图文DEMO并茂讲解RecyclerView滑动时回收和复用触发的时机

class RecyclerViewActivity1 : AppCompatActivity() {
    private lateinit var mRecyclerView: RecyclerView
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_recycler_view1)
        mRecyclerView = findViewById(R.id.recyclerview)
        mRecyclerView.setHasFixedSize(true)
        mRecyclerView.setItemViewCacheSize(0)
        mRecyclerView.layoutManager =
            LinearLayoutManager(this).apply {
                orientation = LinearLayoutManager.VERTICAL
                isItemPrefetchEnabled = false
            }
        val list: MutableList<String> =
            ArrayList()
        repeat(100) {
            list.add("item $it")
        }
        mRecyclerView.adapter = MyAdapter(list)
    }
    inner class MyAdapter(val mStrings: MutableList<String>) :
        RecyclerView.Adapter<RecyclerView.ViewHolder>() {
        override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): RecyclerView.ViewHolder {
            println("RecyclerView 场景一 onCreateViewHolder ")
            val view = LayoutInflater.from(parent.context)
                .inflate(R.layout.view_item, parent, false)
            return object : RecyclerView.ViewHolder(view) {}
        }
        override fun getItemCount(): Int {
            return mStrings.size
        }
        override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) {
            println("RecyclerView 场景一 onBindViewHolder $position ")
            val textView = holder.itemView as TextView
            textView.layoutParams.height = (resources.displayMetrics.density * 100).toInt()
            textView.text = mStrings[position]
        }
        override fun onViewRecycled(holder: RecyclerView.ViewHolder) {
            println("RecyclerView 场景一 发生回收 " + (holder.itemView as TextView).text)
            super.onViewRecycled(holder)
        }
    }
    fun scroll120(view: View) {
        mRecyclerView.scrollBy(0, (resources.displayMetrics.density * 120).toInt())
    }
    fun scroll60(view: View) {
        mRecyclerView.scrollBy(0, (resources.displayMetrics.density * 60).toInt())
    }
    fun scroll40(view: View) {
        mRecyclerView.scrollBy(0, (resources.displayMetrics.density * 40).toInt())
    }
}

class RecyclerViewActivity2 : AppCompatActivity() {
    private lateinit var mRecyclerView: RecyclerView
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_recycler_view2)
        mRecyclerView = findViewById(R.id.recyclerview)
        mRecyclerView.setHasFixedSize(true)
        mRecyclerView.setItemViewCacheSize(0)
        mRecyclerView.layoutManager =
            LinearLayoutManager(this).apply {
                orientation = LinearLayoutManager.VERTICAL
                isItemPrefetchEnabled = false
            }
        val list: MutableList<String> =
            ArrayList()
        repeat(100) {
            list.add("item $it")
        }
        mRecyclerView.adapter = MyAdapter(list)
    }
    inner class MyAdapter(val mStrings: MutableList<String>) :
        RecyclerView.Adapter<RecyclerView.ViewHolder>() {
        override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): RecyclerView.ViewHolder {
            println("RecyclerView 场景二 onCreateViewHolder ")
            val view = LayoutInflater.from(parent.context)
                .inflate(R.layout.view_item, parent, false)
            return object : RecyclerView.ViewHolder(view) {}
        }
        override fun getItemCount(): Int {
            return mStrings.size
        }
        override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) {
            println("RecyclerView 场景二 onBindViewHolder $position ")
            val textView = holder.itemView as TextView
            textView.layoutParams.height =
                if (position == 0) (resources.displayMetrics.density * 50).toInt() else (resources.displayMetrics.density * 100).toInt()
            textView.text = mStrings[position]
        }
        override fun onViewRecycled(holder: RecyclerView.ViewHolder) {
            println("RecyclerView 场景二 发生回收 " + (holder.itemView as TextView).text)
            super.onViewRecycled(holder)
        }
    }
    fun scroll120(view: View) {
        mRecyclerView.scrollBy(0, (resources.displayMetrics.density * 120).toInt())
    }
    fun scroll60(view: View) {
        mRecyclerView.scrollBy(0, (resources.displayMetrics.density * 60).toInt())
    }
    fun scroll40(view: View) {
        mRecyclerView.scrollBy(0, (resources.displayMetrics.density * 40).toInt())
    }
}

//LinearLayoutManager.java
int scrollBy(int delta, RecyclerView.Recycler recycler, RecyclerView.State state) {
        if (getChildCount() == 0 || delta == 0) {
            return 0;
        }
        ensureLayoutState();
        mLayoutState.mRecycle = true;
        final int layoutDirection = delta > 0 ? LayoutState.LAYOUT_END : LayoutState.LAYOUT_START;
        final int absDelta = Math.abs(delta);
        ##代码1 updateLayoutState方法，主要是计算mOffset等参数。
        updateLayoutState(layoutDirection, absDelta, true, state);
        ##代码2 fill方法，根据剩余空间，填充View
        final int consumed = mLayoutState.mScrollingOffset
                + 
                fill(recycler, mLayoutState, state, false);
        if (consumed < 0) {
            if (DEBUG) {
                Log.d(TAG, "Don't have any more elements to scroll");
            }
            return 0;
        }
        final int scrolled = absDelta > consumed ? layoutDirection * consumed : delta;
        ##代码3 offsetChildren，整体移动RV的子View
        mOrientationHelper.offsetChildren(-scrolled);
        if (DEBUG) {
            Log.d(TAG, "scroll req: " + delta + " scrolled: " + scrolled);
        }
        mLayoutState.mLastScrollDelta = scrolled;
        return scrolled;
    }

//主要是计算
private void updateLayoutState(int layoutDirection, int requiredSpace,
            boolean canUseExistingSpace, RecyclerView.State state) {
        // If parent provides a hint, don't measure unlimited.
        mLayoutState.mInfinite = resolveIsInfinite();
        mLayoutState.mLayoutDirection = layoutDirection;
        mReusableIntPair[0] = 0;
        mReusableIntPair[1] = 0;
        calculateExtraLayoutSpace(state, mReusableIntPair);
        int extraForStart = Math.max(0, mReusableIntPair[0]);
        int extraForEnd = Math.max(0, mReusableIntPair[1]);
        boolean layoutToEnd = layoutDirection == LayoutState.LAYOUT_END;
        mLayoutState.mExtraFillSpace = layoutToEnd ? extraForEnd : extraForStart;
        mLayoutState.mNoRecycleSpace = layoutToEnd ? extraForStart : extraForEnd;
        int scrollingOffset;
        if (layoutToEnd) {
            mLayoutState.mExtraFillSpace += mOrientationHelper.getEndPadding();
            // get the first child in the direction we are going
            final View child = getChildClosestToEnd();
            // the direction in which we are traversing children
            mLayoutState.mItemDirection = mShouldReverseLayout ? LayoutState.ITEM_DIRECTION_HEAD
                    : LayoutState.ITEM_DIRECTION_TAIL;
            mLayoutState.mCurrentPosition = getPosition(child) + mLayoutState.mItemDirection;
            mLayoutState.mOffset = mOrientationHelper.getDecoratedEnd(child);
            // calculate how much we can scroll without adding new children (independent of layout)
            scrollingOffset = mOrientationHelper.getDecoratedEnd(child)
                    - mOrientationHelper.getEndAfterPadding();
        } else {
            final View child = getChildClosestToStart();
            mLayoutState.mExtraFillSpace += mOrientationHelper.getStartAfterPadding();
            mLayoutState.mItemDirection = mShouldReverseLayout ? LayoutState.ITEM_DIRECTION_TAIL
                    : LayoutState.ITEM_DIRECTION_HEAD;
            mLayoutState.mCurrentPosition = getPosition(child) + mLayoutState.mItemDirection;
            mLayoutState.mOffset = mOrientationHelper.getDecoratedStart(child);
            scrollingOffset = -mOrientationHelper.getDecoratedStart(child)
                    + mOrientationHelper.getStartAfterPadding();
        }
        mLayoutState.mAvailable = requiredSpace;
        if (canUseExistingSpace) {
            mLayoutState.mAvailable -= scrollingOffset;
        }
        mLayoutState.mScrollingOffset = scrollingOffset;
    }

int fill(RecyclerView.Recycler recycler, LayoutState layoutState,
            RecyclerView.State state, boolean stopOnFocusable) {
        // max offset we should set is mFastScroll + available
        final int start = layoutState.mAvailable;
        if (layoutState.mScrollingOffset != LayoutState.SCROLLING_OFFSET_NaN) {
            // TODO ugly bug fix. should not happen
            if (layoutState.mAvailable < 0) {
                layoutState.mScrollingOffset += layoutState.mAvailable;
            }
            ##代码1 首先判断是否需要回收View
            recycleByLayoutState(recycler, layoutState);
        }
        int remainingSpace = layoutState.mAvailable + layoutState.mExtraFillSpace;
        LayoutChunkResult layoutChunkResult = mLayoutChunkResult;
        ##代码2 根据剩余空间，判断是否需要填充View
        while ((layoutState.mInfinite || remainingSpace > 0) && layoutState.hasMore(state)) {
            layoutChunkResult.resetInternal();
            if (RecyclerView.VERBOSE_TRACING) {
                TraceCompat.beginSection("LLM LayoutChunk");
            }
            ##代码3 是具体的layout方法
            layoutChunk(recycler, state, layoutState, layoutChunkResult);
            if (RecyclerView.VERBOSE_TRACING) {
                TraceCompat.endSection();
            }
            if (layoutChunkResult.mFinished) {
                break;
            }
            layoutState.mOffset += layoutChunkResult.mConsumed * layoutState.mLayoutDirection;
            /**
             * Consume the available space if:
             * * layoutChunk did not request to be ignored
             * * OR we are laying out scrap children
             * * OR we are not doing pre-layout
             */
            if (!layoutChunkResult.mIgnoreConsumed || layoutState.mScrapList != null
                    || !state.isPreLayout()) {
                layoutState.mAvailable -= layoutChunkResult.mConsumed;
                // we keep a separate remaining space because mAvailable is important for recycling
                remainingSpace -= layoutChunkResult.mConsumed;
            }
            if (layoutState.mScrollingOffset != LayoutState.SCROLLING_OFFSET_NaN) {
                layoutState.mScrollingOffset += layoutChunkResult.mConsumed;
                if (layoutState.mAvailable < 0) {
                    layoutState.mScrollingOffset += layoutState.mAvailable;
                }
                ##代码4 是layout完成后判断是否需要回收View
                recycleByLayoutState(recycler, layoutState);
            }
            if (stopOnFocusable && layoutChunkResult.mFocusable) {
                break;
            }
        }
        if (DEBUG) {
            validateChildOrder();
        }
        return start - layoutState.mAvailable;
    }