λ:

# 仓库地址: https://github.com/lzyprime/android_demos/tree/recyclerview

git clone -b recyclerview https://github.com/lzyprime/android_demos

RecyclerView作 Android 列表项的展示组件。相比ListView,缓存机制做的更细致,提升流畅度。以空间换时间

两个重要参数:

  1. LayoutManager: 排版
  2. RecyclerView.Adapter: 列表项获取方式

LayoutManager

LayoutManager 可以在xml中直接配置. 也可在逻辑代码中设置。

// xml
   <androidx.recyclerview.widget.RecyclerView
        ...
        // LayoutManager类型
        app:layoutManager="androidx.recyclerview.widget.GridLayoutManager"
        // 几栏
        app:spanCount="1"
        />

全部可配参数:

1. LinearLayoutManager

public class LinearLayoutManager extends RecyclerView.LayoutManager implements
        ItemTouchHelper.ViewDropHandler, RecyclerView.SmoothScroller.ScrollVectorProvider

单栏线性布局。无法多栏展示。构造函数参数:

  1. orientation: 方向
  2. reverseLayout: 反转,倒序列表项

stackFromEnd 用来兼容 android.widget.AbsListView.setStackFromBottom(boolean)。相当于reverseLayout 的效果。

同时实现了ItemTouchHelper.ViewDropHandler, RecyclerView.SmoothScroller.ScrollVectorProvider

2. GridLayoutManager

public class GridLayoutManager extends LinearLayoutManager

网格布局。LinearLayoutManager 升级版,可以通过spanCount设置分几栏

3. StaggeredGridLayoutManager

public class StaggeredGridLayoutManager extends RecyclerView.LayoutManager implements
        RecyclerView.SmoothScroller.ScrollVectorProvider

流布局。 当列表项尺寸不一致时, GridLayoutManager 根据尺寸较大项确定网格尺寸。导致较小项会有空白部分。StaggeredGridLayoutManager 则紧凑拼接每一项。 通过 setGapStrategy(int) 设置间隙处理策略。

Adapter

RecyclerView.Adapter<VH : RecyclerView.ViewHolder>

public abstract static class Adapter<VH extends ViewHolder> {
    ...
    @NonNull
    public abstract VH onCreateViewHolder(@NonNull ViewGroup parent, int viewType);

    public abstract void onBindViewHolder(@NonNull VH holder, int position);

    public abstract int getItemCount();
}

public abstract static class ViewHolder {
    public ViewHolder(@NonNull View itemView) { ... }
}

一个Adapter至少需要override这三个函数。

getItemCount

返回列表项的个数。

onCreateViewHolder, getItemViewType

创建一个ViewHolder, 如果 ViewHolder 有多种类型,可以通过viewType参数判断。 viewType 的值来自 getItemViewType(position: Int) 函数。默认返回0。 0 <= position < getItemCount()

以聊天消息为例:

sealed class Msg {
    data class Text(val content: String) : Msg()
    data class Image(val url: String) : Msg()
    data class Video(...) : Msg()
    ...
}

class MsgListAdapter : RecyclerView.Adapter<MsgListAdapter.MsgViewHolder>() {
    sealed class MsgViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
        class Text(...) : MsgViewHolder(...)
        class Image(...) : MsgViewHolder(...)
        ...
    }

    private var dataList: List<Msg> = listOf()

    override fun getItemViewType(position: Int): Int =
        when (dataList[position]) {
            is Msg.Text -> 1
            is Msg.Image -> 2
            ...
        }

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): MsgViewHolder =
        when (viewType) {
            1 -> MsgViewHolder.Text(...)
            2 -> MsgViewHolder.Image(...)
            ...
        }
}

onBindViewHolder

View 创建完成,开始绑定数据。包括事件监听注册。

class VBViewHolder<VB : ViewBinding>(private val binding : VB) : ViewHolder(binding.root) {
    fun bind(data: T, onClick:() -> Unit) {
        binding.data = data
        ...
        binding.anyView.setOnClickListener { onClick() }
        ...
    }
}

class Adapter(private val onItemClick: () -> Unit) : RecyclerView.Adapter<VBViewHolder<XXX>>() {
    override fun onBindViewHolder(holder: VBViewHolder<XXX>, position: Int) =
        holder.bindHolder(dataList[position], onItemClick)
}

更新

由于缓存机制,更新完数据源, ViewHolder 也并不会立刻刷新。需要通过Adapter的一系列方法,显式通知发生变化的列表项。

  • notifyDataSetChanged()
  • notifyItemChanged(position: Int), notifyItemChanged(position: Int, payload: Any?)
  • notifyItemRangeChanged(positionStart: Int, itemCount: Int), notifyItemRangeChanged(positionStart: Int, itemCount: Int, payload: Any?)
  • notifyItemMoved(fromPosition: Int, toPosition: Int)
  • notifyItemInserted(position: Int)
  • notifyItemRangeInserted(positionStart: Int, itemCount: Int)
  • notifyItemRemoved(position: Int)
  • notifyItemRangeRemoved(positionStart: Int, itemCount: Int)

payload: Any? 要配合 AdapteronBindViewHolder(holder: VH, position: Int, payloads: MutableList<Any>) 实现 View 的局部刷新。否则,执行 onBindViewHolder(holder: VBViewHolder<VH>, position: Int)

缓存机制

主要逻辑在 RecyclerView.Recycler。 缓存主要有 Scrap, CachedView, RecycledViewPoolViewCacheExtension 用于额外自定义缓存。

  • Scrap: 当前正在展示的部分。
  • CachedView: 刚划出展示区域的部分,默认最大存储 DEFAULT_CACHE_SIZE = 2FIFO更新
  • RecycledViewPool: CachedView 淘汰后,只保留 ViewHolder, 清空数据绑定。 复用时需要重新执行onBindViewHolder

RecycledViewPool 内部是一个SparseArray<ScrapData> 下标为 holder.viewTypeScrapData 内嵌ArrayList<ViewHolder>, 默认最大存储 DEFAULT_MAX_SCRAP = 5ViewHolder。 所以简化一下RecycledViewPool ~= SparseArray<ArrayList<ViewHolder>>

public final class Recycler {
    final ArrayList<ViewHolder> mAttachedScrap = new ArrayList<>();
    ArrayList<ViewHolder> mChangedScrap = null;

    final ArrayList<ViewHolder> mCachedViews = new ArrayList<ViewHolder>();

    private final List<ViewHolder>
            mUnmodifiableAttachedScrap = Collections.unmodifiableList(mAttachedScrap);

    private int mRequestedCacheMax = DEFAULT_CACHE_SIZE;
    int mViewCacheMax = DEFAULT_CACHE_SIZE;

    RecycledViewPool mRecyclerPool;

    private ViewCacheExtension mViewCacheExtension;

    static final int DEFAULT_CACHE_SIZE = 2;

    ...
}
public static class RecycledViewPool {
    private static final int DEFAULT_MAX_SCRAP = 5;

    static class ScrapData {
        final ArrayList<ViewHolder> mScrapHeap = new ArrayList<>();
        int mMaxScrap = DEFAULT_MAX_SCRAP;
        long mCreateRunningAverageNs = 0;
        long mBindRunningAverageNs = 0;
    }

    SparseArray<ScrapData> mScrap = new SparseArray<>();

    private int mAttachCount = 0;
    ...
}

取, getViewForPosition

跟一下该函数就大概知道各级缓存如何配合。

@NonNull
public View getViewForPosition(int position) {
    return getViewForPosition(position, false);
}

View getViewForPosition(int position, boolean dryRun) {
    return tryGetViewHolderForPositionByDeadline(position, dryRun, FOREVER_NS).itemView;
}
@Nullable
RecyclerView.ViewHolder tryGetViewHolderForPositionByDeadline(int position, boolean dryRun, long deadlineNs) {
    ...
    boolean fromScrapOrHiddenOrCache = false;
    RecyclerView.ViewHolder holder = null;
    // 0) If there is a changed scrap, try to find from there
    if (mState.isPreLayout()) {
        holder = getChangedScrapViewForPosition(position);
        fromScrapOrHiddenOrCache = holder != null;
    }
    // 1) Find by position from scrap/hidden list/cache
    if (holder == null) {
        holder = getScrapOrHiddenOrCachedHolderForPosition(position, dryRun);
        ...
    }
    if (holder == null) {
        final int offsetPosition = mAdapterHelper.findPositionOffset(position);
        ...
        final int type = mAdapter.getItemViewType(offsetPosition);
        // 2) Find from scrap/cache via stable ids, if exists
        if (mAdapter.hasStableIds()) {
            holder = getScrapOrCachedViewForId(mAdapter.getItemId(offsetPosition), type, dryRun);
            ...
        }
        if (holder == null && mViewCacheExtension != null) {
            // We are NOT sending the offsetPosition because LayoutManager does not
            // know it.
            final View view = mViewCacheExtension.getViewForPositionAndType(this, position, type);
            ...
        }
        if (holder == null) { // fallback to pool
            ...
            holder = getRecycledViewPool().getRecycledView(type);
            ...
        }
        if (holder == null) {
            ...
            holder = mAdapter.createViewHolder(RecyclerView.this, type);
            ...
        }
    }
    ...

    return holder;
}
  • getChangedScrapViewForPosition
  • getScrapOrHiddenOrCachedHolderForPosition
  • getScrapOrCachedViewForId
  • mViewCacheExtension.getViewForPositionAndType
  • getRecycledViewPool().getRecycledView(type)
  • mAdapter.createViewHolder(RecyclerView.this, type)

放,recycleView

跟一下该函数,了解放入缓存过程和策略

public void recycleView(@NonNull View view) {
    ViewHolder holder = getChildViewHolderInt(view);
    ... // 清空flag

    recycleViewHolderInternal(holder);
    ...
}

void recycleViewHolderInternal(ViewHolder holder) {
    ... 
    final boolean transientStatePreventsRecycling = holder.doesTransientStatePreventRecycling();
    @SuppressWarnings("unchecked") final boolean forceRecycle = mAdapter != null && transientStatePreventsRecycling && mAdapter.onFailedToRecycleView(holder);
    boolean cached = false;
    boolean recycled = false;
    
    if (forceRecycle || holder.isRecyclable()) {
        if (mViewCacheMax > 0 && !holder.hasAnyOfTheFlags(...)) {
            // Retire oldest cached view
            int cachedViewSize = mCachedViews.size();
            if (cachedViewSize >= mViewCacheMax && cachedViewSize > 0) {
                recycleCachedViewAt(0);
                cachedViewSize--;
            }

            int targetCacheIndex = cachedViewSize;
            if (ALLOW_THREAD_GAP_WORK && cachedViewSize > 0 && !mPrefetchRegistry.lastPrefetchIncludedPosition(holder.mPosition)) {
                // when adding the view, skip past most recently prefetched views
                int cacheIndex = cachedViewSize - 1;
                while (cacheIndex >= 0) {
                    int cachedPos = mCachedViews.get(cacheIndex).mPosition;
                    if (!mPrefetchRegistry.lastPrefetchIncludedPosition(cachedPos)) {
                        break;
                    }
                    cacheIndex--;
                }
                targetCacheIndex = cacheIndex + 1;
            }
            mCachedViews.add(targetCacheIndex, holder);
            cached = true;
        }
        if (!cached) {
            addViewHolderToRecycledViewPool(holder, true);
            recycled = true;
        }
    } else {
        ... // Log
    }
    // even if the holder is not removed, we still call this method so that it is removed
    // from view holder lists.
    mViewInfoStore.removeViewHolder(holder);
    if (!cached && !recycled && transientStatePreventsRecycling) {
        holder.mBindingAdapter = null;
        holder.mOwnerRecyclerView = null;
    }
}
  • mCachedViews.add(targetCacheIndex, holder)
  • addViewHolderToRecycledViewPool

简化 & 封装 & 工具

一个 Adapter 的实现,大多数时候只关注 onBindViewHolder 的过程,以及数据更新时 notify 更新逻辑。剩下的操作,基本是重复的。

ListAdapter

默认实现了fun getItemCount() = dataList.size()

需要一个 DiffUtil.ItemCallback<T>,内部构造mDiffer: AsyncListDiffer<T>, 用于比较列表项的变化,然后自动刷新。

通过 submitList(List<T>?) 提交数据。

通过 getItem(position: Int): T = dataList[position] 获取当前位置对应数据。

省去了数据更新和notify的过程, 只需要关注onCreateViewHolder, onBindViewHolder

PS: 注意 submitList()和传引用问题。 做数据比较时 previousList, currentList 以及 Item 的比较,全是靠引用拿到,diff(previousList[index], currentList[index])。所以如果 submitList() 如果提交的同一份List, diff比较就会失效。

如果使用 Paging3 分页库, 在View层会有 PagingDataAdapter, 与 ListAdapter 类似。 将数据源 PagingData 等设置好后,列表便可以自动刷新,加载更多等。

public abstract class ListAdapter<T, VH extends RecyclerView.ViewHolder>
extends RecyclerView.Adapter<VH> {
    final AsyncListDiffer<T> mDiffer;
    private final AsyncListDiffer.ListListener<T> mListener = ...;

    protected ListAdapter(@NonNull DiffUtil.ItemCallback<T> diffCallback) { ... }
    protected ListAdapter(@NonNull AsyncDifferConfig<T> config) { ... }

    public void submitList(@Nullable List<T> list) { mDiffer.submitList(list); }
    public void submitList(@Nullable List<T> list, @Nullable final Runnable commitCallback) { mDiffer.submitList(list, commitCallback); }

    protected T getItem(int position) { return mDiffer.getCurrentList().get(position); }

    @Override public int getItemCount() { return mDiffer.getCurrentList().size(); }

    @NonNull public List<T> getCurrentList() { return mDiffer.getCurrentList(); }

    public void onCurrentListChanged(@NonNull List<T> previousList, @NonNull List<T> currentList) {}
}

DSL + ViewBinding

继续简化。

  • 大部分ViewHolderViewBinding 实现。 那 onCreateViewHolder() 也基本是重复的操作。

  • ViewBinding的创建过程也基本一致:ViewBinding.inflate(...)。 可以用 《android ViewBinding, DataBinding》 中的老方法,靠反射拿到。所以只需要 Adapter<VB : ViewBinding>onCreateViewHolder() 也可以省了。

  • DiffUtil.ItemCallback<T> 的实现也基本重复。 通常只需要两个lambda表达式说明情况。

// ViewHolder
data class BindingViewHolder<VB : ViewBinding>(val binding: VB) : RecyclerView.ViewHolder(binding.root)
// DiffUtil.ItemCallback<T>
inline fun <reified T> diffItemCallback(
    crossinline areItemsTheSame: (oldItem: T, newItem: T) -> Boolean,
    crossinline areContentsTheSame: (oldItem: T, newItem: T) -> Boolean = { o, n -> o == n },
) = object : DiffUtil.ItemCallback<T>() {
    override fun areItemsTheSame(oldItem: T, newItem: T): Boolean =
        areItemsTheSame(oldItem, newItem)

    override fun areContentsTheSame(oldItem: T, newItem: T): Boolean =
        areContentsTheSame(oldItem, newItem)
}
// ListAdapter<T, VH : ViewHolder>
fun <T, VH : RecyclerView.ViewHolder> dslListAdapter(
    diffItemCallback: DiffUtil.ItemCallback<T>,
    createHolder: (parent: ViewGroup, viewType: Int) -> VH,
    bindHolder: VH.(position: Int, data: T) -> Unit,
) = object : ListAdapter<T, VH>(diffItemCallback) {
    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): VH =
        createHolder(parent, viewType)

    override fun onBindViewHolder(holder: VH, position: Int) =
        holder.bindHolder(position, getItem(position))
}
/** 
* ListAdapter<T, BindingViewHolder<VB : ViewBinding>>
* 
* inflate 不传时,通过反射拿到VB的inflate 
* */
inline fun <T, reified VB : ViewBinding> dslBindingListAdapter(
    diffItemCallback: DiffUtil.ItemCallback<T>,
    noinline inflate: ((parent: ViewGroup, viewType: Int) -> VB)? = null,
    crossinline bindHolder: VB.(position: Int, data: T) -> Unit,
) = dslListAdapter(
    diffItemCallback,
    { p, v ->
        BindingViewHolder(
            inflate?.invoke(p, v) ?: VB::class.java.getMethod(
                "inflate",
                LayoutInflater::class.java,
                ViewGroup::class.java,
                Boolean::class.java
            ).invoke(null, LayoutInflater.from(p.context), p, false) as VB
        )
    },
    { p, d -> binding.bindHolder(p, d) },
)

使用:

val adapter = dslBindingListAdapter<Comment, ListItemSingleLineTextBinding>(
    diffItemCallback({ o, n -> o.id == n.id }, { o, n -> o == n }),
) { _, data ->
    // this is ListItemSingleLineTextBinding, 
    // data: Comment(id: Int, content: String)
    titleText.text = data
}

此外还有各种库也做了封装。最好靠(ksp,kapt)注解和编译器插件在编译期做代码生成,靠反射不保险还额外费资源

ItemTouchHelper

列表项滑动和拖拽。

public class ItemTouchHelper extends RecyclerView.ItemDecoration implements RecyclerView.OnChildAttachStateChangeListener
// use:
ItemTouchHelper(callback: ItemTouchHelper.Callback).attachToRecyclerView(recyclerView: RecyclerView?)

ItemTouchHelper.Callback

需要设定滑动和拖拽的方向START(LEFT), END(RIGHT), UP, DOWN

可通过onChildDraw(), onChildDrawOver() 等自定义滑动和拖拽过程中的行为。

object: ItemTouchHelper.Callback() {
    override fun getMovementFlags(recyclerView: RecyclerView, viewHolder: RecyclerView.ViewHolder): Int {
        // 返回滑动和拖拽的方向
    }

    override fun onMove(recyclerView: RecyclerView, viewHolder: RecyclerView.ViewHolder, target: RecyclerView.ViewHolder): Boolean {
        viewHolder // 被拖拽holder
        target // 正在经过holder
        // 返回是否允许滑动
    }

    override fun onSwiped(viewHolder: RecyclerView.ViewHolder, direction: Int) {
        direction // 滑动方向
    }
}

ItemTouchHelper.SimpleCallback

ItemTouchHelper.Callback 简版实现。构造函数传入滑动和拖拽方向。只需要关注onMove()onSwiped()过程。

public abstract static class SimpleCallback extends Callback {
    public SimpleCallback(int dragDirs, int swipeDirs)
    ...
}

自定义行为

override fun onChildDraw(
    c: Canvas, // holder所占区域的Canvas
    recyclerView: RecyclerView,
    viewHolder: RecyclerView.ViewHolder,
    dX: Float, // 用户动作引起的x移量
    dY: Float, // 用户动作引起的y移量
    actionState: Int, // 交互类型,swipe | drag
    isCurrentlyActive: Boolean, // 用户是否正在控制
) { ... }

onChildDraw的默认实现:translationX = dX, translationY = dY

public void onDraw(Canvas c, RecyclerView recyclerView, View view, float dX, float dY,
        int actionState, boolean isCurrentlyActive) {
    ...
    view.setTranslationX(dX);
    view.setTranslationY(dY);
}

dX, dY:

关于dX, dY的计算规则,要从头一点点看,attachToRecyclerView() 之后。

public void attachToRecyclerView(@Nullable RecyclerView recyclerView) { ...
    setupCallbacks();
... }

private void setupCallbacks() { ...
    mRecyclerView.addOnItemTouchListener(mOnItemTouchListener);
... }

private final OnItemTouchListener mOnItemTouchListener = new OnItemTouchListener() { ...
    select(...)
... };

void select(@Nullable ViewHolder selected, int actionState) { ...
    swipeIfNecessary(...)
... }

private int swipeIfNecessary(ViewHolder viewHolder) {
    checkHorizontalSwipe(...)
    checkVerticalSwipe(...)
}

// flags: 方向
private int checkHorizontalSwipe(ViewHolder viewHolder, int flags) { ...
    mCallback.getSwipeVelocityThreshold(...) // 速度临界点
    mCallback.getSwipeEscapeVelocity(...) // 最小速度
    final float threshold = mRecyclerView.getWidth() * 
    mCallback.getSwipeThreshold(viewHolder); // 位置临界点,默认0.5
... }

主要关注swipe过程,以及松手之后。

// 以水平滑动为例:
// 如果是默认行为: dx == holder.translationX
val oldDX // 开始滑动时的位置,也就是上次停止的位置。 abs(oldDX) == 0 || abs(oldDX) == holder.width

// 正在滑动时
val diffX: Int // 手指滑动偏移量
dX = oldDX + diffX

// 松开时:
val isSwiped = 是否超过了速度临界点或者位置临界点
if(true) {
    // 如果超过,dx 最终值根据oldDX和滑动方向确定。 
    // 最终值 = 如果之前为未滑动状态,则划出屏幕。如果之前未划出屏幕,则置为未滑动
    // 值变化靠动画补全
    dx = anim(curDX -> (abs(oldDX) == 0 ? holder.width : 0) * (direction == LEFT ? -1 : 1))
} else {
    // 如果未超过,dx开始还原回初始值
    dx = anim(curDX -> oldDX)
}

松手后会根据是否超过临界值,而选择最终位置。

demo: 添加震动, 半透明效果, 自定义绘制等

override fun onChildDraw(
    c: Canvas,
    recyclerView: RecyclerView,
    viewHolder: RecyclerView.ViewHolder,
    dX: Float,
    dY: Float,
    actionState: Int,
    isCurrentlyActive: Boolean
) {
    val midWidth = c.width / 2
    val absCurrentX = abs(viewHolder.itemView.translationX)

    // 震动
    if (absCurrentX < midWidth && abs(dX) >= midWidth) {
        val vibrator = requireContext().getSystemService(Vibrator::class.java) as Vibrator
        if (vibrator.hasVibrator()) {
            vibrator.vibrate(VibrationEffect.createOneShot(50, 255))
        }
    }

    // 半透明
    viewHolder.itemView.alpha = if (absCurrentX >= midWidth) 0.5f else 1f

    // 背景
    if (dX != 0f) {
        c.drawRect(
            0f,
            viewHolder.itemView.top.toFloat(),
            c.width.toFloat(),
            viewHolder.itemView.bottom.toFloat(),
            Paint().apply { color = Color.RED },
        )
    }

    super.onChildDraw(c, recyclerView, viewHolder, dX, dY, actionState, isCurrentlyActive)
}

二次滑动,展示侧滑菜单

最难控的就是dX, dY的变化。可以把getSwipeVelocityThreshold 速度临界点禁掉,只靠位置推算是否滑动成功。同时还要判断失去焦点时还原。

虽然能写出来,但是并不稳定。如果真有需求,不如自己实现ItemTouchHelper,大部分代码不用动,修改滑动判定,和松手后anim动画设置即可。

ConcatAdapter

Adapter 拼接。

需要引入recyclerview

implementation("androidx.recyclerview:recyclerview:latest")
public ConcatAdapter(@NonNull Adapter<? extends ViewHolder>... adapters)

public ConcatAdapter(@NonNull List<? extends Adapter<? extends ViewHolder>> adapters)

@SafeVarargs
public ConcatAdapter(
        @NonNull Config config,
        @NonNull Adapter<? extends ViewHolder>... adapters)

public ConcatAdapter(
        @NonNull Config config,
        @NonNull List<? extends Adapter<? extends ViewHolder>> adapters)

~λ:

2.25 开始写,现在 3.4 了。虽然内容多,中间也断断续续,但写总结仍然很耗时。 单纯看这些源码,写demo,也就花一下午,但整理要花这么久。

没有需求一阵子了。公司客户端开发需求并不多。而且本来也是我的个人爱好,当初只是人手不够暂时支援,结果越走越远,快要回不去后端了。

现在没有需求,打算回后端,从工作以来,写客户端(kotlin, flutter)的时间比后端还多。顶多LeetCode刷题用一下语言(kotlin, scala, c++, rust),平时自己玩一下Linux,但真正的开发,没怎么正经写过。

所以,就算换工作,也只能投递客户端,投后端基本没戏。