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。

发表5 分钟读完 (大约793个字)

Problems专题：RecyclerView

Problems专题：RecyclerView

0x01 Called attach on a child which is not detached

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java.lang.IllegalArgumentException: Called attach on a child which is not detached: BaseViewHolder{2b241e1 position=12 id=-1, oldPos=-1, pLpos:-1} androidx.recyclerview.widget.RecyclerView{afecb06 VFED..... ......ID 0,0-1080,2055 #7f09236e app:id/recycler_view_xxx}, adapter:com.xxxx.adapter.XxxAdapter@cfc75c7, layout:androidx.recyclerview.widget.LinearLayoutManager@24af7f4, context:com.xxxx.XxxActivity@1ed75e2
	at androidx.recyclerview.widget.RecyclerView$5.attachViewToParent(RecyclerView.java:917)
	at androidx.recyclerview.widget.ChildHelper.attachViewToParent(ChildHelper.java:239)
	at androidx.recyclerview.widget.RecyclerView.addAnimatingView(RecyclerView.java:1438)
	at androidx.recyclerview.widget.RecyclerView.animateDisappearance(RecyclerView.java:4377)
	at androidx.recyclerview.widget.RecyclerView$4.processDisappeared(RecyclerView.java:616)
	at androidx.recyclerview.widget.ViewInfoStore.process(ViewInfoStore.java:242)
	at androidx.recyclerview.widget.RecyclerView.dispatchLayoutStep3(RecyclerView.java:4210)
	at androidx.recyclerview.widget.RecyclerView.dispatchLayout(RecyclerView.java:3864)
	at androidx.recyclerview.widget.RecyclerView.onLayout(RecyclerView.java:4410)
	at android.view.View.layout(View.java:22213)
	at android.view.ViewGroup.layout(ViewGroup.java:6340)
	at android.widget.RelativeLayout.onLayout(RelativeLayout.java:1131)
	at android.view.View.layout(View.java:22213)
	at android.view.ViewGroup.layout(ViewGroup.java:6340)
	at androidx.viewpager.widget.ViewPager.onLayout(ViewPager.java:1775)
	at android.view.View.layout(View.java:22213)
	at android.view.ViewGroup.layout(ViewGroup.java:6340)
	at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1829)
	at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1673)
	at android.widget.LinearLayout.onLayout(LinearLayout.java:1582)
	at android.view.View.layout(View.java:22213)
	at android.view.ViewGroup.layout(ViewGroup.java:6340)
	at android.widget.RelativeLayout.onLayout(RelativeLayout.java:1131)
	at android.view.View.layout(View.java:22213)
	at android.view.ViewGroup.layout(ViewGroup.java:6340)
	at android.widget.FrameLayout.layoutChildren(FrameLayout.java:332)
	at android.widget.FrameLayout.onLayout(FrameLayout.java:270)
	at android.view.View.layout(View.java:22213)
	at android.view.ViewGroup.layout(ViewGroup.java:6340)
	at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1829)
	at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1673)
	at android.widget.LinearLayout.onLayout(LinearLayout.java:1582)
	at android.view.View.layout(View.java:22213)
	at android.view.ViewGroup.layout(ViewGroup.java:6340)
	at android.widget.FrameLayout.layoutChildren(FrameLayout.java:332)
	at android.widget.FrameLayout.onLayout(FrameLayout.java:270)
	at android.view.View.layout(View.java:22213)
	at android.view.ViewGroup.layout(ViewGroup.java:6340)
	at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1829)
	at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1673)
	at android.widget.LinearLayout.onLayout(LinearLayout.java:1582)
	at android.view.View.layout(View.java:22213)
	at android.view.ViewGroup.layout(ViewGroup.java:6340)
	at android.widget.FrameLayout.layoutChildren(FrameLayout.java:332)
	at android.widget.FrameLayout.onLayout(FrameLayout.java:270)
	at com.android.internal.policy.DecorView.onLayout(DecorView.java:905)
	at android.view.View.layout(View.java:22213)
	at android.view.ViewGroup.layout(ViewGroup.java:6340)
	at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:3286)
	at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2757)
	at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1865)
	at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:7933)
	at android.view.Choreographer$CallbackRecord.run(Choreographer.java:1018)
	at android.view.Choreographer.doCallbacks(Choreographer.java:837)
	at android.view.Choreographer.doFrame(Choreographer.java:767)
	at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:1003)
	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:7951)
	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)

问题分析

对同一个 position 位置同时进行notifyItemRemoved(position) 和 notifyItemInserted(position) 操作导致。

解决方案

避免同时对同一个位置先 notifyItemRemoved 再 notifyItemInserted，使用 notifyItemChanged。

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adapter?.notifyItemChanged(position)

0x02 RecyclerView设置最大高度、宽度

当RecyclerView属性设置为wrap_content+maxHeight时，maxHeight没有效果。

问题分析

当RecyclerView的LayoutManager#isAutoMeasureEnabled()返回true时，RecyclerView高度取决于children view的布局高度，并非取决于RecyclerView自身的测量高度。

解决方案

因此，我们只需要重写LayoutManager的public void setMeasuredDimension(Rect childrenBounds, int wSpec, int hSpec)方法即可为RecyclerView设置最大宽高。

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recyclerView.layoutManager = object : LinearLayoutManager(context, RecyclerView.VERTICAL, false) {
    override fun setMeasuredDimension(childrenBounds: Rect?, wSpec: Int, hSpec: Int) {
        val height = View.MeasureSpec.getSize(hSpec)
        val maxHeight = getScreenHeight() * 4 / 5
        val realHeight = height.coerceAtMost(maxHeight)
        val realHeightSpec = View.MeasureSpec.makeMeasureSpec(realHeight, AT_MOST)
        super.setMeasuredDimension(childrenBounds, wSpec, realHeightSpec)
    }
}

作者：猫爸iYao
链接：https://www.jianshu.com/p/0dec79ff70df
来源：简书
著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。

发表6 分钟读完 (大约838个字)

Problems专题：Parcel

Problems专题：Parcel

0x01 Unmarshalling unknown type

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Thread Name: 'main' 
java.lang.RuntimeException: Parcel android.os.Parcel@bbfcc04: Unmarshalling unknown type code 2131296928 at offset 1088
	at android.os.Parcel.readValue(Parcel.java:2750)
	at android.os.Parcel.readSparseArrayInternal(Parcel.java:3126)
	at android.os.Parcel.readSparseArray(Parcel.java:2354)
	at android.os.Parcel.readValue(Parcel.java:2728)
	at android.os.Parcel.readArrayMapInternal(Parcel.java:3045)
	at android.os.BaseBundle.initializeFromParcelLocked(BaseBundle.java:288)
	at android.os.BaseBundle.unparcel(BaseBundle.java:232)
	at android.os.Bundle.getSparseParcelableArray(Bundle.java:1010)
	at com.android.internal.policy.PhoneWindow.restoreHierarchyState(PhoneWindow.java:2133)
	at android.app.Activity.onRestoreInstanceState(Activity.java:1173)
	at android.app.Activity.performRestoreInstanceState(Activity.java:1128)
	at android.app.Instrumentation.callActivityOnRestoreInstanceState(Instrumentation.java:1318)
	at android.app.ActivityThread.handleStartActivity(ActivityThread.java:3025)
	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)

问题分析

情况1 Parcelable 对象为空，反序列化异常

情况2 Parcelable 序列化和反序列化的字段和顺序没有完全对应

情况3 自定义View的数据保存与恢复

解决方案

情况1 Parcelable 对象在序列化和反序列化增加 null 值判断

情况2 Parcelable 对象 read 和 write 字段的类型和顺序保持一直

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public class Account implements Parcelable {
    public Account(Parcel in) {
        this.name = in.readString();
        this.type = in.readInt();
        if (TextUtils.isEmpty(name)) {
            throw new android.os.BadParcelableException("the name must not be empty: " + name);
        }
        // ...
        this.accessId = in.readString();
        // ...
    }

    public int describeContents() {
        return 0;
    }

    public void writeToParcel(Parcel dest, int flags) {
        dest.writeString(name);
        dest.writeInt(type);
        dest.writeString(accessId);
    }
    // ...
}

情况3 自定义View的数据保存与恢复

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override fun onSaveInstanceState(): Parcelable? {
    Log.d("NavigationBar", "onSaveInstanceState: selectedId=${mSelectedId}")
    return SavedState(super.onSaveInstanceState(), mSelectedId)
}

override fun onRestoreInstanceState(state: Parcelable?) {
    if (state is SavedState) {
        val id = state.selectedId
        Log.d("NavigationBar", "onRestoreInstanceState: selectedId=${state.selectedId}")
        super.onRestoreInstanceState(state.superState)
        select(id)
        return
    }
    return super.onRestoreInstanceState(state)
}

internal class SavedState : BaseSavedState {
    var selectedId: Int = View.NO_ID

    constructor(source: Parcel) : super(source) {
        selectedId = source.readInt()
        Log.d("NavigationBar", "readFromParcel: selectedId=$selectedId")
    }

    constructor(superState: Parcelable?, selectedId: Int) : super(superState) {
        this.selectedId = selectedId
    }

    override fun writeToParcel(parcel: Parcel, flags: Int) {
        super.writeToParcel(parcel, flags)
        parcel.writeInt(selectedId)
        Log.d("NavigationBar", "writeToParcel: selectedId=$selectedId")
    }

    override fun describeContents(): Int {
        return 0
    }

    companion object CREATOR : Parcelable.Creator<SavedState> {
        override fun createFromParcel(parcel: Parcel): SavedState {
            return SavedState(parcel)
        }

        override fun newArray(size: Int): Array<SavedState?> {
            return arrayOfNulls(size)
        }
    }
}

0x02 android.os.TransactionTooLargeException

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java.lang.RuntimeException: android.os.TransactionTooLargeException: data parcel size 542688 bytes
	at android.app.servertransaction.PendingTransactionActions$StopInfo.run(PendingTransactionActions.java:160)
	at android.os.Handler.handleCallback(Handler.java:873)
	at android.os.Handler.dispatchMessage(Handler.java:99)
	at android.os.Looper.loop(Looper.java:207)
	at android.app.ActivityThread.main(ActivityThread.java:6878)
	at java.lang.reflect.Method.invoke(Native Method)
	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:876)
Caused by: android.os.TransactionTooLargeException: data parcel size 542688 bytes
	at android.os.BinderProxy.transactNative(Native Method)
	at android.os.BinderProxy.transact(BinderProxy.java:479)
	at android.app.IActivityManager$Stub$Proxy.activityStopped(IActivityManager.java:3941)
	at java.lang.reflect.Method.invoke(Native Method)
	at com.taobao.monitor.impl.common.c.invoke(ActivityManagerHook.java:89)
	at java.lang.reflect.Proxy.invoke(Proxy.java:1006)
	at $Proxy2.activityStopped(Unknown Source)
	at android.app.servertransaction.PendingTransactionActions$StopInfo.run(PendingTransactionActions.java:144)
	... 7 more
android.os.TransactionTooLargeException: data parcel size 542688 bytes
	at android.os.BinderProxy.transactNative(Native Method)
	at android.os.BinderProxy.transact(BinderProxy.java:479)
	at android.app.IActivityManager$Stub$Proxy.activityStopped(IActivityManager.java:3941)
	at java.lang.reflect.Method.invoke(Native Method)
	at com.taobao.monitor.impl.common.c.invoke(ActivityManagerHook.java:89)
	at java.lang.reflect.Proxy.invoke(Proxy.java:1006)
	at $Proxy2.activityStopped(Unknown Source)
	at android.app.servertransaction.PendingTransactionActions$StopInfo.run(PendingTransactionActions.java:144)
	at android.os.Handler.handleCallback(Handler.java:873)
	at android.os.Handler.dispatchMessage(Handler.java:99)
	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)

问题分析

1 Intent 传递的数据过大。

2 onSaveInstance 保存的数据过大。

解决方案

尽可能的使用少量的数据。

大数据考虑持久化和其他传递形式。

发表7 分钟读完 (大约1034个字)

链表相交

编写一个程序，找到两个单链表相交的起始节点。

例如，下面的两个链表：

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A:          a1 → a2
                   ↘
                     c1 → c2 → c3
                   ↗            
B:     b1 → b2 → b3

在节点 c1 开始相交。

注意：

  • 如果两个链表没有交点，返回 null.
  • 在返回结果后，两个链表仍须保持原有的结构。
  • 可假定整个链表结构中没有循环。
  • 程序尽量满足 O(n) 时间复杂度，且仅用 O(1) 内存。

方法一：哈希集合

思路和算法

判断两个链表是否相交，可以使用哈希集合存储链表节点。

首先遍历链表 headA，并将链表 headA 中的每个节点加入哈希集合中。然后遍历链表 headB，对于遍历到的每个节点，判断该节点是否在哈希集合中：

  • 如果当前节点不在哈希集合中，则继续遍历下一个节点；

  • 如果当前节点在哈希集合中，则后面的节点都在哈希集合中，即从当前节点开始的所有节点都在两个链表的相交部分，因此在链表 headB 中遍历到的第一个在哈希集合中的节点就是两个链表相交的节点，返回该节点。

如果链表 headB 中的所有节点都不在哈希集合中，则两个链表不相交，返回 null。

复杂度分析

时间复杂度：O(m+n)，其中 m 和 n 是分别是链表 headA 和 headB 的长度。需要遍历两个链表各一次。

空间复杂度：O(m)，其中 m 是链表 headA 的长度。需要使用哈希集合存储链表 headA 中的全部节点。

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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
        Set<ListNode> visited = new HashSet<ListNode>();
        ListNode temp = headA;
        while (temp != null) {
            visited.add(temp);
            temp = temp.next;
        }
        temp = headB;
        while (temp != null) {
            if (visited.contains(temp)) {
                return temp;
            }
            temp = temp.next;
        }
        return null;
    }
}

方法二：双指针

思路和算法

使用双指针的方法，可以将空间复杂度降至 O(1)。

只有当链表 headA 和 headB 都不为空时，两个链表才可能相交。因此首先判断链表 headA 和 headB 是否为空，如果其中至少有一个链表为空，则两个链表一定不相交，返回 null。

当链表 headA 和 headB 都不为空时，创建两个指针 pA 和 pB，初始时分别指向两个链表的头节点 headA 和 headB，然后将两个指针依次遍历两个链表的每个节点。具体做法如下：

  • 每步操作需要同时更新指针 pA 和 pB；

  • 如果指针 pA 不为空，则将指针 pA 移到下一个节点；如果指针 pB 不为空，则将指针 pB 移到下一个节点。

  • 如果指针 pA 为空，则将指针 pA 移到链表 headB 的头节点；如果指针 pB 为空，则将指针 pB 移到链表 headA 的头节点。

  • 当指针 pA 和 pB 指向同一个节点或者都为空时，返回它们指向的节点或者 null。

证明

考虑两种情况，第一种情况是两个链表相交，第二种情况是两个链表不相交。

复杂度分析

时间复杂度：O(m+n)，其中 m 和 n 是分别是链表 headA 和 headB 的长度。两个指针同时遍历两个链表，每个指针遍历两个链表各一次。

空间复杂度：O(1)。

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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
  public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
    if (headA == null || headB == null) {
      return null;
    }
    ListNode pA = headA, pB = headB;
    while (pA != pB) {
      pA = pA == null ? headB : pA.next;
      pB = pB == null ? headA : pB.next;
    }
    return pA;
  }
}

参考连接：

https://leetcode-cn.com/problems/intersection-of-two-linked-lists/solution/xiang-jiao-lian-biao-by-leetcode-solutio-a8jn/

https://zhuanlan.zhihu.com/p/48313122

发表1 分钟读完 (大约141个字)

RecyclerView Item 嵌套 ScrollView

RecyclerView Item 嵌套 ScrollView

RecyclerView Item 嵌套 ScrollView 产生 Touch 事件冲突，通过自定义ScrollView来拦截和处理事件

image-20210908211849925

自定义 ItemScrollView 代码如下

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import android.content.Context
import android.util.AttributeSet
import android.view.MotionEvent
import android.widget.ScrollView

class ItemScrollView @JvmOverloads constructor(
    context: Context,
    attrs: AttributeSet? = null,
    defStyleAttr: Int = 0
) : ScrollView(context, attrs, defStyleAttr) {

    override fun onInterceptTouchEvent(ev: MotionEvent?): Boolean {
        parent.requestDisallowInterceptTouchEvent(true)
        return super.onInterceptTouchEvent(ev)
    }

    private var lastY: Float = 0f

    override fun onTouchEvent(ev: MotionEvent?): Boolean {
        when (ev?.action) {
            MotionEvent.ACTION_DOWN -> {
                lastY = ev.y
            }

            MotionEvent.ACTION_MOVE -> {
                val currentY = ev.y
                this.scrollBy(0, (lastY - currentY).toInt())
                lastY = currentY
            }

            MotionEvent.ACTION_UP -> {
                lastY = 0f
            }
        }

        return canScroll()
    }

    private fun canScroll(): Boolean {
        val child = getChildAt(0)
        child?.let {
            return height < child.height
        }
        return false
    }
}
发表2 分钟读完 (大约275个字)

PowerShell最佳实践

PowerShell最佳实践

Windows 10 系统自带 PowerShell，美化教程

image-20210909150114264

0x01 安装Fluent Terminal

在Windows 应用商店安装，或者Github

0x02 安装powershell模块

1、安装posh-git、oh-my-posh和Get-ChildItemColor(美化ls命令)：

在powershell管理员模式下：

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$ Install-Module posh-git -Scope CurrentUser
$ Install-Module oh-my-posh -Scope CurrentUser
$ Install-Module DirColors

2、设置修改powershell的配置文件：

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$ if (!(Test-Path -Path $PROFILE )) { New-Item -Type File -Path $PROFILE -Force }
$ notepad $PROFILE

输入内容：

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Import-Module DirColors
Import-Module posh-git
Import-Module oh-my-posh
Set-PoshPrompt -Theme PowerLine

其中主题名可以在下面的路径里找到,可以自行切换主题。

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C:\Program Files\WindowsPowerShell\Modules\oh-my-posh\3.163.0\themes

0x03 安装Powerline字体

在Fluent Terminal设置 powerline 字体和字体大小。

0x04 文件管理器命令

在文件夹中打开：

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# I 在文件夹中打开
$ explorer (gl)
# II 在文件夹中打开
$ start .
# III 在文件夹中打开
$ ii .
# 打开当前根目录
$ ii /

在当前文件夹打开PowerShell：

空白处 Shift + 鼠标右键，在弹出的菜单中选择PowerShell.

0x05 VS Code 命令

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