ViewPager2

官方Demo

Github 地址：https://github.com/android/views-widgets-samples/tree/master/ViewPager2

基本使用

ViewPager2 是基于RecyclerView+LinearLayoutManager+PageSnapHelper实现，因此对于普通的布局可以直接使用RecyclerView的Adapter/ViewHolder来填充。

与TabLayout

实现ViewPager2 与 TabLayout联动需要用到TabLayoutMediator，注意需要在给ViewPager设置Adapter之后在设置。

import com.google.android.material.tabs.TabLayoutMediator

tabLayout = findViewById(R.id.tabs)
//传入tablayout、ViewPager2实例
TabLayoutMediator(tabLayout, viewPager) { tab, position ->
    //初始化tab显示内容
    tab.text = Card.DECK[position].toString()
}.attach()

attach 方法实现，主要是创建ViewPager2和Tablayout的滑动监听，用于在滑动时更新两者的显示情况。

public void attach() {
    if (attached) {
      throw new IllegalStateException("TabLayoutMediator is already attached");
    }
    adapter = viewPager.getAdapter();
    //需要先给ViewPager2设置Adapter
    if (adapter == null) {
      throw new IllegalStateException(
          "TabLayoutMediator attached before ViewPager2 has an " + "adapter");
    }
    attached = true;

    // 给ViewPager 设置监听
    onPageChangeCallback = new TabLayoutOnPageChangeCallback(tabLayout);
    viewPager.registerOnPageChangeCallback(onPageChangeCallback);

    // 给Tablayout 设置监听
    onTabSelectedListener = new ViewPagerOnTabSelectedListener(viewPager);
    tabLayout.addOnTabSelectedListener(onTabSelectedListener);

    // Now we'll populate ourselves from the pager adapter, adding an observer if
    // autoRefresh is enabled
    if (autoRefresh) {
      // Register our observer on the new adapter
      pagerAdapterObserver = new PagerAdapterObserver();
      adapter.registerAdapterDataObserver(pagerAdapterObserver);
    }

    //开始初始化tab
    populateTabsFromPagerAdapter();

    // 根据ViewPager2的当前位置更新TabLayout
    tabLayout.setScrollPosition(viewPager.getCurrentItem(), 0f, true);
  }

populateTabsFromPagerAdapter 方法会回调 onConfigureTab 方法，用于初始化所有Tab。

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));
        }
      }
    }
  }

与Fragment使用

Adapter需要使用 androidx.viewpager2.adapter.FragmentStateAdapter

不要手动创建并在外部持有fragment的引用，在滑动过程中，ViewPager2会执行Fragment的销毁和重建，这时候如果外部还持有引用实际上已经发生了泄露。

应该把Fragment的创建交给Adapter：

class ProfilePagerAdapter(activity: FragmentActivity, private val fragmentIds: List<Long>)
    : FragmentStateAdapter(activity) {

    override fun getItemCount(): Int = fragmentIds.size

    override fun createFragment(position: Int): Fragment {
        //创建新的Fragment
        return BaseProfileFragment.create(getItemId(position))
    }

    override fun containsItem(itemId: Long): Boolean {
        return fragmentIds.contains(itemId)
    }

    override fun getItemId(position: Int): Long {
        return fragmentIds[position]
    }
}

Fragment 状态管理

对于 fragment 销毁后的状态恢复 ，可以在 oncreateView 方法中的 savedInstanceState 恢复，在 onSaveInstanceState中保存需要保持的数据。

获取当前Fragment

获取当前Fragment，可以通过fragmentManager.findFragmentByTag,tag为 "f" + holder.getItemId()，具体逻辑在FragmentStateAdapter 的placeFragmentInViewHolder 方法中，需要指定itemId

// { f:notAdded, v:notCreated, v:notAttached } -> add, create, attach
        if (!shouldDelayFragmentTransactions()) {
            scheduleViewAttach(fragment, container);
            mFragmentManager.beginTransaction()
                    .add(fragment, "f" + holder.getItemId())
                    .setMaxLifecycle(fragment, STARTED)
                    .commitNow();
            mFragmentMaxLifecycleEnforcer.updateFragmentMaxLifecycle(false);
        } else {
            if (mFragmentManager.isDestroyed()) {
                return; // nothing we can do
            }
            mLifecycle.addObserver(new LifecycleEventObserver() {
                @Override
                public void onStateChanged(@NonNull LifecycleOwner source,
                        @NonNull Lifecycle.Event event) {
                    if (shouldDelayFragmentTransactions()) {
                        return;
                    }
                    source.getLifecycle().removeObserver(this);
                    if (ViewCompat.isAttachedToWindow(holder.getContainer())) {
                        placeFragmentInViewHolder(holder);
                    }
                }
            });
        }

可变集合

需要重写 getItemId 和 containsItem 方法（特别是可变的Fragment集合），如Demo 所示：

override fun createViewPagerAdapter(): RecyclerView.Adapter<*> {
        val items = items // avoids resolving the ViewModel multiple times
        return object : FragmentStateAdapter(this) {
            override fun createFragment(position: Int): PageFragment {
                Log.d(TAG, "createFragment: $position")
                val itemId = items.itemId(position)
                val itemText = items.getItemById(itemId)
                return PageFragment.create(itemText)
            }

            override fun getItemCount(): Int = items.size
            override fun getItemId(position: Int): Long = items.itemId(position)
            override fun containsItem(itemId: Long): Boolean = items.contains(itemId)
        }
    }

PageTransformer

实现ViewPager2.PageTransformer接口

/**
     * A PageTransformer is invoked whenever a visible/attached page is scrolled.
     * This offers an opportunity for the application to apply a custom transformation
     * to the page views using animation properties.
     */
    public interface PageTransformer {

        /**
         * Apply a property transformation to the given page.
         *
         * @param page Apply the transformation to this page
         * @param position Position of page relative to the current front-and-center
         *                 position of the pager. 0 is front and center. 1 is one full
         *                 page position to the right, and -2 is two pages to the left.
         *                 Minimum / maximum observed values depend on how many pages we keep
         *                 attached, which depends on offscreenPageLimit.
         *
         * @see #setOffscreenPageLimit(int)
         */
        void transformPage(@NonNull View page, float position);
    }

通过viewPager.setPageTransformer来设置transformer，如需设置多个transformer，可以通过CompositePageTransformer来进行组合。