1. ViewPager2和RecyclerView滑动冲突。

背景

ViewPager2中使用了水平的RecyclerView。

现象

在网上看，大部分人碰见的都是RecyclerView无法滑动或者是ViewPager2无法滑动。但我碰到的现象是：如果点击RecyclerView后立刻就进行左右滑动，则是ViewPager2被滑动，而非RecyclerView被滑动；如果点击RecyclerView后停顿一下，再滑动，则可以正常滑动RecyclerView。

解决办法

使用Google提供的NestedScrollableHost.kt，包装一下RecyclerView即可。


/*
 * Copyright 2019 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
import android.content.Context
import android.util.AttributeSet
import android.view.MotionEvent
import android.view.View
import android.view.ViewConfiguration
import android.widget.FrameLayout
import androidx.viewpager2.widget.ViewPager2
import androidx.viewpager2.widget.ViewPager2.ORIENTATION_HORIZONTAL
import kotlin.math.absoluteValue
import kotlin.math.sign

/**
 * Layout to wrap a scrollable component inside a ViewPager2. Provided as a solution to the problem
 * where pages of ViewPager2 have nested scrollable elements that scroll in the same direction as
 * ViewPager2. The scrollable element needs to be the immediate and only child of this host layout.
 *
 * This solution has limitations when using multiple levels of nested scrollable elements
 * (e.g. a horizontal RecyclerView in a vertical RecyclerView in a horizontal ViewPager2).
 */
class NestedScrollableHost : FrameLayout {
    constructor(context: Context) : super(context)
    constructor(context: Context, attrs: AttributeSet?) : super(context, attrs)

    private var touchSlop = 0
    private var initialX = 0f
    private var initialY = 0f
    private val parentViewPager: ViewPager2?
        get() {
            var v: View? = parent as? View
            while (v != null && v !is ViewPager2) {
                v = v.parent as? View
            }
            return v as? ViewPager2
        }

    private val child: View? get() = if (childCount > 0) getChildAt(0) else null

    init {
        touchSlop = ViewConfiguration.get(context).scaledTouchSlop
    }

    private fun canChildScroll(orientation: Int, delta: Float): Boolean {
        val direction = -delta.sign.toInt()
        return when (orientation) {
            0 -> child?.canScrollHorizontally(direction) ?: false
            1 -> child?.canScrollVertically(direction) ?: false
            else -> throw IllegalArgumentException()
        }
    }

    override fun onInterceptTouchEvent(e: MotionEvent): Boolean {
        handleInterceptTouchEvent(e)
        return super.onInterceptTouchEvent(e)
    }

    private fun handleInterceptTouchEvent(e: MotionEvent) {
        val orientation = parentViewPager?.orientation ?: return

        // Early return if child can't scroll in same direction as parent
        if (!canChildScroll(orientation, -1f) && !canChildScroll(orientation, 1f)) {
            return
        }

        if (e.action == MotionEvent.ACTION_DOWN) {
            initialX = e.x
            initialY = e.y
            parent.requestDisallowInterceptTouchEvent(true)
        } else if (e.action == MotionEvent.ACTION_MOVE) {
            val dx = e.x - initialX
            val dy = e.y - initialY
            val isVpHorizontal = orientation == ORIENTATION_HORIZONTAL

            // assuming ViewPager2 touch-slop is 2x touch-slop of child
            val scaledDx = dx.absoluteValue * if (isVpHorizontal) .5f else 1f
            val scaledDy = dy.absoluteValue * if (isVpHorizontal) 1f else .5f

            if (scaledDx > touchSlop || scaledDy > touchSlop) {
                if (isVpHorizontal == (scaledDy > scaledDx)) {
                    // Gesture is perpendicular, allow all parents to intercept
                    parent.requestDisallowInterceptTouchEvent(false)
                } else {
                    // Gesture is parallel, query child if movement in that direction is possible
                    if (canChildScroll(orientation, if (isVpHorizontal) dx else dy)) {
                        // Child can scroll, disallow all parents to intercept
                        parent.requestDisallowInterceptTouchEvent(true)
                    } else {
                        // Child cannot scroll, allow all parents to intercept
                        parent.requestDisallowInterceptTouchEvent(false)
                    }
                }
            }
        }
    }
}

使用方法：

    <com.example.NestedScrollableHost
        android:id="@+id/nsh"
        android:layout_width="match_parent"
        android:layout_height="72dp"
        android:orientation="horizontal">
        <androidx.recyclerview.widget.RecyclerView
            android:id="@+id/rv"
            android:layout_width="match_parent"
            android:layout_height="72dp"/>
    </com.example.NestedScrollableHost>

2.ViewPager2滑动至边缘阴影取消。

现象

不仅是ViewPager2，类似ScrollView、RecyclerView类的控件，都会存在滑动至第一项或最后一项时，再继续滑动，会出现淡紫色波纹阴影。

解决办法

如果是RecyclerView，则直接设置：

binding.rvRoom.overScrollMode = View.OVER_SCROLL_NEVER

或在xml中设置：

android:overScrollMode="never"

即可。
但ViewPager2这么设置不可以，需用如下办法：

 //ViewPager 取消滑动到边缘阴影效果
        val child = binding.viewPager.getChildAt(0)
        if (child is RecyclerView) child.overScrollMode = View.OVER_SCROLL_NEVER

3. TabLayout和ViewPager2滑动效果

背景

当在TabLayout中间隔多个Tab点击时，ViewPager2会连续滑动过多个页面。
需要点击Tab时无滑动动画效果，滑动ViewPager2时依旧有动画效果。（类似Android端微信效果）

解决办法

重写TabLayoutMediator

public class TabLayoutMediators {

    private TabLayout tabLayout;
    private ViewPager2 viewPager;
    private boolean autoRefresh;
    private static boolean smoothScroll;
    private 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 TabLayoutMediators(
            @NonNull TabLayout tabLayout,
            @NonNull ViewPager2 viewPager,
            @NonNull TabConfigurationStrategy tabConfigurationStrategy) {
        this(tabLayout, viewPager, /* autoRefresh= */ true, tabConfigurationStrategy);
    }

    public TabLayoutMediators(
            @NonNull TabLayout tabLayout,
            @NonNull ViewPager2 viewPager,
            boolean autoRefresh,
            @NonNull TabConfigurationStrategy tabConfigurationStrategy) {
        this(tabLayout, viewPager, autoRefresh, /* smoothScroll= */ true, tabConfigurationStrategy);
    }

    public TabLayoutMediators(
            @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) {
            adapter.unregisterAdapterDataObserver(pagerAdapterObserver);
            pagerAdapterObserver = null;
        }
        tabLayout.removeOnTabSelectedListener(onTabSelectedListener);
        viewPager.unregisterOnPageChangeCallback(onPageChangeCallback);
        onTabSelectedListener = null;
        onPageChangeCallback = null;
        adapter = null;
        attached = false;
    }

    /**
     * Returns whether the {@link TabLayout} and the {@link ViewPager2} are linked together.
     */
    public boolean isAttached() {
        return attached;
    }

    @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) {
            if (state == SCROLL_STATE_DRAGGING) {
                smoothScroll = true;
            } else if (state == SCROLL_STATE_IDLE) {
                smoothScroll = false;
            }
            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 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
        }
    }

    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();
        }
    }
}

使用

注意：使用的是自定义的TabLayoutMediators ，而非TabLayoutMediator

 //定义TabLayout
        val tabLayoutMediator = TabLayoutMediators(
            binding.tabLayout,
            binding.viewPager
        ) { tab, position ->
            when (position) {
                0 -> tab.text = "智能"
                1 -> tab.text = "情景"
                2 -> tab.text = " 我 "
            }
        }