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为Fragment换装ViewPager2

为Fragment换装ViewPager2">">

1. 开启ViewPager2之旅

image.png

距离ViewPager2正式版的发布已经一年多了,目前ViewPager早已停止更新,官方鼓励使用ViewPager2替代。 ViewPager2底层基于RecyclerView实现,因此可以获得RecyclerView带来的诸多收益:

  • 抛弃传统的PagerAdapter,统一了Adapter的API
  • 通过LinearLayoutManager可以实现类似抖音的纵向滑动
  • 支持DiffUitl,可以实现局部刷新
  • 支持RTL(right-to-left),对于一些有出海需求的APP非常有用
  • 支持ItemDecorator

2. ViewPager2 + Fragment

跟ViewPager一样,除了View以外,ViewPager2更多的是配合Fragment使用,这需要借助于FragmentStateAdapter

image.png

接下来,本文简单介绍一下FragmentStateAdapter的使用及实现原理:

首先在gradle中引入ViewPager2:

 implementation 'androidx.viewpager2:viewpager2:1.1.0'
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然后在xml中布局:

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

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FragmentStateAdapter

import androidx.appcompat.app.AppCompatActivity
import androidx.fragment.app.Fragment
import androidx.viewpager2.adapter.FragmentStateAdapter

class DoppelgangerAdapter(activity: AppCompatActivity, val doppelgangerList: List<DoppelgangerItem>) :
FragmentStateAdapter(activity) {

override fun getItemCount(): Int {
return doppelgangerList.size
}

override fun createFragment(position: Int): Fragment {
return DoppelgangerFragment.getInstance(doppelgangerList[position])
}
}
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FragmentStateAdapter的API跟旧的Adapter很相似:

  • getItemCount:返回Item的数量
  • createFragment:用来根据position创建fragment
  • DoppelgangerFragment:创建的具体Fragment类型

MainActivity

在Activity中为ViewPager2设置Adapter:

val doppelgangerAdapter = DoppelgangerAdapter(this, doppelgangerList) 
doppelgangerViewPager.adapter = doppelgangerAdapter
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3. 揭秘FragmentStateAdapter的实现

因为ViewPager2继承自RecyclerView,因此可以推断出FragmentStateAdapter继承自RecyclerView.Adapter

public abstract class FragmentStateAdapter extends 
RecyclerView.Adapter<FragmentViewHolder> implements StatefulAdapter {
}
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虽说是继承关系,但两者的API却不一致,RecyclerView.Adapter关注的是ViewHolder的复用,而在FragmentStateAdapter中Framgent是不会复用的,即有多少个item就应该创建多少个Fragment,那么这其中是如何转换的呢?

onCreateViewHolder

通过FragmentStateAdapter声明中的泛型可以知道,ViewPager2之所以能够在RecyclerView的基础上对外屏蔽对ViewHolder的使用,其内部是借助FragmentViewHolder实现的。

onCreateViewHolder中会创建一个FragmentViewHolder

@NonNull
@Override
public final FragmentViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
return FragmentViewHolder.create(parent);
}
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FragmentViewHolder的主要作用是通过FrameLayout为Fragment提供用作容器的container:

@NonNull static FragmentViewHolder create(@NonNull ViewGroup parent) {
FrameLayout container = new FrameLayout(parent.getContext());
container.setLayoutParams(
new ViewGroup.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,
ViewGroup.LayoutParams.MATCH_PARENT));
container.setId(ViewCompat.generateViewId());
container.setSaveEnabled(false);
return new FragmentViewHolder(container);
}
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onBindViewHolder

@Override
public final void onBindViewHolder(final @NonNull FragmentViewHolder holder, int position) {
...
ensureFragment(position);
...
gcFragments();
}
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ensureFragment(position),其内部会最终回调用createFragment创建当前Fragment

   private void ensureFragment(int position) {
long itemId = getItemId(position);
if (!mFragments.containsKey(itemId)) {
// TODO(133419201): check if a Fragment provided here is a new Fragment
Fragment newFragment = createFragment(position);
newFragment.setInitialSavedState(mSavedStates.get(itemId));
mFragments.put(itemId, newFragment);
}
}
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mFragments缓存创建的Fragment,供后面placeFramentInViewholder使用; gcFragments回收已经不再使用的的Fragment(对应的item已经删除),节省内存开销。

placeFragmentInViewHolder

  @Override
public final void onViewAttachedToWindow(@NonNull final FragmentViewHolder holder) {
placeFragmentInViewHolder(holder);
gcFragments();
}
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onViewAttachToWindow的时候调用placeFragmentInViewHolder,将FragmentViewHolder的container与当前Fragment绑定

    void placeFragmentInViewHolder(@NonNull final FragmentViewHolder holder) {
Fragment fragment = mFragments.get(holder.getItemId());
if (fragment == null) {
throw new IllegalStateException("Design assumption violated.");
}
FrameLayout container = holder.getContainer();
View view = fragment.getView();

...
if (fragment.isAdded() && view.getParent() != null) {
if (view.getParent() != container) {
addViewToContainer(view, container);
}
return;
}
...
}
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void addViewToContainer(@NonNull View v, @NonNull FrameLayout container) {
...

if (container.getChildCount() > 0) {
container.removeAllViews();
}

if (v.getParent() != null) {
((ViewGroup) v.getParent()).removeView(v);
}

container.addView(v);
}
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通过上面源码分析可以知道,虽然Fragment没有被复用,但是通过复用了ViewHolder的container实现了Framgent的交替显示


4. 滑动监听

监听页面滑动是一个常见需求,ViewPager2的API也发生了变化,使用OnPageChangeCallback

使用效果如下:

var doppelgangerPageChangeCallback = object : ViewPager2.OnPageChangeCallback() {
override fun onPageSelected(position: Int) {
Toast.makeText(this@MainActivity, "Selected position: ${position}",
Toast.LENGTH_SHORT).show()
}
}
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OnPageChangeCallback同样也有三个方法:

  • onPageScrolled: 当前页面开始滑动时
  • onPageSelected: 当页面被选中时
  • onPageScrollStateChanged: 当前页面滑动状态变动时


5. 纵向滑动

设置纵向滑动很简单,一行代码搞定

doppelgangerViewPager.orientation = ViewPager2.ORIENTATION_VERTICAL
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源码也很简单

/**
* Sets the orientation of the ViewPager2.
*
* @param orientation {@link #ORIENTATION_HORIZONTAL} or {@link #ORIENTATION_VERTICAL}
*/

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


配合TabLayout的使用也是一个常见需求,TabLayout需要引入material

implementation 'com.google.android.material:material:1.2.0-alpha04'
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然后在xml中声明

<com.google.android.material.tabs.TabLayout
android:id="@+id/tabLayout"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:background="@color/colorPrimary"
app:tabMode="scrollable"
app:tabTextColor="@android:color/white" />

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TabsLayoutMediator

要关联TabLayout和ViewPager2需要借助TabLayoutMediator

public TabLayoutMediator(
@NonNull TabLayout tabLayout,
@NonNull ViewPager2 viewPager,
@NonNull TabConfigurationStrategy tabConfigurationStrategy)
{
this(tabLayout, viewPager, true, tabConfigurationStrategy);
}
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其中,TabConfigurationStrategy定义如下:根据position配置当前tab

/**
* A callback interface that must be implemented to set the text and styling of newly created
* tabs.
*/

public interface TabConfigurationStrategy {
/**
* Called to configure the tab for the page at the specified position. Typically calls {@link
* TabLayout.Tab#setText(CharSequence)}, but any form of styling can be applied.
*
* @param tab The Tab which should be configured to represent the title of the item at the given
* position in the data set.
* @param position The position of the item within the adapter's data set.
*/

void onConfigureTab(@NonNull TabLayout.Tab tab, int position);
}
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在MainActivity中具体使用如下:

TabLayoutMediator(tabLayout, doppelgangerViewPager) { tab, position ->
//To get the first name of doppelganger celebrities
tab.text = doppelgangerList[position].title
}.attach()
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attach方法很关键,经过前面一系列配置后最终需要通过它关联两个组件。

加入TabLayout后的最终效果如下:


7. DiffUtil 局部更新

RecyclerView基于DiffUtil可以实现局部更新,如今,FragmentStateAdapter也可以对Fragment实现局部更新。

首先定义DiffUtil.Callback

class PagerDiffUtil(private val oldList: List<DoppelgangerItem>, private val newList: List<DoppelgangerItem>) : DiffUtil.Callback() {

enum class PayloadKey {
VALUE
}

override fun getOldListSize() = oldList.size

override fun getNewListSize() = newList.size

override fun areItemsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean {
return oldList[oldItemPosition].id == newList[newItemPosition].id
}

override fun areContentsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean {
return oldList[oldItemPosition].value == newList[newItemPosition].value
}

override fun getChangePayload(oldItemPosition: Int, newItemPosition: Int): Any? {
return listOf(PayloadKey.VALUE)
}
}
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然后在Adapter中使用DiffUtil更新数据


class DoppelgangerAdapter(private val activity: FragmentActivity) : FragmentStateAdapter(activity) {

private val items: ArrayList<DoppelgangerItem> = arrayListOf()


override fun createFragment(position: Int): Fragment {
return DoppelgangerFragment.getInstance(doppelgangerList[position])
}

override fun getItemCount() = items.size

override fun getItemId(position: Int): Long {
return items[position].id.toLong()
}

override fun containsItem(itemId: Long): Boolean {
return items.any { it.id.toLong() == itemId }
}

fun setItems(newItems: List<PagerItem>) {
//不借助DiffUtil更新数据
//items.clear()
//items.addAll(newItems)
//notifyDataSetChanged()

//使用DiffUtil更新数据
val callback = PagerDiffUtil(items, newItems)
val diff = DiffUtil.calculateDiff(callback)
items.clear()
items.addAll(newItems)
diff.dispatchUpdatesTo(this)
}
}
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8. 总结

本文主要介绍了ViewPager2配合Fragment的使用方法以及FragmentStateAdapter的实现原理,顺带介绍了TabLayoutOnPageChangeCallbackDiffUtil等常见功能的用法。ViewPager2的使用非常简单,在性能以及使用体验等各方面都要优于传统的ViewPager,没尝试的小伙伴抓紧用起来吧~

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