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
Adapter
由于ViewPager2 基于RecyclerView实现，所以Adapter应继承于RecyclerView.Adapter
public class BaseAdapter extends RecyclerView.Adapter {
@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
/**
* 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}.
*
* 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}.
*
* 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.
*
* You should keep this limit low, especially if your pages have complex layouts. By default
* it is set to {@code OFFSCREEN_PAGE_LIMIT_DEFAULT}.
*
* @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 fragments;
public BaseFragmentStateAdapter(@NonNull FragmentActivity fragmentActivity, List 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
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 titles;
public TabAdapter(@NonNull FragmentActivity fragmentActivity, List 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));
}
});