发表13 分钟读完 (大约1985个字)

Problems专题：ViewPager2

ViewPager2

0x01 FragmentManager is already executing transactions

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java.lang.IllegalStateException: FragmentManager is already executing transactions
	at androidx.fragment.app.FragmentManager.ensureExecReady(FragmentManager.java:1778)
	at androidx.fragment.app.FragmentManager.execSingleAction(FragmentManager.java:1814)
	at androidx.fragment.app.BackStackRecord.commitNow(BackStackRecord.java:297)

	at androidx.viewpager2.adapter.FragmentStateAdapter.removeFragment(FragmentStateAdapter.java:464)
	at androidx.viewpager2.adapter.FragmentStateAdapter.gcFragments(FragmentStateAdapter.java:228)
	at androidx.viewpager2.adapter.FragmentStateAdapter.restoreState(FragmentStateAdapter.java:569)
	at androidx.viewpager2.widget.ViewPager2.restorePendingState(ViewPager2.java:350)
	at androidx.viewpager2.widget.ViewPager2.dispatchRestoreInstanceState(ViewPager2.java:375)

	at android.view.ViewGroup.dispatchRestoreInstanceState(ViewGroup.java:3829)
	at android.view.ViewGroup.dispatchRestoreInstanceState(ViewGroup.java:3829)
	at android.view.ViewGroup.dispatchRestoreInstanceState(ViewGroup.java:3829)
	at android.view.ViewGroup.dispatchRestoreInstanceState(ViewGroup.java:3829)
	at android.view.ViewGroup.dispatchRestoreInstanceState(ViewGroup.java:3829)

	at android.view.View.restoreHierarchyState(View.java:18613)

	at androidx.fragment.app.Fragment.restoreViewState(Fragment.java:573)
	at androidx.fragment.app.FragmentStateManager.restoreViewState(FragmentStateManager.java:356)
	at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1189)
	at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1356)
	at androidx.fragment.app.FragmentManager.moveFragmentToExpectedState(FragmentManager.java:1434)
	at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1497)
	at androidx.fragment.app.FragmentManager.dispatchStateChange(FragmentManager.java:2625)
	at androidx.fragment.app.FragmentManager.dispatchActivityCreated(FragmentManager.java:2577)

	at androidx.fragment.app.FragmentController.dispatchActivityCreated(FragmentController.java:247)

	at androidx.fragment.app.FragmentActivity.onStart(FragmentActivity.java:541)
	at androidx.appcompat.app.AppCompatActivity.onStart(AppCompatActivity.java:210)
	at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1392)
	at android.app.Activity.performStart(Activity.java:7260)
	at android.app.ActivityThread.handleStartActivity(ActivityThread.java:3009)

	at android.app.servertransaction.TransactionExecutor.performLifecycleSequence(TransactionExecutor.java:180)
	at android.app.servertransaction.TransactionExecutor.cycleToPath(TransactionExecutor.java:165)
	at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:142)
	at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:70)
	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1840)
	at android.os.Handler.dispatchMessage(Handler.java:106)
	at android.os.Looper.loop(Looper.java:207)
	at android.app.ActivityThread.main(ActivityThread.java:6878)
	at java.lang.reflect.Method.invoke(Native Method)
	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:876)

解决方案

如果在 Fragment 中使用 ViewPager2，那么 FragmentStateAdapter 应该使用 childFragmentManager。将

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FragmentStateAdapter viewPagerAdapter = new FragmentStateAdapter(getActivity().getSupportFragmentManager(), titles);

改为

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FragmentStateAdapter viewPagerAdapter = new FragmentStateAdapter(getChildFragmentManager(), titles);

0x02 ViewPager2+FragmentStateAdapter 的 notifyDataSetChanged 方法失效

原因分析：

因为 FragmentStateAdapter 会保存所有 Fragment 实例，当调用 Adapter.notifyDataSetChanged() 方法时，Fragment 并没有走 onCreate 方法。

解决方案：

方案一（这个方法会导致内存泄漏，不推荐）

在调用 notifyDataSetChanged 之前，清空 FragmentStateAdapter 的 Fragment 列表。

方案二

重写 getItemId() containsItem() 这两个方法，并确保 getItemId() 的值是唯一的。

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override fun createViewPagerAdapter(): RecyclerView.Adapter<*> {
    val items = items // avoids resolving the ViewModel multiple times
    return object : FragmentStateAdapter(this) {
        override fun createFragment(position: Int): PageFragment {
            val itemId = items.itemId(position)
            val itemText = items.getItemById(itemId)
            return PageFragment.create(itemText)
        }
        override fun getItemCount(): Int = items.size
        override fun getItemId(position: Int): Long = items.itemId(position)
        override fun containsItem(itemId: Long): Boolean = items.contains(itemId)
    }
}

/** A very simple collection of items. Optimized for simplicity (i.e. not performance). */
class ItemsViewModel : ViewModel() {
    private var nextValue = 1L

    private val items = (1..9).map { longToItem(nextValue++) }.toMutableList()

    fun getItemById(id: Long): String = items.first { itemToLong(it) == id }
    fun itemId(position: Int): Long = itemToLong(items[position])
    fun contains(itemId: Long): Boolean = items.any { itemToLong(it) == itemId }
    fun addNewAt(position: Int) = items.add(position, longToItem(nextValue++))
    fun removeAt(position: Int) = items.removeAt(position)
    fun createIdSnapshot(): List<Long> = (0 until size).map { position -> itemId(position) }
    val size: Int get() = items.size

    private fun longToItem(value: Long): String = "item#$value"
    private fun itemToLong(value: String): Long = value.split("#")[1].toLong()
}

0x03 Design assumption violated

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java.lang.IllegalStateException: Design assumption violated.
    at androidx.viewpager2.widget.ViewPager2.updateCurrentItem(ViewPager2.java:538)
    at androidx.viewpager2.widget.ViewPager2$4.onAnimationsFinished(ViewPager2.java:518)
    at androidx.recyclerview.widget.RecyclerView$ItemAnimator.isRunning(RecyclerView.java:13244)
    at androidx.viewpager2.widget.ViewPager2.onLayout(ViewPager2.java:515)
    at android.view.View.layout(View.java:15596)

解决方案：

如果重写了 getItemId() containsItem() 这两个方法，确保 getItemId() 的值是唯一的。代码同 0x02

0x04 ViewPager2 嵌套 RecyclerView 手势冲突问题

img

原因分析：

同方向滚动事件被 ViewPager2 拦截了。

解决方案：

方案一 自定义 NestedScrollableHost

采用官方提供的自定义 NestedScrollableHost 来包一层 RecyclerView

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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)
                    }
                }
            }
        }
    }
}

对应的 layout 代码：

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<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:tools="http://schemas.android.com/tools"
    android:orientation="vertical">
... 水平
    <androidx.viewpager2.integration.testapp.NestedScrollableHost
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="8dp">
        <androidx.recyclerview.widget.RecyclerView
            android:id="@+id/first_rv"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:background="#FFFFFF" />
    </androidx.viewpager2.integration.testapp.NestedScrollableHost>
 ... 竖直
    <androidx.viewpager2.integration.testapp.NestedScrollableHost
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_marginLeft="20dp"
        android:layout_marginRight="20dp"
        android:layout_marginTop="8dp"
        android:layout_weight="1">
        <androidx.recyclerview.widget.RecyclerView
            android:id="@+id/second_rv"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:background="#FFFFFF" />
    </androidx.viewpager2.integration.testapp.NestedScrollableHost>

</LinearLayout>

方案二 自定义 RecyclerView

自定义 NestedRecyclerView 的分发事件通过 requestDisallowInterceptTouchEvent() 方法来限制父布类的拦截事件

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public class NestedRecyclerView extends RecyclerView {
    public NestedRecyclerView(@NonNull Context context) {
        super(context);
    }
    public NestedRecyclerView(@NonNull Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }
    public NestedRecyclerView(@NonNull Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    private int startX, startY;

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        switch (ev.getAction()) {
            case MotionEvent.ACTION_DOWN:
                startX = (int) ev.getX();
                startY = (int) ev.getY();
                getParent().requestDisallowInterceptTouchEvent(true);
                break;
            case MotionEvent.ACTION_MOVE:
                int endX = (int) ev.getX();
                int endY = (int) ev.getY();
                int disX = Math.abs(endX - startX);
                int disY = Math.abs(endY - startY);

                if (disX > disY) {
                    getParent().requestDisallowInterceptTouchEvent(canScrollHorizontally(startX - endX));
                } else {
                    getParent().requestDisallowInterceptTouchEvent(canScrollVertically(startX - endX));
                }
                break;
            case MotionEvent.ACTION_UP:
            case MotionEvent.ACTION_CANCEL:
                getParent().requestDisallowInterceptTouchEvent(false);
                break;
        }
        return super.dispatchTouchEvent(ev);
    }
}

方法三 使用 ViewPager

降级，使用 ViewPager 来嵌套 RecyclerView ，可以避免事件冲突，亲测有效。

0x05 ViewPager2 两边保留上一页预览

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import android.os.Bundle
import android.view.LayoutInflater
import android.view.ViewGroup
import android.widget.Toast
import androidx.fragment.app.FragmentActivity
import androidx.recyclerview.widget.RecyclerView
import androidx.viewpager2.widget.ViewPager2

class PreviewPagesActivity : FragmentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_viewpager2)
        findViewById<ViewPager2>(R.id.view_pager).apply {
            // Set offscreen page limit to at least 1, so adjacent pages are always laid out
            offscreenPageLimit = 1
            val recyclerView = getChildAt(0) as RecyclerView
            recyclerView.apply {
                clipToPadding = false
                val leftPadding = resources.getDimensionPixelOffset(R.dimen.halfPageMargin) +
                resources.getDimensionPixelOffset(R.dimen.peekOffset)
                // setting padding on inner RecyclerView puts overscroll effect in the right place
                // TODO: expose in later versions not to rely on
                //  getChildAt(0) which might break
                setPadding(leftPadding, 0, leftPadding, 0)
            }
            adapter = Adapter()
        }
    }

    class ViewHolder(parent: ViewGroup) : RecyclerView.ViewHolder(
        LayoutInflater.from(parent.context).inflate(R.layout.item_preview_pages, parent, false)
    )

    class Adapter : RecyclerView.Adapter<RecyclerView.ViewHolder>() {
        override fun getItemCount(): Int {
            return 10
        }

        override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): RecyclerView.ViewHolder {
            return ViewHolder(parent)
        }

        override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) {
            holder.itemView.tag = position
            holder.itemView.setOnClickListener {
                Toast.makeText(it.context, "position=$position", Toast.LENGTH_LONG).show()
            }
        }
    }
}

0x06 Fragment no longer exists for key f1

在 Fragment 中使用 ViewPager 的时候，切换 Fragment 导致 ViewPager 无法正确恢复异常

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java.lang.IllegalStateException: Fragment no longer exists for key f1: unique id 55efaee5-a65c-4e57-9281-7c8f8f6e4156
    at androidx.fragment.app.FragmentManager.getFragment(FragmentManager.java:960)
    at androidx.fragment.app.FragmentStatePagerAdapter.restoreState(FragmentStatePagerAdapter.java:328)
    at androidx.viewpager.widget.ViewPager.onRestoreInstanceState(ViewPager.java:1461)
    at android.view.View.dispatchRestoreInstanceState(View.java:20032)
    at android.view.ViewGroup.dispatchRestoreInstanceState(ViewGroup.java:3922)
    at android.view.ViewGroup.dispatchRestoreInstanceState(ViewGroup.java:3928)
    at android.view.View.restoreHierarchyState(View.java:20010)
    at androidx.fragment.app.Fragment.restoreViewState(Fragment.java:639)
    at androidx.fragment.app.Fragment.restoreViewState(Fragment.java:3010)
    at androidx.fragment.app.Fragment.performActivityCreated(Fragment.java:3001)
    at androidx.fragment.app.FragmentStateManager.activityCreated(FragmentStateManager.java:580)
    at androidx.fragment.app.FragmentStateManager.moveToExpectedState(FragmentStateManager.java:285)
    at androidx.fragment.app.FragmentManager.executeOpsTogether(FragmentManager.java:2189)
    at androidx.fragment.app.FragmentManager.removeRedundantOperationsAndExecute(FragmentManager.java:2100)
    at androidx.fragment.app.FragmentManager.execPendingActions(FragmentManager.java:2002)
    at androidx.fragment.app.FragmentManager$5.run(FragmentManager.java:524)
    at android.os.Handler.handleCallback(Handler.java:883)
    at android.os.Handler.dispatchMessage(Handler.java:100)
    at android.os.Looper.loop(Looper.java:230)
    at android.app.ActivityThread.main(ActivityThread.java:8018)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:526)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1034)

20240301183554

在这个页面中，内容列表使用 ViewPager 嵌套 Fragment 实现，并和时间选择 Tab 绑定。切换【即将上线】和【播出时间表】Tab，实际是使用 FragmentManager 的 replace 方法，动态切换两个 Fragment，然后就报了上面的异常。

网上流行的解决方案是使用 FragmentPagerAdapter 或者添加

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@Override
public Parcelable saveState() {
    return null;
}

但是这样处理会导致 ViewPager 中的 fragments 全部无法恢复，导致 ViewPager 白屏。

本例中的解决方案是：

在切换【即将上线】和【播出时间表】Tab 时，不使用 FragmentManager 的 replace 方法，而采用动态的 Hide 和 show 方法暂时规避 Fragment 被回收的问题。

Problems专题：ViewPager2

https://denchopen.github.io/blog/2021/06/20/Problems专题之ViewPager2/

作者

Dench

发布于

2021-06-20

更新于

2021-06-20

许可协议

CC BY-NC-SA 4.0

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