will be recycled as usual by
     * {@link RecyclerView}.</p>
     *
     * <p>This is offered as an optimization. If you know in advance the number of pages you will
     * need to support or have lazy-loading mechanisms in place on your pages, tweaking this setting
     * can have benefits in perceived smoothness of paging animations and interaction. If you have a
     * small number of pages (3-4) that you can keep active all at once, less time will be spent in
     * layout for newly created view subtrees as the user pages back and forth.</p>
     *
     * <p>You should keep this limit low, especially if your pages have complex layouts. By default
     * it is set to {@code OFFSCREEN_PAGE_LIMIT_DEFAULT}.</p>
     *
     * @param limit How many pages will be kept offscreen on either side. Valid values are all
     *        values {@code >= 1} and {@link #OFFSCREEN_PAGE_LIMIT_DEFAULT}
     * @throws IllegalArgumentException If the given limit is invalid
     * @see #getOffscreenPageLimit()
     */
    public void setOffscreenPageLimit(@OffscreenPageLimit int limit) {
        if (limit < 1 && limit != OFFSCREEN_PAGE_LIMIT_DEFAULT) {
            throw new IllegalArgumentException(
                    "Offscreen page limit must be OFFSCREEN_PAGE_LIMIT_DEFAULT or a number > 0");
        }
        mOffscreenPageLimit = limit;
        // Trigger layout so prefetch happens through getExtraLayoutSize()
        mRecyclerView.requestLayout();
    }

其中OFFSCREEN_PAGE_LIMIT_DEFAULT默认为-1,如果不设置，默认使用RecyclerView的缓存机制，详细点击查看【Android进阶】RecyclerView之缓存（二）
如果设置为大于的数，则会预加载limit个

PageTransformer

ViewPager2延续了ViewPager中的PageTransformer，内置的有MarginPageTransformer和CompositePageTransformer

使用方式如下：

        BaseAdapter baseAdapter = new BaseAdapter();
        vpBase.setPageTransformer(new MarginPageTransformer(DensityUtils.dp2px(this, 20)));
        vpBase.setAdapter(baseAdapter);

当然，我们也可以自己实现，只需要实现ViewPager2.PageTransformer

    class ScalePageTransformer implements ViewPager2.PageTransformer {
        private static final float DEFAULT_MIN_SCALE = 0.85f;
        private static final float DEFAULT_CENTER = 0.5f;
        private float mMinScale = DEFAULT_MIN_SCALE;

        @Override
        public void transformPage(@NonNull View view, float position) {

            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
                view.setElevation(-Math.abs(position));
            }

            int pageWidth = view.getWidth();
            int pageHeight = view.getHeight();

            view.setPivotY(pageHeight / 2);
            view.setPivotX(pageWidth / 2);
            if (position < -1) {
                view.setScaleX(mMinScale);
                view.setScaleY(mMinScale);
                view.setPivotX(pageWidth);
            } else if (position <= 1) {
                if (position < 0) {
                    float scaleFactor = (1 + position) * (1 - mMinScale) + mMinScale;
                    view.setScaleX(scaleFactor);
                    view.setScaleY(scaleFactor);
                    view.setPivotX(pageWidth * (DEFAULT_CENTER + DEFAULT_CENTER * -position));
                } else {
                    float scaleFactor = (1 - position) * (1 - mMinScale) + mMinScale;
                    view.setScaleX(scaleFactor);
                    view.setScaleY(scaleFactor);
                    view.setPivotX(pageWidth * ((1 - position) * DEFAULT_CENTER));
                }
            } else {
                view.setPivotX(0);
                view.setScaleX(mMinScale);
                view.setScaleY(mMinScale);
            }
        }
    }

使用方式如下

        BaseAdapter baseAdapter = new BaseAdapter();
        CompositePageTransformer pageTransformer = new CompositePageTransformer();
        pageTransformer.addTransformer(new MarginPageTransformer(DensityUtils.dp2px(this, 20)));
        pageTransformer.addTransformer(new ScalePageTransformer());
        vpBase.setPageTransformer(pageTransformer);
        vpBase.setAdapter(baseAdapter);

Fragment

ViewPager2中使用Fragment只需要Adapter继承于FragmentStateAdapter

public class BaseFragmentStateAdapter extends FragmentStateAdapter {

    private List<DemoFragment> fragments;

    public BaseFragmentStateAdapter(@NonNull FragmentActivity fragmentActivity, List<DemoFragment> fragments) {
        super(fragmentActivity);
        this.fragments = fragments;
    }

    public BaseFragmentStateAdapter(@NonNull FragmentActivity fragmentActivity) {
        super(fragmentActivity);
    }

    @NonNull
    @Override
    public Fragment createFragment(int position) {
        return fragments.get(position);
    }

    @Override
    public int getItemCount() {
        return fragments == null ? 0 : fragments.size();
    }
}

然后这样使用

        fragmentList = new ArrayList<>();
        fragmentList.add(new DemoFragment());
        fragmentList.add(new DemoFragment());
        fragmentList.add(new DemoFragment());
        fragmentList.add(new DemoFragment());
        fragmentList.add(new DemoFragment());
        fragmentList.add(new DemoFragment());
        fragmentList.add(new DemoFragment());
        fragmentList.add(new DemoFragment());

        BaseFragmentStateAdapter stateAdapter = new BaseFragmentStateAdapter(this, fragmentList);
        vpFragment.setAdapter(stateAdapter);

这样就完了吗？恭喜你，收获内存泄漏一枚

Screenshot_2020-01-11-11-51-25-206_com.ch.viewpager2demo.png

ArrayList持有DemoFragment的强引用，导致DemoFragment无法销毁。
所以正确的写法是

public class TabAdapter extends FragmentStateAdapter {
    private List<String> titles;

    public TabAdapter(@NonNull FragmentActivity fragmentActivity, List<String> titles) {
        super(fragmentActivity);
        this.titles = titles;
    }

    @NonNull
    @Override
    public Fragment createFragment(int position) {
        return new DemoFragment(titles.get(position));
    }

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

TabLayout

ViewPager2配合TabLayout的联动可以使用TabLayoutMediator，记得销毁时，解除绑定tabLayoutMediator.detach();

        tabLayoutMediator = new TabLayoutMediator(tabLayout, vpTablayout, new TabLayoutMediator.TabConfigurationStrategy() {
            @Override
            public void onConfigureTab(@NonNull TabLayout.Tab tab, int position) {
                tab.setText(titles.get(position));
            }
        });
        tabLayoutMediator.attach();

踩坑

TabLayout使用自定义布局和ViewPager2联动时会出现问题，TabLayout的自定义效果会无效，关键代码如下

  void populateTabsFromPagerAdapter() {
    tabLayout.removeAllTabs();

    if (adapter != null) {
      int adapterCount = adapter.getItemCount();
      for (int i = 0; i < adapterCount; i++) {
        TabLayout.Tab tab = tabLayout.newTab();
        tabConfigurationStrategy.onConfigureTab(tab, i);
        tabLayout.addTab(tab, false);
      }
      // Make sure we reflect the currently set ViewPager item
      if (adapterCount > 0) {
        int lastItem = tabLayout.getTabCount() - 1;
        int currItem = Math.min(viewPager.getCurrentItem(), lastItem);
        if (currItem != tabLayout.getSelectedTabPosition()) {
          tabLayout.selectTab(tabLayout.getTabAt(currItem));
        }
      }
    }
  }

可见，onConfigureTab返回的都是tabLayout.newTab(),所以，正确的做法是采用最原始的联动

 tabLayout.addOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
            @Override
            public void onTabSelected(TabLayout.Tab tab) {
                vpTablayout.setCurrentItem(tab.getPosition());
            }

            @Override
            public void onTabUnselected(TabLayout.Tab tab) {

            }

            @Override
            public void onTabReselected(TabLayout.Tab tab) {

            }
        });

        vpTablayout.registerOnPageChangeCallback(new ViewPager2.OnPageChangeCallback() {
            @Override
            public void onPageSelected(int position) {
                tabLayout.selectTab(tabLayout.getTabAt(position));
            }
        });

所涉及到的源码：ViewPagerDemo