TabLayoutMediator的坑
问题描述
当你想取消ViewPager2滑动时的动画，首先会去查看构造函数有没有设置这个的参数，一查果然有：
public TabLayoutMediator(
@NonNull TabLayout tabLayout,
@NonNull ViewPager2 viewPager,
boolean autoRefresh,
boolean smoothScroll,
@NonNull TabConfigurationStrategy tabConfigurationStrategy) {
this.tabLayout = tabLayout;
this.viewPager = viewPager;
this.autoRefresh = autoRefresh;
this.smoothScroll = smoothScroll;
this.tabConfigurationStrategy = tabConfigurationStrategy;
}
然而接着你欣喜若狂地将smoothScroll设置为false时却发现ViewPager2失去了原来的惯性滑动效果，惯性滑动效果是指比如你将第一个页面向左滑动到一定程度就会自动转向第二页。
那么为什么会失去这种效果呢？这里我们可以去分析TabLayoutMediator的源码：
问题分析
ViewPager2翻页的监听
private static class TabLayoutOnPageChangeCallback extends ViewPager2.OnPageChangeCallback {
@NonNull private final WeakReference<TabLayout> tabLayoutRef;
private int previousScrollState;
private int scrollState;
TabLayoutOnPageChangeCallback(TabLayout tabLayout) {
tabLayoutRef = new WeakReference<>(tabLayout);
reset();
}
@Override
public void onPageScrollStateChanged(final int state) {
previousScrollState = scrollState;
scrollState = state;
}
@Override
public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
TabLayout tabLayout = tabLayoutRef.get();
if (tabLayout != null) {
// Only update the text selection if we're not settling, or we are settling after
// being dragged
boolean updateText =
scrollState != SCROLL_STATE_SETTLING || previousScrollState == SCROLL_STATE_DRAGGING;
// Update the indicator if we're not settling after being idle. This is caused
// from a setCurrentItem() call and will be handled by an animation from
// onPageSelected() instead.
boolean updateIndicator =
!(scrollState == SCROLL_STATE_SETTLING && previousScrollState == SCROLL_STATE_IDLE);
tabLayout.setScrollPosition(position, positionOffset, updateText, updateIndicator);
}
}
@Override
public void onPageSelected(final int position) {
TabLayout tabLayout = tabLayoutRef.get();
if (tabLayout != null
&& tabLayout.getSelectedTabPosition() != position
&& position < tabLayout.getTabCount()) {
// Select the tab, only updating the indicator if we're not being dragged/settled
// (since onPageScrolled will handle that).
boolean updateIndicator =
scrollState == SCROLL_STATE_IDLE
|| (scrollState == SCROLL_STATE_SETTLING
&& previousScrollState == SCROLL_STATE_IDLE);
tabLayout.selectTab(tabLayout.getTabAt(position), updateIndicator);
}
}
void reset() {
previousScrollState = scrollState = SCROLL_STATE_IDLE;
}
}
以上是对ViewPager2翻页的监听，包括滑动状态改变、滑动中和选定页面三种不同类型的监听,
滑动状态改变会记录当前和现在的状态，滑动中会依据状态同步TabLayout的indicator（指示线）位置，选定页面会同步选定Tab。
滑动状态解释如下
ViewPager.SCROLL_STATE_DRAGGING ：
当用户按下ViewPager视图并且需要滑动第一下时;
ViewPager.SCROLL_STATE_SETTLING：
当用户滑动的放手让其惯性滑动的时候，比如滑了放手触发。如果用户滑了左手边一点然后不松手滑回原点 将不会触发；
ViewPager.SCROLL_STATE_IDLE：
当用户滑动的时候松手。
TabLayout选择Tab的监听
private static class ViewPagerOnTabSelectedListener implements TabLayout.OnTabSelectedListener {
private final ViewPager2 viewPager;
private final boolean smoothScroll;
ViewPagerOnTabSelectedListener(ViewPager2 viewPager, boolean smoothScroll) {
this.viewPager = viewPager;
this.smoothScroll = smoothScroll;
}
@Override
public void onTabSelected(@NonNull TabLayout.Tab tab) {
viewPager.setCurrentItem(tab.getPosition(), smoothScroll);
}
@Override
public void onTabUnselected(TabLayout.Tab tab) {
// No-op
}
@Override
public void onTabReselected(TabLayout.Tab tab) {
// No-op
}
}
以上是对TabLayout选择Tab的监听，没什么好说的，就只是设置了选定Tab时同步选定ViewPager2的页面（smoothScroll决定是否带翻页动画）。
当我们将smoothScroll设为false时，滑动ViewPager2同步将TabLayout的indicator（指示线）位置定位到刚刚触碰第二个Tab的位置即选中了第二个Tab，紧接着监听调用
setCurrentItem函数设置ViewPager2页面然而没有动画辅助纠正位置导致ViewPager2停留在之前第二个Tab的indicator（指示线）位置。
解决方法
复制TabLayoutMediator源码自己重写一个MyTabLayoutMediator
设置滑动ViewPager2时如果之前状态为ViewPager.SCROLL_STATE_DRAGGING（拖曳状态)
smoothScroll设为true，否则为false
tips：
原本源码中的ViewPagerOnTabSelectedListener被static所修饰，因为要访问TabLayoutOnPageChangeCallback中的previousScrollState（ViewPager2之前状态）所以我将static去掉，也许会稍微影响性能。
自写的MyTabLayoutMediator源码如下：
import static androidx.viewpager2.widget.ViewPager2.SCROLL_STATE_DRAGGING;
import static androidx.viewpager2.widget.ViewPager2.SCROLL_STATE_IDLE;
import static androidx.viewpager2.widget.ViewPager2.SCROLL_STATE_SETTLING;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.recyclerview.widget.RecyclerView;
import androidx.viewpager2.widget.ViewPager2;
import com.google.android.material.tabs.TabLayout;
import java.lang.ref.WeakReference;
public final class MyTabLayoutMediator {
@NonNull
private final TabLayout tabLayout;
@NonNull
private final ViewPager2 viewPager;
private final boolean autoRefresh;
private final boolean smoothScroll;
private final TabConfigurationStrategy tabConfigurationStrategy;
@Nullable
private RecyclerView.Adapter<?> adapter;
private boolean attached;
@Nullable
private TabLayoutOnPageChangeCallback onPageChangeCallback;
@Nullable
private TabLayout.OnTabSelectedListener onTabSelectedListener;
@Nullable
private RecyclerView.AdapterDataObserver pagerAdapterObserver;
/**
* 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);
}
public MyTabLayoutMediator(
@NonNull TabLayout tabLayout,
@NonNull ViewPager2 viewPager,
@NonNull TabConfigurationStrategy tabConfigurationStrategy) {
this(tabLayout, viewPager, true, tabConfigurationStrategy);
}
public MyTabLayoutMediator(
@NonNull TabLayout tabLayout,
@NonNull ViewPager2 viewPager,
boolean autoRefresh,
@NonNull TabConfigurationStrategy tabConfigurationStrategy) {
this(tabLayout, viewPager, autoRefresh, true, tabConfigurationStrategy);
}
public MyTabLayoutMediator(
@NonNull TabLayout tabLayout,
@NonNull ViewPager2 viewPager,
boolean autoRefresh,
boolean smoothScroll,
@NonNull TabConfigurationStrategy tabConfigurationStrategy) {
this.tabLayout = tabLayout;
this.viewPager = viewPager;
this.autoRefresh = autoRefresh;
this.smoothScroll = smoothScroll;
this.tabConfigurationStrategy = tabConfigurationStrategy;
}
/**
* Link the TabLayout and the ViewPager2 together. Must be called after ViewPager2 has an adapter
* set. To be called on a new instance of TabLayoutMediator or if the ViewPager2's adapter
* changes.
*
* @throws IllegalStateException If the mediator is already attached, or the ViewPager2 has no
* adapter.
*/
public void attach() {
if (attached) {
throw new IllegalStateException("TabLayoutMediator is already attached");
}
adapter = viewPager.getAdapter();
if (adapter == null) {
throw new IllegalStateException(
"TabLayoutMediator attached before ViewPager2 has an " + "adapter");
}
attached = true;
// Add our custom OnPageChangeCallback to the ViewPager
onPageChangeCallback = new TabLayoutOnPageChangeCallback(tabLayout);
viewPager.registerOnPageChangeCallback(onPageChangeCallback);
// Now we'll add a tab selected listener to set ViewPager's current item
onTabSelectedListener = new ViewPagerOnTabSelectedListener(viewPager, smoothScroll);
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);
}
populateTabsFromPagerAdapter();
// Now update the scroll position to match the ViewPager's current item
tabLayout.setScrollPosition(viewPager.getCurrentItem(), 0f, true);
}
/**
* Unlink the TabLayout and the ViewPager. To be called on a stale TabLayoutMediator if a new one
* is instantiated, to prevent holding on to a view that should be garbage collected. Also to be
* called before {@link #attach()} when a ViewPager2's adapter is changed.
*/
public void detach() {
if (autoRefresh && adapter != null) {
if (pagerAdapterObserver != null) {
adapter.unregisterAdapterDataObserver(pagerAdapterObserver);
}
pagerAdapterObserver = null;
}
if (onTabSelectedListener != null) {
tabLayout.removeOnTabSelectedListener(onTabSelectedListener);
}
if (onPageChangeCallback != null) {
viewPager.unregisterOnPageChangeCallback(onPageChangeCallback);
}
onTabSelectedListener = null;
onPageChangeCallback = null;
adapter = null;
attached = false;
}
@SuppressWarnings("WeakerAccess")
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));
}
}
}
}
/**
* A {@link ViewPager2.OnPageChangeCallback} class which contains the necessary calls back to the
* provided {@link TabLayout} so that the tab position is kept in sync.
*
* <p>This class stores the provided TabLayout weakly, meaning that you can use {@link
* ViewPager2#registerOnPageChangeCallback(ViewPager2.OnPageChangeCallback)} without removing the
* callback and not cause a leak.
*/
private static class TabLayoutOnPageChangeCallback extends ViewPager2.OnPageChangeCallback {
@NonNull
private final WeakReference<TabLayout> tabLayoutRef;
private int previousScrollState;
private int scrollState;
TabLayoutOnPageChangeCallback(TabLayout tabLayout) {
tabLayoutRef = new WeakReference<>(tabLayout);
reset();
}
@Override
public void onPageScrollStateChanged(final int state) {
previousScrollState = scrollState;
scrollState = state;
}
@Override
public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
TabLayout tabLayout = tabLayoutRef.get();
if (tabLayout != null) {
// Only update the text selection if we're not settling, or we are settling after
// being dragged
boolean updateText =
scrollState != SCROLL_STATE_SETTLING || previousScrollState == SCROLL_STATE_DRAGGING;
// Update the indicator if we're not settling after being idle. This is caused
// from a setCurrentItem() call and will be handled by an animation from
// onPageSelected() instead.
boolean updateIndicator =
!(scrollState == SCROLL_STATE_SETTLING && previousScrollState == SCROLL_STATE_IDLE);
tabLayout.setScrollPosition(position, positionOffset, updateText, updateIndicator);
}
}
@Override
public void onPageSelected(final int position) {
TabLayout tabLayout = tabLayoutRef.get();
if (tabLayout != null
&& tabLayout.getSelectedTabPosition() != position
&& position < tabLayout.getTabCount()) {
// Select the tab, only updating the indicator if we're not being dragged/settled
// (since onPageScrolled will handle that).
boolean updateIndicator =
scrollState == SCROLL_STATE_IDLE
|| (scrollState == SCROLL_STATE_SETTLING
&& previousScrollState == SCROLL_STATE_IDLE);
tabLayout.selectTab(tabLayout.getTabAt(position), updateIndicator);
}
}
void reset() {
previousScrollState = scrollState = SCROLL_STATE_IDLE;
}
}
/**
* A {@link TabLayout.OnTabSelectedListener} class which contains the necessary calls back to the
* provided {@link ViewPager2} so that the tab position is kept in sync.
*/
private class ViewPagerOnTabSelectedListener implements TabLayout.OnTabSelectedListener {
private final ViewPager2 viewPager;
private final boolean smoothScroll;
ViewPagerOnTabSelectedListener(ViewPager2 viewPager, boolean smoothScroll) {
this.viewPager = viewPager;
this.smoothScroll = smoothScroll;
}
@Override
public void onTabSelected(@NonNull TabLayout.Tab tab) {
if (onPageChangeCallback != null) {
if (onPageChangeCallback.previousScrollState == SCROLL_STATE_DRAGGING) {
viewPager.setCurrentItem(tab.getPosition(), true);
} else {
viewPager.setCurrentItem(tab.getPosition(), smoothScroll);
}
}
}
@Override
public void onTabUnselected(TabLayout.Tab tab) {
// No-op
}
@Override
public void onTabReselected(TabLayout.Tab tab) {
// No-op
}
}
private class PagerAdapterObserver extends RecyclerView.AdapterDataObserver {
PagerAdapterObserver() {
}
@Override
public void onChanged() {
populateTabsFromPagerAdapter();
}
@Override
public void onItemRangeChanged(int positionStart, int itemCount) {
populateTabsFromPagerAdapter();
}
@Override
public void onItemRangeChanged(int positionStart, int itemCount, @Nullable Object payload) {
populateTabsFromPagerAdapter();
}
@Override
public void onItemRangeInserted(int positionStart, int itemCount) {
populateTabsFromPagerAdapter();
}
@Override
public void onItemRangeRemoved(int positionStart, int itemCount) {
populateTabsFromPagerAdapter();
}
@Override
public void onItemRangeMoved(int fromPosition, int toPosition, int itemCount) {
populateTabsFromPagerAdapter();
}
}
}
重要代码如下：
@Override
public void onTabSelected(@NonNull TabLayout.Tab tab) {
if (onPageChangeCallback != null) {
if (onPageChangeCallback.previousScrollState == SCROLL_STATE_DRAGGING) {
viewPager.setCurrentItem(tab.getPosition(), true);
} else {
viewPager.setCurrentItem(tab.getPosition(), smoothScroll);
}
}
}