TabLayoutMediator的坑
- 問題描述
- 問題分析
- ViewPager2翻頁的監聽
- TabLayout選擇Tab的監聽
- 解決方法
問題描述
當你想取消ViewPager2滑動時的影片，首先會去查看建構式有沒有設定這個的引數，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;
}
然而接著你欣喜若狂地將smoothScroll設定為false時卻發現ViewPager2失去了原來的慣性滑動效果，慣性滑動效果是指比如你將第一個頁面向左滑動到一定程度就會自動轉向第二頁，
那么為什么會失去這種效果呢？這里我們可以去分析TabLayoutMediator的原始碼：
問題分析
ViewPager2翻頁的監聽
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;
}
}
以上是對ViewPager2翻頁的監聽，包括滑動狀態改變、滑動中和選定頁面三種不同型別的監聽,
滑動狀態改變會記錄當前和現在的狀態，滑動中會依據狀態同步TabLayout的indicator（指示線）位置，選定頁面會同步選定Tab，
滑動狀態解釋如下
ViewPager.SCROLL_STATE_DRAGGING ：
當用戶按下ViewPager視圖并且需要滑動第一下時;
ViewPager.SCROLL_STATE_SETTLING：
當用戶滑動的放手讓其慣性滑動的時候，比如滑了放手觸發，如果用戶滑了左手邊一點然后不松手滑回原點 將不會觸發；
ViewPager.SCROLL_STATE_IDLE：
當用戶滑動的時候松手，
TabLayout選擇Tab的監聽
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
}
}
以上是對TabLayout選擇Tab的監聽，沒什么好說的，就只是設定了選定Tab時同步選定ViewPager2的頁面（smoothScroll決定是否帶翻頁影片），
當我們將smoothScroll設為false時，滑動ViewPager2同步將TabLayout的indicator（指示線）位置定位到剛剛觸碰第二個Tab的位置即選中了第二個Tab，緊接著監聽呼叫
setCurrentItem函式設定ViewPager2頁面然而沒有影片輔助糾正位置導致ViewPager2停留在之前第二個Tab的indicator（指示線）位置，
解決方法
復制TabLayoutMediator原始碼自己重寫一個MyTabLayoutMediator
設定滑動ViewPager2時如果之前狀態為ViewPager.SCROLL_STATE_DRAGGING（拖曳狀態)
smoothScroll設為true，否則為false
tips：
原本原始碼中的ViewPagerOnTabSelectedListener被static所修飾，因為要訪問TabLayoutOnPageChangeCallback中的previousScrollState（ViewPager2之前狀態）所以我將static去掉，也許會稍微影響性能，
自寫的MyTabLayoutMediator原始碼如下：
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();
}
}
}
重要代碼如下：
@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);
}
}
}
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